path: root/lib/mnesia/doc/src/mnesia.xml

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<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2021</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at

          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>mnesia</title>
    <prepared>Claes Wikstr&ouml;m and H&aring;kan Mattsson</prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev></rev>
    <file></file>
  </header>
  <module since="">mnesia</module>
  <modulesummary>A distributed telecommunications DBMS</modulesummary>
  <description>

    <p>The following are some of the most important and attractive
      capabilities provided by Mnesia:</p>
    <list type="bulleted">
      <item>A relational/object hybrid data model that is suitable
       for telecommunications applications.
      </item>
      <item>A DBMS query language, Query List Comprehension (QLC) as
       an add-on library.
      </item>
      <item>Persistence. Tables can be coherently kept on disc and
       in the main memory.
      </item>
      <item>Replication. Tables can be replicated at several nodes.
      </item>
      <item>Atomic transactions. A series of table manipulation
       operations can be grouped into a single atomic transaction.
      </item>
      <item>Location transparency. Programs can be written without
       knowledge of the actual data location.
      </item>
      <item>Extremely fast real-time data searches.
      </item>
      <item>Schema manipulation routines. The DBMS can be
       reconfigured at runtime without stopping the system.
      </item>
    </list>
    <p>This Reference Manual describes the Mnesia API. This
      includes functions that define and manipulate Mnesia
      tables.</p>
    <p>All functions in this Reference Manual can be used in any
      combination with queries using the list comprehension notation.
      For information about the query notation, see the
      <seeerl marker="stdlib:qlc">qlc</seeerl>
      manual page in STDLIB.</p>
    <p>Data in Mnesia is organized as a set of tables. Each table
      has a name that must be an atom. Each table is made up of
      Erlang records. The user is responsible for the record
      definitions. Each table also has a set of properties. The
      following are some of the properties that are associated with each
      table:</p>
    <list type="bulleted">
      <item>
        <p><c>type</c>. Each table can have <c>set</c>,
          <c>ordered_set</c>, or <c>bag</c> semantics. Notice that
          currently <c>ordered_set</c> is not supported for
          <c>disc_only_copies</c>.</p>
        <p>If a table is of type <c>set</c>, each key leads to
          either one or zero records.</p>
        <p>If a new item is inserted with the same key as an
          existing record, the old record is overwritten. However,
          if a table is of type <c>bag</c>, each key can map to
          several records. All records in type <c>bag</c> tables are
          unique, only the keys can be duplicated.</p>
      </item>
      <item>
        <p><c>record_name</c>. All records stored in a table must
          have the same name. The records must be instances of the
          same record type.</p>
      </item>
      <item>
        <p><c>ram_copies</c>. A table can be replicated on a number
          of Erlang nodes. Property <c>ram_copies</c> specifies a
          list of Erlang nodes where RAM copies are kept. These
          copies can be dumped to disc at regular intervals. However,
          updates to these copies are not written to disc on a
          transaction basis.</p>
      </item>
      <item>
        <p><c>disc_copies</c>. This property
          specifies a list of Erlang nodes where the table is kept in
          RAM and on disc. All updates of the table are
          performed in the actual table and are also logged to disc.
          If a table is of type <c>disc_copies</c> at a certain node,
          the entire table is resident in RAM memory and on disc.
          Each transaction performed on the table is appended to a
          <c>LOG</c> file and written into the RAM table.</p>
      </item>
      <item>
        <p><c>disc_only_copies</c>. Some, or all, table replicas
          can be kept on disc only. These replicas are considerably
          slower than the RAM-based replicas.</p>
      </item>
      <item>
        <p><c>index</c>. This is a list of attribute names, or
          integers, which specify the tuple positions on which
          Mnesia is to build and maintain an extra index
          table.</p>
      </item>
      <item>
        <p><c>local_content</c>. When an application requires
          tables whose contents are local to each node,
          <c>local_content</c> tables can be used. The table name
          is known to all Mnesia nodes, but its content is
          unique on each node. This means that access to such a table
          must be done locally. Set field <c>local_content</c> to
          <c>true</c> to enable the <c>local_content</c>
          behavior. Default is <c>false</c>.</p>
      </item>
      <item>
        <p><c>majority</c>. This attribute is <c>true</c> or
        <c>false</c>; default is <c>false</c>. When <c>true</c>,
        a majority of the table replicas must be available for an
        update to succeed. Majority checking can be enabled on
        tables with mission-critical data, where it is vital to
        avoid inconsistencies because of network splits.</p>
      </item>
      <item>
        <p><c>snmp</c>. Each (set-based) Mnesia table can be
          automatically turned into a Simple Network Management
          Protocol (SNMP) ordered table as well.
          This property specifies the types of the SNMP keys.</p>
      </item>
      <item>
        <p><c>attributes</c>. The names of the attributes for the
          records that are inserted in the table.</p>
      </item>
    </list>
    <p>For information about the complete set of table properties
      and their details, see <c>mnesia:create_table/2</c>.</p>
    <p>This Reference Manual uses a table of persons to illustrate
      various examples. The following record definition is assumed:</p>
    <code type="none">
-record(person, {name,
                 age = 0,
                 address = unknown,
                 salary = 0,
                 children = []}),</code>
    <p>The first record attribute is the primary key, or key
      for short.</p>
    <p>The function descriptions are sorted in alphabetical order.
      It is recommended to start to read about
      <c>mnesia:create_table/2</c>, <c>mnesia:lock/2</c>, and
      <c>mnesia:activity/4</c> before you continue and learn
      about the rest.</p>
    <p>Writing or deleting in transaction-context creates a local
      copy of each modified record during the transaction. During
      iteration, that is, <c>mnesia:fold[lr]/4</c>,
      <c>mnesia:next/2</c>, <c>mnesia:prev/2</c>, and
      <c>mnesia:snmp_get_next_index/2</c>, Mnesia
      compensates for every written or deleted record, which can
      reduce the performance.</p>
    <p>If possible, avoid writing or deleting records in the same
      transaction before iterating over the table.</p>
  </description>

  <datatypes>
    <datatype>
      <name name="table"/>
    </datatype>
    <datatype>
      <name name="activity"/>
    </datatype>
    <datatype>
      <name name="create_option"/>
    </datatype>
    <datatype>
      <name name="storage_type"/>
    </datatype>
    <datatype>
      <name name="t_result" n_vars="1"/>
    </datatype>
    <datatype>
      <name name="result"/>
    </datatype>
    <datatype>
      <name name="index_attr"/>
    </datatype>
    <datatype>
      <name name="write_locks"/>
    </datatype>
    <datatype>
      <name name="read_locks"/>
    </datatype>
    <datatype>
      <name name="lock_kind"/>
    </datatype>
    <datatype>
      <name name="select_continuation"/>
    </datatype>
    <datatype>
      <name name="snmp_struct"/>
    </datatype>
    <datatype>
      <name name="snmp_type"/>
    </datatype>
    <datatype>
      <name name="tuple_of" n_vars="1"/>
    </datatype>
    <datatype>
      <name name="config_key"/>
    </datatype>
    <datatype>
      <name name="config_value"/>
    </datatype>
    <datatype>
      <name name="config_result"/>
    </datatype>
    <datatype>
      <name name="debug_level"/>
    </datatype>

  </datatypes>

  <funcs>
    <func>
      <name name="abort" arity="1" since=""/>
      <fsummary>Terminates the current transaction.</fsummary>
      <desc>
        <p>Makes the transaction silently
          return the tuple <c>{aborted, Reason}</c>.
          Termination of a Mnesia transaction means that
          an exception is thrown to an enclosing <c>catch</c>.
          Thus, the expression <c>catch mnesia:abort(x)</c> does
          not terminate the transaction.</p>
      </desc>
    </func>
    <func>
      <name name="activate_checkpoint" arity="1" since=""/>
      <fsummary>Activates a checkpoint.</fsummary>
      <desc>
      <marker id="activate_checkpoint"></marker>
        <p>A checkpoint is a consistent view of the system.
          A checkpoint can be activated on a set of tables.
          This checkpoint can then be traversed and
          presents a view of the system as it existed at the time when
          the checkpoint was activated, even if the tables are
          being or have been manipulated.</p>
        <p><c>Args</c> is a list of the following tuples:</p>
        <list type="bulleted">
          <item>
            <p><c>{name,Name}</c>. <c>Name</c> is the checkpoint name.
              Each checkpoint must have a name that is unique to the
              associated nodes. The name can be reused only once the
              checkpoint has been deactivated. By default, a name
              that is probably unique is generated.</p>
          </item>
          <item>
            <p><c>{max,MaxTabs}</c>. <c>MaxTabs</c> is a list of
              tables that are to be included in the checkpoint.
              Default is <c>[]</c>. For these tables, the redundancy is
              maximized and checkpoint information is retained together
              with all replicas. The checkpoint becomes more fault
              tolerant if the tables have several replicas. When a new
              replica is added by the schema manipulation
              function <c>mnesia:add_table_copy/3</c>, a retainer is
              also attached automatically.</p>
          </item>
          <item>
            <p><c>{min,MinTabs}</c>. <c>MinTabs</c> is a list of
              tables that are to be included in the checkpoint.
              Default is []. For these tables, the redundancy is
              minimized and the checkpoint information is only retained
              with one replica, preferably on the local node.</p>
          </item>
          <item>
            <p><c>{allow_remote,Bool}</c>. <c>false</c> means that
              all retainers must be local. The checkpoint cannot be
              activated if a table does not reside locally.
              <c>true</c> allows retainers to be allocated on any
              node. Default is <c>true</c>.</p>
          </item>
          <item>
            <p><c>{ram_overrides_dump,Bool}</c>. Only applicable
              for <c>ram_copies</c>. <c>Bool</c> allows you to choose
              to back up the table state as it is in RAM, or as it is
              on disc. <c>true</c> means that the latest committed
              records in RAM are to be included in the checkpoint.
              These are the records that the application accesses.
              <c>false</c> means that the records dumped to <c>DAT</c>
              files are to be included in the checkpoint. These
              records are loaded at startup. Default is <c>false</c>.</p>
          </item>
        </list>
        <p>Returns <c>{ok,Name,Nodes}</c> or <c>{error,Reason}</c>.
          <c>Name</c> is the (possibly generated) checkpoint name.
          <c>Nodes</c> are the nodes that
          are involved in the checkpoint. Only nodes that keep a
          checkpoint retainer know about the checkpoint.</p>
      </desc>
    </func>
    <func>
      <name name="activity" arity="2" since=""/>
        <!--<name name="activity" arity="3" since=""/>-->
      <fsummary>Executes <c>Fun</c> in <c>AccessContext</c>.</fsummary>
      <desc>
      <marker id="activity_2_3"></marker>
        <p>Calls <c>mnesia:activity(AccessContext, Fun, Args,
          AccessMod)</c>, where <c>AccessMod</c> is the default
          access callback module obtained by
          <c>mnesia:system_info(access_module)</c>. <c>Args</c>
          defaults to <c>[]</c> (empty list).</p>
      </desc>
    </func>
    <func>
      <name name="activity" arity="4" since=""/>
      <fsummary>Executes <c>Fun</c> in <c>AccessContext</c>.</fsummary>
      <desc>
      <marker id="activity_4"></marker>
        <p>Executes the functional object <c>Fun</c>
          with argument <c>Args</c>.</p>
        <p>The code that executes inside the activity can
          consist of a series of table manipulation functions, which are
          performed in an <c>AccessContext</c>. Currently, the following
          access contexts are supported:</p>
        <taglist>
          <tag><c>transaction</c></tag>
          <item>
            <p>Short for <c>{transaction, infinity}</c></p>
          </item>
          <tag><c>{transaction, Retries}</c></tag>
          <item>
            <p>Calls <c>mnesia:transaction(Fun, Args, Retries)</c>.
              Notice that the result from <c>Fun</c> is
              returned if the transaction is successful (atomic),
              otherwise the function exits with an abort reason.</p>
          </item>
          <tag><c>sync_transaction</c></tag>
          <item>
            <p>Short for <c>{sync_transaction, infinity}</c></p>
          </item>
          <tag><c>{sync_transaction, Retries}</c></tag>
          <item>
            <p>Calls <c>mnesia:sync_transaction(Fun, Args, Retries)</c>.
              Notice that the result from <c>Fun</c> is
              returned if the transaction is successful (atomic),
              otherwise the function exits with an abort reason.</p>
          </item>
          <tag><c>async_dirty</c></tag>
          <item>
            <p>Calls <c>mnesia:async_dirty(Fun, Args)</c>.</p>
          </item>
          <tag><c>sync_dirty</c></tag>
          <item>
            <p>Calls <c>mnesia:sync_dirty(Fun, Args)</c>.</p>
          </item>
          <tag><c>ets</c></tag>
          <item>
            <p>Calls <c>mnesia:ets(Fun, Args)</c>.</p>
          </item>
        </taglist>
        <p>This function (<c>mnesia:activity/4</c>) differs in an
          important way from the functions <c>mnesia:transaction</c>,
          <c>mnesia:sync_transaction</c>,
          <c>mnesia:async_dirty</c>, <c>mnesia:sync_dirty</c>, and
          <c>mnesia:ets</c>. Argument <c>AccessMod</c>
          is the name of a callback module, which implements the
          <c>mnesia_access</c> behavior.</p>
        <p>Mnesia forwards calls to the following functions:</p>
        <list type="bulleted">
          <item>mnesia:lock/2 (read_lock_table/1, write_lock_table/1)
          </item>
          <item>mnesia:write/3 (write/1, s_write/1)
          </item>
          <item>mnesia:delete/3 (delete/1, s_delete/1)
          </item>
          <item>mnesia:delete_object/3 (delete_object/1, s_delete_object/1)
          </item>
          <item>mnesia:read/3 (read/1, wread/1)
          </item>
          <item>mnesia:match_object/3 (match_object/1)
          </item>
          <item>mnesia:all_keys/1
          </item>
          <item>mnesia:first/1
          </item>
          <item>mnesia:last/1
          </item>
          <item>mnesia:prev/2
          </item>
          <item>mnesia:next/2
          </item>
          <item>mnesia:index_match_object/4 (index_match_object/2)
          </item>
          <item>mnesia:index_read/3
          </item>
          <item>mnesia:table_info/2
          </item>
        </list>
        <p>to the corresponding:</p>
        <list type="bulleted">
          <item>AccessMod:lock(ActivityId, Opaque, LockItem, LockKind)
          </item>
          <item>AccessMod:write(ActivityId, Opaque, Tab, Rec, LockKind)
          </item>
          <item>AccessMod:delete(ActivityId, Opaque, Tab, Key, LockKind)
          </item>
          <item>AccessMod:delete_object(ActivityId, Opaque, Tab, RecXS,
           LockKind)
          </item>
          <item>AccessMod:read(ActivityId, Opaque, Tab, Key, LockKind)
          </item>
          <item>AccessMod:match_object(ActivityId, Opaque, Tab, Pattern,
           LockKind)
          </item>
          <item>AccessMod:all_keys(ActivityId, Opaque, Tab, LockKind)
          </item>
          <item>AccessMod:first(ActivityId, Opaque, Tab)
          </item>
          <item>AccessMod:last(ActivityId, Opaque, Tab)
          </item>
          <item>AccessMod:prev(ActivityId, Opaque, Tab, Key)
          </item>
          <item>AccessMod:next(ActivityId, Opaque, Tab, Key)
          </item>
          <item>AccessMod:index_match_object(ActivityId, Opaque, Tab,
           Pattern, Attr, LockKind)
          </item>
          <item>AccessMod:index_read(ActivityId, Opaque, Tab,
           SecondaryKey, Attr, LockKind)
          </item>
          <item>AccessMod:table_info(ActivityId, Opaque, Tab, InfoItem)
          </item>
        </list>
        <p><c>ActivityId</c> is a record that represents the identity
          of the enclosing Mnesia activity. The first field
          (obtained with <c>element(1, ActivityId)</c>) contains an
          atom, which can be interpreted as the activity type:
          <c>ets</c>, <c>async_dirty</c>, <c>sync_dirty</c>, or
          <c>tid</c>. <c>tid</c> means that the activity is a
          transaction. The structure of the rest of the identity
          record is internal to Mnesia.</p>
        <p><c>Opaque</c> is an opaque data structure that is internal
          to Mnesia.</p>
      </desc>
    </func>
    <func>
      <name name="add_table_copy" arity="3" since=""/>
      <fsummary>Copies a table to a remote node.</fsummary>
      <desc>
      <marker id="add_table_copy"></marker>
        <p>Makes another copy of a table at the node <c>Node</c>.
          Argument <c>Type</c> must be either of the atoms
          <c>ram_copies</c>, <c>disc_copies</c>, or
          <c>disc_only_copies</c>. For example, the following call
          ensures that a disc replica of the <c>person</c> table also
          exists at node <c>Node</c>:</p>
        <code type="none">
mnesia:add_table_copy(person, Node, disc_copies)</code>
        <p>This function can also be used to add a replica of the
          table named <c>schema</c>.</p>
      </desc>
    </func>
    <func>
      <name name="add_table_index" arity="2" since=""/>
      <fsummary>Creates an index for a table.</fsummary>
      <desc>
      <marker id="add_table_index"></marker>
        <p>Table indexes can be used whenever the user
          wants to use frequently some other field than the key field
          to look up records. If this other field has an associated
          index, these lookups can occur in constant time
          and space. For example, if your application wishes to use
          field <c>age</c> to find efficiently all persons with
          a specific age, it can be a good idea to have an index on
          field <c>age</c>. This can be done with the following
          call:</p>
        <code type="none">
mnesia:add_table_index(person, age)</code>
        <p>Indexes do not come for free. They occupy space that is
	  proportional to the table size, and they cause insertions
          into the table to execute slightly slower.</p>
      </desc>
    </func>
    <func>
      <name name="all_keys" arity="1" since=""/>
      <fsummary>Returns all keys in a table.</fsummary>
      <desc>
      <marker id="all_keys"></marker>
        <p>Returns a list of all keys in the table named <c>Tab</c>.
          The semantics of this function is context-sensitive.
          For more information, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a read lock on the entire
          table.</p>
      </desc>
    </func>
    <func>
      <name name="async_dirty" arity="1" since=""/>
      <name name="async_dirty" arity="2" since=""/>
      <fsummary>Calls the <c>Fun</c> in a context that is not protected by a transaction.</fsummary>
      <desc>
      <marker id="async_dirty"></marker>
        <p>Calls the <c>Fun</c> in a context that is not protected by
          a transaction. The Mnesia function calls performed in
          the <c>Fun</c> are mapped to the corresponding dirty
          functions. This still involves logging, replication, and
          subscriptions, but there is no locking, local transaction
          storage, or commit protocols involved. Checkpoint retainers
          and indexes are updated, but they are updated dirty. As
          for normal <c>mnesia:dirty_*</c> operations, the operations
          are performed semi-asynchronously. For details, see
          <c>mnesia:activity/4</c> and the User's Guide.</p>
        <p>The Mnesia tables can be manipulated without
          using transactions. This has some serious disadvantages, but
          is considerably faster, as the transaction manager is not
          involved and no locks are set. A dirty operation does,
          however, guarantee a certain level of consistency, and
          the dirty operations cannot return garbled records.
          All dirty operations provide location transparency
          to the programmer, and a program does not have to be aware
          of the whereabouts of a certain table to function.</p>
        <p>Notice that it is more than ten times more efficient to
          read records dirty than within a transaction.</p>
        <p>Depending on the application, it can be a good idea to use
          the dirty functions for certain operations. Almost all
          Mnesia functions that can be called within
          transactions have a dirty equivalent, which is much more
          efficient.</p>
        <p>However, notice that there is a risk that the database can
          be left in an inconsistent state if dirty operations are
          used to update it. Dirty operations are only to be used
          for performance reasons when it is absolutely necessary.</p>
        <p>Notice that calling (nesting) <c>mnesia:[a]sync_dirty</c>
          inside a transaction-context inherits the transaction
          semantics.</p>
      </desc>
    </func>
    <func>
      <name name="backup" arity="1" since=""/>
      <name name="backup" arity="2" since=""/>
      <fsummary>Backs up all tables in the database.</fsummary>
      <desc>
      <marker id="backup"></marker>
        <p>Activates a new checkpoint covering all Mnesia tables,
          including the schema, with maximum degree of redundancy, and
          performs a backup using <c>backup_checkpoint/2/3</c>. The
          default value of the backup callback module <c>BackupMod</c>
          is obtained by <c>mnesia:system_info(backup_module)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="backup_checkpoint" arity="2" since=""/>
      <name name="backup_checkpoint" arity="3" since=""/>
      <fsummary>Backs up all tables in a checkpoint.</fsummary>
      <desc>
      <marker id="backup_checkpoint"></marker>
        <p>The tables are backed up to external media using backup
          module <c>BackupMod</c>. Tables with the local contents
          property are backed up as they exist on the current
          node. <c>BackupMod</c> is the default backup callback
          module obtained by
          <c>mnesia:system_info(backup_module)</c>. For information
          about the exact callback interface (the
          <c>mnesia_backup behavior</c>), see the User's Guide.</p>
      </desc>
    </func>
    <func>
      <name name="change_config" arity="2" since=""/>
      <fsummary>Changes a configuration parameter.</fsummary>
      <desc>
      <marker id="change_config"></marker>
        <p><c>Config</c> is to be an atom of the following
          configuration parameters:</p>
        <taglist>
          <tag><c>extra_db_nodes</c></tag>
          <item>
            <p><c>Value</c> is a list of nodes that Mnesia
              is to try to connect to. <c>ReturnValue</c> is those
              nodes in <c>Value</c> that Mnesia is connected
              to.</p>
            <p>Notice that this function must only be used to connect
              to newly started RAM nodes (N.D.R.S.N.) with an empty
              schema. If, for example, this function is used after
              the network has been partitioned, it can lead to
              inconsistent tables.</p>
            <p>Notice that Mnesia can be connected to other
              nodes than those returned in <c>ReturnValue</c>.</p>
          </item>
          <tag><c>dc_dump_limit</c></tag>
          <item>
            <p><c>Value</c> is a number. See the description in
            <seeerl marker="#configuration_parameters">Section
            Configuration Parameters</seeerl>. <c>ReturnValue</c>
            is the new value. Notice that this configuration
            parameter is not persistent. It is lost when
            Mnesia has stopped.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="change_table_access_mode" arity="2" since=""/>
      <fsummary>Changes the access mode for the table.</fsummary>
      <desc>
      <marker id="change_table_access_mode"></marker>
        <p><c>AcccessMode</c> is by default the atom
          <c>read_write</c> but it can also be set to the atom
          <c>read_only</c>. If <c>AccessMode</c> is set to
          <c>read_only</c>, updates to the table cannot be
          performed. At startup, Mnesia always loads
          <c>read_only</c> tables locally regardless of when and if
          Mnesia is terminated on other nodes.</p>
      </desc>
    </func>
    <func>
      <name name="change_table_copy_type" arity="3" since=""/>
      <fsummary>Changes the storage type of a table.</fsummary>
      <desc>
      <marker id="change_table_copy_type"></marker>
        <p>For example:</p>
        <code type="none">
mnesia:change_table_copy_type(person, node(), disc_copies)</code>
        <p>Transforms the <c>person</c> table from a RAM table into
          a disc-based table at <c>Node</c>.</p>
        <p>This function can also be used to change the storage type
          of the table named <c>schema</c>. The schema table can only
          have <c>ram_copies</c> or <c>disc_copies</c> as the storage
          type. If the storage type of the schema is <c>ram_copies</c>,
          no other table can be disc-resident on that node.</p>
      </desc>
    </func>
    <func>
      <name name="change_table_load_order" arity="2" since=""/>
      <fsummary>Changes the load order priority for the table.</fsummary>
      <desc>
      <marker id="change_table_load_order"></marker>
        <p>The <c>LoadOrder</c> priority is by default <c>0</c> (zero)
          but can be set to any integer. The tables with the highest
          <c>LoadOrder</c> priority are loaded first at startup.</p>
      </desc>
    </func>
    <func>
      <name name="change_table_majority" arity="2" since="OTP R14B03"/>
      <fsummary>Changes the majority check setting for the table.</fsummary>
      <desc>
        <p><c>Majority</c> must be a boolean. Default is <c>false</c>.
          When <c>true</c>, a majority of the table replicas must be
          available for an update to succeed. When used on fragmented
          tables, <c>Tab</c> must be the base table name. Directly
          changing the majority setting on individual fragments is
          not allowed.</p>
      </desc>
    </func>
    <func>
      <name name="clear_table" arity="1" since=""/>
      <fsummary>Deletes all entries in a table.</fsummary>
      <desc>
      <marker id="clear_table"></marker>
        <p>Deletes all entries in the table <c>Tab</c>.</p>
      </desc>
    </func>
    <func>
      <name name="create_schema" arity="1" since=""/>
      <fsummary>Creates a new schema on the specified nodes.</fsummary>
      <desc>
      <marker id="create_schema"></marker>
        <p>Creates a new database on disc. Various files are
          created in the local Mnesia directory of each node.
          Notice that the directory must be unique for each node.
          Two nodes must never share the same directory. If possible,
          use a local disc device to improve performance.</p>
        <p><c>mnesia:create_schema/1</c> fails if any of the
          Erlang nodes given as <c>DiscNodes</c> are not alive, if
          Mnesia is running on any of the nodes, or if any
          of the nodes already have a schema. Use
          <c>mnesia:delete_schema/1</c> to get rid of old faulty
          schemas.</p>
        <p>Notice that only nodes with disc are to be included in
          <c>DiscNodes</c>. Disc-less nodes, that is, nodes where
          all tables including the schema only resides in RAM,
          must not be included.</p>
      </desc>
    </func>
    <func>
      <name name="create_table" arity="2" since=""/>
      <fsummary>Creates a Mnesia table called <c>Name</c>with properties as described by argument <c>TabDef</c>.</fsummary>
      <desc>
      <marker id="create_table"></marker>
        <p>Creates a Mnesia table called
          <c>Name</c> according to argument <c>TabDef</c>. This
          list must be a list of <c>{Item, Value}</c> tuples,
          where the following values are allowed:</p>
        <list type="bulleted">
          <item>
            <p><c>{access_mode, Atom}</c>. The access mode is by
              default the atom <c>read_write</c> but it can also be
              set to the atom <c>read_only</c>. If <c>AccessMode</c>
              is set to <c>read_only</c>, updates to the table
              cannot be performed.</p>
            <p>At startup, Mnesia always loads <c>read_only</c>
              table locally regardless of when and if Mnesia is
              terminated on other nodes. This argument returns the
              access mode of the table. The access mode can be
              <c>read_only</c> or <c>read_write</c>.</p>
          </item>
          <item>
            <p><c>{attributes, AtomList}</c> is a list of the
              attribute names for the records that are supposed to
              populate the table. Default is <c>[key, val]</c>.
              The table must at least have one extra attribute in
              addition to the key.</p>
            <p>When accessing single attributes in a record, it is
              not necessary, or even recommended, to hard code any
              attribute names as atoms. Use construct
              <c>record_info(fields, RecordName)</c> instead. It can
              be used for records of type <c>RecordName</c>.</p>
          </item>
          <item>
            <p><c>{disc_copies, Nodelist}</c>, where
              <c>Nodelist</c> is a list of the nodes where this table
              is supposed to have disc copies. If a table replica is
              of type <c>disc_copies</c>, all write operations on this
              particular replica of the table are written to disc and
              to the RAM copy of the table.</p>
            <p>It is possible to have a replicated table of type
              <c>disc_copies</c> on one node and another type on
              another node.  Default is <c>[]</c>.</p>
          </item>
          <item>
            <p><c>{disc_only_copies, Nodelist}</c>, where
              <c>Nodelist</c> is a list of the nodes where this table
              is supposed to have <c>disc_only_copies</c>. A disc only
              table replica is kept on disc only and unlike the other
              replica types, the contents of the replica do not
              reside in RAM. These replicas are considerably slower
              than replicas held in RAM.</p>
          </item>
          <item>
            <p><c>{index, Intlist}</c>, where
              <c>Intlist</c> is a list of attribute names (atoms) or
              record fields for which Mnesia is to build and
              maintain an extra index table. The <c>qlc</c> query
              compiler <em>may</em> be able to optimize queries
              if there are indexes available.</p>
          </item>
          <item>
            <p><c>{load_order, Integer}</c>. The load order
              priority is by default <c>0</c> (zero) but can be set
              to any integer. The tables with the highest load order
              priority are loaded first at startup.</p>
          </item>
          <item>
            <p><c>{majority, Flag}</c>, where <c>Flag</c> must be a
              boolean. If <c>true</c>, any (non-dirty) update to the
              table is aborted, unless a majority of the table
              replicas are available for the commit. When used on a
              fragmented table, all fragments are given the same
              the same majority setting.</p>
          </item>
          <item>
            <p><c>{ram_copies, Nodelist}</c>, where
              <c>Nodelist</c> is a list of the nodes where this table
              is supposed to have RAM copies. A table replica of type
              <c>ram_copies</c> is not written to disc on a per
              transaction basis. <c>ram_copies</c> replicas can be
              dumped to disc with the function
              <c>mnesia:dump_tables(Tabs)</c>. Default value for
              this attribute is <c>[node()]</c>.</p>
          </item>
          <item>
            <p><c>{record_name, Name}</c>, where <c>Name</c> must
              be an atom. All records stored in the table must have
              this name as the first element. It defaults to the same
              name as the table name.</p>
          </item>
          <item>
            <p><c>{snmp, SnmpStruct}</c>. For a description of
              <c>SnmpStruct</c>, see <c>mnesia:snmp_open_table/2</c>.
              If this attribute is present in <c>ArgList</c> to
              <c>mnesia:create_table/2</c>, the table is immediately
              accessible by SNMP. Therefore applications that use
              SNMP to manipulate and control the system can be
              designed easily, since Mnesia provides a
              direct mapping between the logical tables that make up
              an SNMP control application and the physical data that
              makes up a Mnesia table.</p>
          </item>
          <item>
            <p><c>{storage_properties, [{Backend, Properties}]</c>
            forwards more properties to the back end storage.
            <c>Backend</c> can currently be <c>ets</c> or <c>dets</c>.
            <c>Properties</c> is a list of options sent to the
            back end storage during table creation. <c>Properties</c>
            cannot contain properties already used by Mnesia,
            such as <c>type</c> or <c>named_table</c>.</p>
            <p>For example:</p>
            <code type="none">
mnesia:create_table(table, [{ram_copies, [node()]}, {disc_only_copies, nodes()},
       {storage_properties,
        [{ets, [compressed]}, {dets, [{auto_save, 5000}]} ]}])</code>
          </item>
          <item>
            <p><c>{type, Type}</c>, where <c>Type</c> must be
              either of the atoms <c>set</c>, <c>ordered_set</c>, or
              <c>bag</c>. Default is <c>set</c>. In a
              <c>set</c>, all records have unique keys. In a
              <c>bag</c>, several records can have the same key, but
              the record content is unique. If a non-unique record is
              stored, the old conflicting records are overwritten.</p>
            <p>Notice that currently <c>ordered_set</c> is not
              supported for <c>disc_only_copies</c>.</p>
          </item>
          <item>
            <p><c>{local_content, Bool}</c>, where <c>Bool</c> is
              <c>true</c> or <c>false</c>. Default is <c>false</c>.</p>
          </item>
        </list>
        <p>For example, the following call creates the <c>person</c>
          table (defined earlier) and replicates it on two nodes:</p>
        <code type="none">
mnesia:create_table(person,
    [{ram_copies, [N1, N2]},
     {attributes, record_info(fields, person)}]).</code>
        <p>If it is required that Mnesia must build and
          maintain an extra index table on attribute <c>address</c>
          of all the <c>person</c> records that are inserted in the
          table, the following code would be issued:</p>
        <code type="none">
mnesia:create_table(person,
    [{ram_copies, [N1, N2]},
     {index, [address]},
     {attributes, record_info(fields, person)}]).
   </code>
        <p>The specification of <c>index</c> and <c>attributes</c>
          can be hard-coded as <c>{index, [2]}</c> and
          <c>{attributes, [name, age, address, salary, children]}</c>,
          respectively.</p>
        <p><c>mnesia:create_table/2</c> writes records into the
          table <c>schema</c>. This function, and all other
          schema manipulation functions, are implemented with the
          normal transaction management system. This guarantees that
          schema updates are performed on all nodes in an atomic
          manner.</p>
      </desc>
    </func>
    <func>
      <name name="deactivate_checkpoint" arity="1" since=""/>
      <fsummary>Deactivates a checkpoint.</fsummary>
      <desc>
      <marker id="deactivate_checkpoint"></marker>
        <p>The checkpoint is automatically deactivated when some of
          the tables involved have no retainer attached to them. This
          can occur when nodes go down or when a replica is deleted.
          Checkpoints are also deactivated with this function.
          <c>Name</c> is the name of an active checkpoint.</p>
      </desc>
    </func>
    <func>
      <name name="del_table_copy" arity="2" since=""/>
      <fsummary>Deletes the replica of table <c>Tab</c> at node <c>Node</c>.</fsummary>
      <desc>
      <marker id="del_table_copy"></marker>
        <p>Deletes the replica of table <c>Tab</c> at node <c>Node</c>.
          When the last replica is deleted with this
          function, the table disappears entirely.</p>
        <p>This function can also be used to delete a replica of
          the table named <c>schema</c>. The Mnesia node is
          then removed. Notice that Mnesia must be
          stopped on the node first.</p>
      </desc>
    </func>
    <func>
      <name name="del_table_index" arity="2" since=""/>
      <fsummary>Deletes an index in a table.</fsummary>
      <desc>
      <marker id="del_table_index"></marker>
        <p>Deletes the index on attribute with name
          <c>AttrName</c> in a table.</p>
      </desc>
    </func>
    <func>
      <name name="delete" arity="1" since=""/>
      <fsummary>Deletes all records in table <c>Tab</c> with the key <c>Key</c>.</fsummary>
      <desc>
      <marker id="delete_1"></marker>
        <p>Calls <c>mnesia:delete(Tab, Key, write)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="delete" arity="3" since=""/>
      <fsummary>Deletes all records in table <c>Tab</c> with the key <c>Key</c>.</fsummary>
      <desc>
      <marker id="delete_3"></marker>
        <p>Deletes all records in table <c>Tab</c> with the key
          <c>Key</c>.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a lock of type
          <c>LockKind</c> in the record.
          Currently, the lock types <c>write</c> and
          <c>sticky_write</c> are supported.</p>
      </desc>
    </func>
    <func>
      <name name="delete_object" arity="1" since=""/>
      <fsummary>Delete a record.</fsummary>
      <desc>
      <marker id="delete_object_1"></marker>
        <p>Calls <c>mnesia:delete_object(Tab, Record, write)</c>, where
          <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="delete_object" arity="3" since=""/>
      <fsummary>Deletes a record.</fsummary>
      <desc>
      <marker id="delete_object_3"></marker>
        <p>If a table is of type <c>bag</c>, it can sometimes be
          needed to delete only some of the records with a certain
          key. This can be done with the function <c>delete_object/3</c>.
          A complete record must be supplied to this function.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a lock of type
          <c>LockKind</c> on the record.
          Currently, the lock types <c>write</c> and
          <c>sticky_write</c> are supported.</p>
      </desc>
    </func>
    <func>
      <name name="delete_schema" arity="1" since=""/>
      <fsummary>Deletes the schema on the given nodes.</fsummary>
      <desc>
      <marker id="delete_schema"></marker>
        <p>Deletes a database created with
          <c>mnesia:create_schema/1</c>.
          <c>mnesia:delete_schema/1</c> fails if any of the Erlang
          nodes given as <c>DiscNodes</c> are not alive, or if
          Mnesia is running on any of the nodes.</p>
        <p>After the database is deleted, it can still be possible
          to start Mnesia as a disc-less node. This depends
          on how configuration parameter <c>schema_location</c> is
          set.</p>
        <warning>
          <p>Use this function with extreme caution, as it makes
            existing persistent data obsolete. Think twice before
            using it.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="delete_table" arity="1" since=""/>
      <fsummary>Deletes permanently all replicas of table <c>Tab</c>.</fsummary>
      <desc>
      <marker id="delete_table"></marker>
        <p>Permanently deletes all replicas of table <c>Tab</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_all_keys" arity="1" since=""/>
      <fsummary>Dirty search for all record keys in table.</fsummary>
      <desc>
      <marker id="delete_all_keys"></marker>
        <p>Dirty equivalent of the function <c>mnesia:all_keys/1</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_delete" arity="1" since=""/>
      <fsummary>Dirty delete of a record.</fsummary>
      <desc>
      <marker id="dirty_delete"></marker>
        <p>Calls <c>mnesia:dirty_delete(Tab, Key)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_delete" arity="2" since=""/>
      <fsummary>Dirty delete of a record.</fsummary>
      <desc>
        <p>Dirty equivalent of the function <c>mnesia:delete/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_delete_object" arity="1" since=""/>
      <fsummary>Dirty delete of a record.</fsummary>
      <desc>
      <marker id="dirty_delete_object_1"></marker>
        <p>Calls <c>mnesia:dirty_delete_object(Tab, Record)</c>,
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_delete_object" arity="2" since=""/>
      <fsummary>Dirty delete of a record.</fsummary>
      <desc>
        <p>Dirty equivalent of the function <c>mnesia:delete_object/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_first" arity="1" since=""/>
      <fsummary>Returns the key for the first record in a table.</fsummary>
      <desc>
      <marker id="dirty_first"></marker>
        <p>Records in <c>set</c> or <c>bag</c> tables are not ordered.
          However, there is an ordering of the records that is unknown
          to the user. Therefore, a table can be traversed by this
          function with the function <c>mnesia:dirty_next/2</c>.
          </p>
        <p>If there are no records in the table, this function
          returns the atom <c>'$end_of_table'</c>. It is therefore
          highly undesirable, but not disallowed, to use this atom
          as the key for any user records.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_index_match_object" arity="2" since=""/>
      <fsummary>Dirty pattern match using index.</fsummary>
      <desc>
      <marker id="dirty_index_match_object_2"></marker>
        <p>Starts
          <c>mnesia:dirty_index_match_object(Tab, Pattern, Pos)</c>,
          where <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_index_match_object" arity="3" since=""/>
      <fsummary>Dirty pattern match using index.</fsummary>
      <desc>
        <p>Dirty equivalent of the function
          <c>mnesia:index_match_object/4</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_index_read" arity="3" since=""/>
      <fsummary>Dirty read using index.</fsummary>
      <desc>
      <marker id="dirty_index_read"></marker>
        <p>Dirty equivalent of the function
          <c>mnesia:index_read/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_last" arity="1" since=""/>
      <fsummary>Returns the key for the last record in a table.</fsummary>
      <desc>
      <marker id="dirty_last"></marker>
        <p>Works exactly like <c>mnesia:dirty_first/1</c> but returns
          the last object in Erlang term order for the <c>ordered_set</c>
          table type. For all other table types,
          <c>mnesia:dirty_first/1</c> and
          <c>mnesia:dirty_last/1</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_match_object" arity="1" since=""/>
      <fsummary>Dirty pattern match pattern.</fsummary>
      <desc>
      <marker id="dirty_match_object_1"></marker>
        <p>Calls <c>mnesia:dirty_match_object(Tab, Pattern)</c>,
          where <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_match_object" arity="2" since=""/>
      <fsummary>Dirty pattern match pattern.</fsummary>
      <desc>
        <p>Dirty equivalent of the function
          <c>mnesia:match_object/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_next" arity="2" since=""/>
      <fsummary>Return the next key in a table.</fsummary>
      <desc>
      <marker id="dirty_next"></marker>
        <p>Traverses a table and
          performs operations on all records in the table.
          When the end of the table is reached, the special key
          <c>'$end_of_table'</c> is returned. Otherwise, the function
          returns a key that can be used to read the actual record. The
          behavior is undefined if another Erlang process performs write
          operations on the table while it is being traversed with the
          function <c>mnesia:dirty_next/2</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_prev" arity="2" since=""/>
      <fsummary>Returns the previous key in a table.</fsummary>
      <desc>
      <marker id="dirty_prev"></marker>
        <p>Works exactly like <c>mnesia:dirty_next/2</c> but returns
          the previous object in Erlang term order for the
          <c>ordered_set</c> table type. For all other table types,
          <c>mnesia:dirty_next/2</c> and
          <c>mnesia:dirty_prev/2</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_read" arity="1" since=""/>
      <fsummary>Dirty read of records.</fsummary>
      <desc>
      <marker id="dirty_read"></marker>
        <p>Calls <c>mnesia:dirty_read(Tab, Key)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_read" arity="2" since=""/>
      <fsummary>Dirty read of records.</fsummary>
      <desc>
        <p>Dirty equivalent of the function <c>mnesia:read/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_select" arity="2" since=""/>
      <fsummary>Dirty matches the objects in <c>Tab</c> against <c>MatchSpec</c>.</fsummary>
      <desc>
      <marker id="dirty_select"></marker>
        <p>Dirty equivalent of the function <c>mnesia:select/2</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_update_counter" arity="2" since=""/>
      <fsummary>Dirty update of a counter record.</fsummary>
      <desc>
      <marker id="dirty_update_counter"></marker>
        <p>Calls <c>mnesia:dirty_update_counter(Tab, Key, Incr)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_update_counter" arity="3" since=""/>
      <fsummary>Dirty update of a counter record.</fsummary>
      <desc>
        <p>Mnesia has no special counter records. However,
          records of the form <c>{Tab, Key, Integer}</c> can be used
          as (possibly disc-resident) counters when <c>Tab</c> is a
          <c>set</c>. This function updates a counter with a positive
          or negative number. However, counters can never become less
          than zero. There are two significant differences between
          this function and the action of first reading the record,
          performing the arithmetic, and then writing the record:</p>
        <list type="bulleted">
          <item>It is much more efficient.
          </item>
          <item><c>mnesia:dirty_update_counter/3</c> is performed
           as an atomic operation although it is not protected
           by a transaction.
          </item>
        </list>
        <p>If two processes perform <c>mnesia:dirty_update_counter/3</c>
          simultaneously, both updates take effect without the
          risk of losing one of the updates. The new value
          <c>NewVal</c> of the counter is returned.</p>
        <p>If <c>Key</c> does not exist, a new record is created with
          value <c>Incr</c> if it is larger than 0, otherwise it is
          set to 0.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_write" arity="1" since=""/>
      <fsummary>Dirty write of a record.</fsummary>
      <desc>
      <marker id="dirty_write_1"></marker>
        <p>Calls <c>mnesia:dirty_write(Tab, Record)</c>,
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dirty_write" arity="2" since=""/>
      <fsummary>Dirty write of a record.</fsummary>
      <desc>
        <p>Dirty equivalent of the function <c>mnesia:write/3</c>.</p>
      </desc>
    </func>
    <func>
      <name name="dump_log" arity="0" since=""/>
      <fsummary>Performs a user-initiated dump of the local log file.</fsummary>
      <desc>
      <marker id="dump_log"></marker>
        <p>Performs a user-initiated dump of the local log file.
          This is usually not necessary, as Mnesia by default
          manages this automatically. See configuration parameters
          <seeerl marker="#dump_log_time_threshold">dump_log_time_threshold</seeerl>
          and
          <seeerl marker="#dump_log_write_threshold">dump_log_write_threshold</seeerl>.
          </p>
      </desc>
    </func>
    <func>
      <name name="dump_tables" arity="1" since=""/>
      <fsummary>Dumps all RAM tables to disc.</fsummary>
      <desc>
      <marker id="dump_tables"></marker>
        <p>Dumps a set of <c>ram_copies</c> tables
          to disc. The next time the system is started, these tables
          are initiated with the data found in the files that are the
          result of this dump. None of the tables can have
          disc-resident replicas.</p>
      </desc>
    </func>
    <func>
      <name name="dump_to_textfile" arity="1" since=""/>
      <fsummary>Dumps local tables into a text file.</fsummary>
      <desc>
      <marker id="dump_to_textfile"></marker>
        <p>Dumps all local tables of a Mnesia system into a
          text file, which can be edited (by a normal text editor)
          and then be reloaded with
          <c>mnesia:load_textfile/1</c>. Only use this function for
          educational purposes. Use other functions to deal with real
          backups.</p>
      </desc>
    </func>
    <func>
      <name name="error_description" arity="1" since=""/>
      <fsummary>Returns a string describing a particular Mnesia error.</fsummary>
      <desc>
      <marker id="error_description"></marker>
        <p>All Mnesia transactions, including all the schema
          update functions, either return value <c>{atomic, Val}</c>
          or the tuple <c>{aborted, Reason}</c>. <c>Reason</c> can
          be either of the atoms in the following list. The
          function <c>error_description/1</c> returns a descriptive
          string that describes the error.</p>
        <list type="bulleted">
          <item><c>nested_transaction</c>. Nested transactions are
           not allowed in this context.
          </item>
          <item><c>badarg</c>. Bad or invalid argument, possibly
           bad type.
          </item>
          <item><c>no_transaction</c>. Operation not allowed
           outside transactions.
          </item>
          <item><c>combine_error</c>. Table options illegally
           combined.
          </item>
          <item><c>bad_index</c>. Index already exists, or was out
           of bounds.
          </item>
          <item><c>already_exists</c>. Schema option to be activated
           is already on.
          </item>
          <item><c>index_exists</c>. Some operations cannot be
           performed on tables with an index.
          </item>
          <item><c>no_exists</c>. Tried to perform operation on
           non-existing (not-alive) item.
          </item>
          <item><c>system_limit</c>. A system limit was exhausted.
          </item>
          <item><c>mnesia_down</c>. A transaction involves records
           on a remote node, which became unavailable before the
           transaction was completed. Records are no longer
           available elsewhere in the network.
          </item>
          <item><c>not_a_db_node</c>. A node was mentioned that does
           not exist in the schema.
          </item>
          <item><c>bad_type</c>. Bad type specified in argument.
          </item>
          <item><c>node_not_running</c>. Node is not running.
          </item>
          <item><c>truncated_binary_file</c>. Truncated binary in file.
          </item>
          <item><c>active</c>. Some delete operations require that
           all active records are removed.
          </item>
          <item><c>illegal</c>. Operation not supported on this
           record.
          </item>
        </list>
        <p><c>Error</c> can be <c>Reason</c>,
          <c>{error, Reason}</c>, or <c>{aborted, Reason}</c>.
          <c>Reason</c> can be an atom or a tuple with <c>Reason</c>
          as an atom in the first field.</p>
        <p>The following examples illustrate a function that returns an error,
          and the method to retrieve more detailed error information:</p>
          <list type="bulleted">
            <item>The function
             <seemfa marker="#create_table/2">mnesia:create_table(bar, [{attributes, 3.14}])</seemfa>
             returns the tuple <c>{aborted,Reason}</c>, where <c>Reason</c> is
             the tuple <c>{bad_type,bar,3.14000}</c>.</item>
            <item>The function
             <seemfa marker="#error_description/1">mnesia:error_description(Reason)</seemfa>
             returns the term <c>{"Bad type on some provided
             arguments",bar,3.14000}</c>, which is an error description
             suitable for display.</item>
      </list>
      </desc>
    </func>
    <func>
      <name name="ets" arity="1" since=""/>
      <name name="ets" arity="2" since=""/>
      <fsummary>Calls the <c>Fun</c> in a raw context that is not protected by a transaction.</fsummary>
      <desc>
      <marker id="ets"></marker>
        <p>Calls the <c>Fun</c> in a raw context that is not protected by
          a transaction. The Mnesia function call is performed in
          the <c>Fun</c> and performed directly on the local ETS
          tables on the assumption that the local storage type is
          <c>ram_copies</c> and the tables are not replicated to other
          nodes. Subscriptions are not triggered and checkpoints are
          not updated, but it is extremely fast. This function can
          also be applied to <c>disc_copies</c> tables if all
          operations are read only. For details, see
          <c>mnesia:activity/4</c> and the User's Guide.</p>
        <p>Notice that calling (nesting) a <c>mnesia:ets</c> inside a
          transaction-context inherits the transaction semantics.</p>
      </desc>
    </func>
    <func>
      <name name="first" arity="1" since=""/>
      <fsummary>Returns the key for the first record in a table.</fsummary>
      <desc>
      <marker id="first"></marker>
        <p>Records in <c>set</c> or <c>bag</c> tables are not ordered.
          However, there is an ordering of the records that is unknown
          to the user. A table can therefore be traversed by this
          function with the function <c>mnesia:next/2</c>.</p>
        <p>If there are no records in the table, this function
          returns the atom <c>'$end_of_table'</c>. It is therefore
          highly undesirable, but not disallowed, to use this atom
          as the key for any user records.</p>
      </desc>
    </func>
    <func>
      <name name="foldl" arity="3" since=""/>
      <fsummary>Calls <c>Function</c> for each record in <c>Table</c>.</fsummary>
      <desc>
      <marker id="foldl"></marker>
        <p>Iterates over the table <c>Table</c> and calls
          <c>Function(Record, NewAcc)</c> for each <c>Record</c> in
          the table. The term returned from <c>Function</c> is used
          as the second argument in the next call to <c>Function</c>.</p>
        <p><c>foldl</c> returns the same term as the last call to
          <c>Function</c> returned.</p>
      </desc>
    </func>
    <func>
      <name name="foldr" arity="3" since=""/>
      <fsummary>Calls <c>Function</c> for each record in <c>Table</c>.</fsummary>
      <desc>
      <marker id="foldr"></marker>
        <p>Works exactly like <c>foldl/3</c> but iterates the table
          in the opposite order for the <c>ordered_set</c> table type.
          For all other table types, <c>foldr/3</c> and
          <c>foldl/3</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name name="force_load_table" arity="1" since=""/>
      <fsummary>Forces a table to be loaded into the system.</fsummary>
      <desc>
      <marker id="force_load_table"></marker>
        <p>The Mnesia algorithm for table load can lead to a
          situation where a table cannot be loaded. This situation
          occurs when a node is started and Mnesia concludes, or
          suspects, that another copy of the table was active after
          this local copy became inactive because of a system crash.</p>
        <p>If this situation is not acceptable, this function can be
          used to override the strategy of the Mnesia table
          load algorithm. This can lead to a situation where some
          transaction effects are lost with an inconsistent database as
          result, but for some applications high availability is more
          important than consistent data.</p>
      </desc>
    </func>
    <func>
      <name name="index_match_object" arity="2" since=""/>
      <fsummary>Matches records and uses index information.</fsummary>
      <desc>
      <marker id="index_match_object_2"></marker>
        <p>Starts
          <c>mnesia:index_match_object(Tab, Pattern, Pos, read)</c>,
          where <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="index_match_object" arity="4" since=""/>
      <fsummary>Matches records and uses index information.</fsummary>
      <desc>
      <marker id="index_match_object_4"></marker>
        <p>In a manner similar to the function <c>mnesia:index_read/3</c>,
          any index information can be used when trying to match records.
          This function takes a pattern that obeys the same rules as the
          function <c>mnesia:match_object/3</c>, except that this function
          requires the following conditions:</p>
        <list type="bulleted">
          <item>
            <p>The table <c>Tab</c> must have an index on
              position <c>Pos</c>.</p>
          </item>
          <item>
            <p>The element in position <c>Pos</c> in
              <c>Pattern</c> must be bound. <c>Pos</c> is
              an integer (<c>#record.Field</c>) or an attribute name.</p>
          </item>
        </list>
        <p>The two index search functions described here are
          automatically started when searching tables with <c>qlc</c>
          list comprehensions and also when using the low-level
          <c>mnesia:[dirty_]match_object</c> functions.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a lock of type
          <c>LockKind</c> on the entire table or on a single record.
          Currently, the lock type <c>read</c> is supported.</p>
      </desc>
    </func>
    <func>
      <name name="index_read" arity="3" since=""/>
      <fsummary>Reads records through index table.</fsummary>
      <desc>
      <marker id="index_read"></marker>
        <p>Assume that there is an index on position <c>Pos</c> for a
          certain record type. This function can be used to read the
          records without knowing the actual key for the record. For
          example, with an index in position 1 of table <c>person</c>,
          the call <c>mnesia:index_read(person, 36, #person.age)</c>
          returns a list of all persons with age 36. <c>Pos</c> can
          also be an attribute name (atom), but if the notation
          <c>mnesia:index_read(person, 36, age)</c> is used, the
          field position is searched for in runtime, for each call.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a read lock on the entire
          table.</p>
      </desc>
    </func>
    <func>
      <name name="info" arity="0" since=""/>
      <fsummary>Prints system information on the terminal.</fsummary>
      <desc>
      <marker id="info"></marker>
        <p>Prints system information on the terminal.
          This function can be used even if Mnesia is not
          started. However, more information is displayed if
          Mnesia is started.</p>
      </desc>
    </func>
    <func>
      <name name="install_fallback" arity="1" since=""/>
      <fsummary>Installs a backup as fallback.</fsummary>
      <desc>
      <marker id="install_fallback_1"></marker>
        <p>Calls <c>mnesia:install_fallback(Opaque, Args)</c>, where
          <c>Args</c> is <c>[{scope, global}]</c>.</p>
      </desc>
    </func>
    <func>
      <name name="install_fallback" arity="2" since=""/>
      <fsummary>Installs a backup as fallback.</fsummary>
      <desc>
        <p>Installs a backup as fallback. The fallback is used to
          restore the database at the next startup. Installation of
          fallbacks requires Erlang to be operational on all the
          involved nodes, but it does not matter if Mnesia
          is running or not. The installation of the fallback fails
          if the local node is not one of the disc-resident nodes
          in the backup.</p>
        <p><c>Args</c> is a list of the following tuples:</p>
        <list type="bulleted">
          <item>
            <p><c>{module, BackupMod}</c>.
              All accesses of the backup media are performed through
              a callback module named <c>BackupMod</c>. Argument
              <c>Opaque</c> is forwarded to the callback module,
              which can interpret it as it wishes. The default
              callback module is called <c>mnesia_backup</c> and it
              interprets argument <c>Opaque</c> as a local
              filename. The default for this module is also
              configurable through configuration parameter
              <c>-mnesia mnesia_backup</c>.</p>
          </item>
          <item>
            <p><c>{scope, Scope}</c>.
              The <c>Scope</c> of a fallback is either
              <c>global</c> for the entire database or <c>local</c>
              for one node. By default, the installation of a fallback
              is a global operation, which either is performed on all
              nodes with a disc-resident schema or none. Which nodes
              that are disc-resident is determined from the
              schema information in the backup.</p>
            <p>If <c>Scope</c> of the operation is <c>local</c>,
              the fallback is only installed on the local node.</p>
          </item>
          <item>
            <p><c>{mnesia_dir, AlternateDir}</c>.
              This argument is only valid if the scope of the
              installation is <c>local</c>. Normally the installation
              of a fallback is targeted to the Mnesia directory,
              as configured with configuration parameter
              <c>-mnesia dir</c>. But by explicitly supplying an
              <c>AlternateDir</c>, the fallback is installed there
              regardless of the Mnesia directory configuration
              parameter setting. After installation of a fallback on
              an alternative Mnesia directory, that directory
              is fully prepared for use as an active Mnesia
              directory.</p>
            <p>This is a dangerous feature that must be
              used with care. By unintentional mixing of directories,
              you can easily end up with an inconsistent database, if
              the same backup is installed on more than one directory.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="is_transaction" arity="0" since=""/>
      <fsummary>Checks if code is running in a transaction.</fsummary>
      <desc>
      <marker id="is_transaction"></marker>
        <p>When this function is executed inside a transaction-context,
          it returns <c>true</c>, otherwise <c>false</c>.</p>
      </desc>
    </func>
    <func>
      <name name="last" arity="1" since=""/>
      <fsummary>Returns the key for the last record in a table.</fsummary>
      <desc>
        <p>Works exactly like
          <c>mnesia:first/1</c>, but returns the last object in
          Erlang term order for the <c>ordered_set</c> table type.
          For all other table types, <c>mnesia:first/1</c> and
          <c>mnesia:last/1</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name name="load_textfile" arity="1" since=""/>
      <fsummary>Loads tables from a text file.</fsummary>
      <desc>
      <marker id="load_textfile"></marker>
        <p>Loads a series of definitions and data found in the
          text file (generated with <c>mnesia:dump_to_textfile/1</c>)
          into Mnesia. This function also starts Mnesia
          and possibly creates a new schema. This function is
          intended for educational purposes only. It is recommended
          to use other functions to deal with real backups.</p>
      </desc>
    </func>
    <func>
      <name name="lock" arity="2" since=""/>
      <fsummary>Explicit grab lock.</fsummary>
      <desc>
      <marker id="lock"></marker>
        <p>Write locks are normally acquired on all nodes where a
          replica of the table resides (and is active). Read locks
          are acquired on one node (the local node if a local
          replica exists). Most of the context-sensitive access
          functions acquire an implicit lock if they are started in a
          transaction-context. The granularity of a lock can either
          be a single record or an entire table.</p>
        <p>The normal use is to call the function without checking
          the return value, as it exits if it fails and the
          transaction is restarted by the transaction manager. It
          returns all the locked nodes if a write lock is acquired
          and <c>ok</c> if it was a read lock.</p>
        <p>The function <c>mnesia:lock/2</c> is intended to support
          explicit locking on tables, but is also intended for
          situations when locks need to be acquired regardless of
          how tables are replicated. Currently, two kinds of
          <c>LockKind</c> are supported:</p>
        <taglist>
          <tag><c>write</c></tag>
          <item>
            <p>Write locks are exclusive. This means that if one
              transaction manages to acquire a write lock on an item,
              no other transaction can acquire any kind of lock on
              the same item.</p>
          </item>
          <tag><c>read</c></tag>
          <item>
            <p>Read locks can be shared. This means that if one
              transaction manages to acquire a read lock on an item,
              other transactions can also acquire a read lock on the
              same item. However, if someone has a read lock, no one
              can acquire a write lock at the same item. If someone
              has a write lock, no one can acquire either a read lock
              or a write lock at the same item.</p>
          </item>
        </taglist>
        <p>Conflicting lock requests are automatically queued if there
          is no risk of a deadlock. Otherwise the transaction must be
          terminated and executed again. Mnesia does this
          automatically as long as the upper limit of the maximum
          <c>retries</c> is not reached. For details, see
          <c>mnesia:transaction/3</c>.</p>
        <p>For the sake of completeness, sticky write locks are also
          described here even if a sticky write lock is not
          supported by this function:</p>
        <taglist>
          <tag><c>sticky_write</c></tag>
          <item>
            <p>Sticky write locks are a mechanism that can be used
              to optimize write lock acquisition. If your application
              uses replicated tables mainly for fault tolerance (as
              opposed to read access optimization purpose), sticky
              locks can be the best option available.</p>
            <p>When a sticky write lock is acquired, all nodes are
              informed which node is locked. Then,
              sticky lock requests from the same node are
              performed as a local operation without any
              communication with other nodes. The sticky lock
              lingers on the node even after the transaction
              ends. For details, see the User's Guide.</p>
          </item>
        </taglist>
        <p>Currently, this function supports two kinds of
          <c>LockItem</c>:</p>
        <taglist>
          <tag><c>{table, Tab}</c></tag>
          <item>
            <p>This acquires a lock of type <c>LockKind</c> on the
              entire table <c>Tab</c>.</p>
          </item>
          <tag><c>{global, GlobalKey, Nodes}</c></tag>
          <item>
            <p>This acquires a lock of type <c>LockKind</c> on the
              global resource <c>GlobalKey</c>. The lock is acquired
              on all active nodes in the <c>Nodes</c> list.</p>
          </item>
        </taglist>
        <p>Locks are released when the outermost transaction ends.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires locks, otherwise it
          ignores the request.</p>
      </desc>
    </func>
    <func>
      <name name="match_object" arity="1" since=""/>
      <fsummary>Matches <c>Pattern</c> for records.</fsummary>
      <desc>
      <marker id="match_object_1"></marker>
        <p>Calls <c>mnesia:match_object(Tab, Pattern, read)</c>, where
          <c>Tab</c> is <c>element(1, Pattern)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="match_object" arity="3" since=""/>
      <fsummary>Matches <c>Pattern</c> for records.</fsummary>
      <desc>
      <marker id="match_object_3"></marker>
        <p>Takes a pattern with "don't care" variables
          denoted as a <c>'_'</c> parameter. This function returns
          a list of records that matched the pattern.
          Since the second element
          of a record in a table is considered to be the key for the
          record, the performance of this function depends on whether
          this key is bound or not.</p>
        <p>For example, the call <c>mnesia:match_object(person,
          {person, '_', 36, '_', '_'}, read)</c> returns a list of
          all person records with an <c>age</c> field of 36.</p>
        <p>The function <c>mnesia:match_object/3</c>
          automatically uses indexes if these exist. However, no
          heuristics are performed to select the best index.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a lock of type
          <c>LockKind</c> on the entire table or a single record.
          Currently, the lock type <c>read</c> is supported.</p>
      </desc>
    </func>
    <func>
      <name name="move_table_copy" arity="3" since=""/>
      <fsummary>Moves the copy of table <c>Tab</c> from node <c>From</c> to node <c>To</c>.</fsummary>
      <desc>
      <marker id="move_table_copy"></marker>
        <p>Moves the copy of table <c>Tab</c> from node
          <c>From</c> to node <c>To</c>.</p>
        <p>The storage type is preserved. For example, a RAM table
          moved from one node remains a RAM on the new node. Other
          transactions can still read and write in
          the table while it is being moved.</p>
        <p>This function cannot be used on <c>local_content</c> tables.</p>
      </desc>
    </func>
    <func>
      <name name="next" arity="2" since=""/>
      <fsummary>Returns the next key in a table.</fsummary>
      <desc>
      <marker id="next"></marker>
        <p>Traverses a table and
          performs operations on all records in the table. When
          the end of the table is reached, the special key
          <c>'$end_of_table'</c> is returned. Otherwise the function
          returns a key that can be used to read the actual record.</p>
      </desc>
    </func>
    <func>
      <name name="prev" arity="2" since=""/>
      <fsummary>Returns the previous key in a table.</fsummary>
      <desc>
        <p>Works exactly like
          <c>mnesia:next/2</c>, but returns the previous object in
          Erlang term order for the <c>ordered_set</c> table type.
          For all other table types, <c>mnesia:next/2</c> and
          <c>mnesia:prev/2</c> are synonyms.</p>
      </desc>
    </func>
    <func>
      <name name="read" arity="1" since=""/>
      <name name="read" arity="2" since=""/>
      <fsummary>Reads records(s) with a given key.</fsummary>
      <desc>
      <marker id="read_2"></marker>
        <p>Calls function <c>mnesia:read(Tab, Key, read)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="read" arity="3" since=""/>
      <fsummary>Reads records(s) with a given key.</fsummary>
      <desc>
      <marker id="read_3"></marker>
        <p>Reads all records from table <c>Tab</c> with
          key <c>Key</c>. This function has the same semantics
          regardless of the location of <c>Tab</c>. If the table is
          of type <c>bag</c>, the function
          <c>mnesia:read(Tab, Key)</c> can
          return an arbitrarily long list. If the table is of type
          <c>set</c>, the list is either of length 1, or <c>[]</c>.</p>
        <p>The semantics of this function is context-sensitive.
          For details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a lock of type
          <c>LockKind</c>. Currently, the lock types <c>read</c>,
          <c>write</c>, and <c>sticky_write</c> are supported.</p>
        <p>If the user wants to update the record, it is more
          efficient to use <c>write/sticky_write</c> as the
          <c>LockKind</c>. If majority checking is active on the
          table, it is checked as soon as a write lock is
          attempted. This can be used to end quickly if the
          majority condition is not met.</p>
      </desc>
    </func>
    <func>
      <name name="read_lock_table" arity="1" since=""/>
      <fsummary>Sets a read lock on an entire table.</fsummary>
      <desc>
      <marker id="read_lock_table"></marker>
        <p>Calls the function
          <c>mnesia:lock({table, Tab}, read)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="report_event" arity="1" since=""/>
      <fsummary>Reports a user event to the Mnesia event handler.</fsummary>
      <desc>
      <marker id="report_event"></marker>
        <p>When tracing a system of Mnesia applications it is
          useful to be able to interleave Mnesia own events with
          application-related events that give information about the
          application context.</p>
        <p>Whenever the application begins a
          new and demanding Mnesia task, or if it enters a new
          interesting phase in its execution, it can be a good idea to
          use <c>mnesia:report_event/1</c>. <c>Event</c> can be
          any term and generates a <c>{mnesia_user, Event}</c> event
          for any processes that subscribe to Mnesia system
          events.</p>
      </desc>
    </func>
    <func>
      <name name="restore" arity="2" since=""/>
      <fsummary>Online restore of backup.</fsummary>
      <desc>
      <marker id="restore"></marker>
        <p>With this function, tables can be restored online from a
          backup without restarting Mnesia.
          <c>Opaque</c> is forwarded to the backup module.
          <c>Args</c> is a list of the following tuples:</p>
        <list type="bulleted">
          <item>
            <c>{module,BackupMod}</c>. The backup module
              <c>BackupMod</c> is used to access the backup media.
              If omitted, the default backup module is used.
          </item>
          <item><c>{skip_tables, TabList}</c>, where <c>TabList</c>
           is a list of tables that is not to be read from the
           backup.
          </item>
          <item><c>{clear_tables, TabList}</c>, where
          <c>TabList</c> is a list of tables that is to be
           cleared before the records from the backup are inserted.
           That is, all records in the tables are deleted before the
           tables are restored. Schema information about the tables
           is not cleared or read from the backup.
          </item>
          <item><c>{keep_tables, TabList}</c>, where <c>TabList</c>
           is a list of tables that is not to be cleared before the
           records from the backup are inserted. That is, the records
           in the backup are added to the records in the table.
           Schema information about the tables is not cleared or read
           from the backup.
          </item>
          <item><c>{recreate_tables, TabList}</c>, where
           <c>TabList</c> is a list of tables that is to be
           recreated before the records from the backup are inserted.
           The tables are first deleted and then created with the
           schema information from the backup. All the nodes in the
           backup need to be operational.
          </item>
          <item><c>{default_op, Operation}</c>, where <c>Operation</c>
           is either of the operations <c>skip_tables</c>,
           <c>clear_tables</c>, <c>keep_tables</c>, or
           <c>recreate_tables</c>. The default operation specifies
           which operation that is to be used on tables from the backup
           that is not specified in any of the mentioned lists. If
           omitted, operation <c>clear_tables</c> is used.
          </item>
        </list>
        <p>The affected tables are write-locked during the
          restoration. However, regardless of the lock conflicts caused
          by this, the applications can continue to do their work while
          the restoration is being performed. The restoration is
          performed as one single transaction.</p>
        <p>If the database is huge,
          it it not always possible to restore it online. In such
          cases, restore the old database by installing a
          fallback and then restart.</p>
      </desc>
    </func>
    <func>
      <name name="s_delete" arity="1" since=""/>
      <fsummary>Sets sticky lock and delete records.</fsummary>
      <desc>
      <marker id="s_delete"></marker>
        <p>Calls the function
          <c>mnesia:delete(Tab, Key, sticky_write)</c></p>
      </desc>
    </func>
    <func>
      <name name="s_delete_object" arity="1" since=""/>
      <fsummary>Sets sticky lock and delete record.</fsummary>
      <desc>
      <marker id="s_delete_object"></marker>
        <p>Calls the function
          <c>mnesia:delete_object(Tab, Record, sticky_write)</c>,
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="s_write" arity="1" since=""/>
      <fsummary>Writes <c>Record</c> and sets sticky lock.</fsummary>
      <desc>
      <marker id="s_write"></marker>
        <p>Calls the function
          <c>mnesia:write(Tab, Record, sticky_write)</c>,
          where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="schema" arity="0" since=""/>
      <fsummary>Prints information about all table definitions on the terminal.</fsummary>
      <desc>
        <p>Prints information about all table definitions on the terminal.</p>
      </desc>
    </func>
    <func>
      <name name="schema" arity="1" since=""/>
      <fsummary>Prints information about one table definition on the terminal.</fsummary>
      <desc>
        <p>Prints information about one table definition on the terminal.</p>
      </desc>
    </func>
    <func>
      <name name="select" arity="2" since=""/>
      <name name="select" arity="3" since=""/>
      <fsummary>Matches the objects in <c>Tab</c> against <c>MatchSpec</c>.</fsummary>
      <desc>
      <marker id="select_2_3"></marker>
        <p>Matches the objects in table <c>Tab</c> using a
          <c>match_spec</c> as described in the
          <seemfa marker="stdlib:ets#select/3">ets:select/3</seemfa>.
          Optionally a lock
          <c>read</c> or <c>write</c> can be given as the third
          argument. Default is <c>read</c>. The return value depends
          on <c>MatchSpec</c>.</p>
        <p>Notice that for best performance, <c>select</c> is to be
          used before any modifying operations are done on that table
          in the same transaction. That is, do not use <c>write</c>
          or <c>delete</c> before a <c>select</c>.</p>
        <p>In its simplest forms, the <c>match_spec</c> look as
          follows:</p>
        <list type="bulleted">
          <item><c>MatchSpec = [MatchFunction]</c></item>
          <item><c>MatchFunction = {MatchHead, [Guard], [Result]}</c></item>
          <item><c>MatchHead = tuple() | record()</c></item>
          <item><c>Guard =  {"Guardtest name", ...}</c></item>
          <item><c>Result = "Term construct"</c></item>
        </list>
        <p>For a complete description of <c>select</c>, see the
          <seeapp marker="erts:index">ERTS</seeapp> User's Guide and the
          <seeerl marker="stdlib:ets">ets</seeerl> manual page in
          STDLIB.</p>
        <p>For example, to find the names of all male persons older
          than 30 in table <c>Tab</c>:</p>
        <code type="none">
MatchHead = #person{name='$1', sex=male, age='$2', _='_'},
Guard = {'>', '$2', 30},
Result = '$1',
mnesia:select(Tab,[{MatchHead, [Guard], [Result]}]),</code>
      </desc>
    </func>
    <func>
      <name name="select" arity="4" since=""/>
      <fsummary>Matches the objects in <c>Tab</c> against <c>MatchSpec</c>.</fsummary>
      <desc>
      <marker id="select_4"></marker>
        <p>Matches the objects in table <c>Tab</c> using a
          <c>match_spec</c> as described in the
          <seeapp marker="erts:index">ERTS</seeapp> User's Guide,
          and returns a chunk of terms and a continuation.
          The wanted number of returned terms is specified by
          argument <c>NObjects</c>. The lock argument can be
          <c>read</c> or <c>write</c>. The continuation is to be
          used as argument to <c>mnesia:select/1</c>,
          if more or all answers are needed.</p>
        <p>Notice that for best performance, <c>select</c> is to be
          used before any modifying operations are done on that table
          in the same transaction. That is, do not use
          <c>mnesia:write</c> or <c>mnesia:delete</c> before a
          <c>mnesia:select</c>. For efficiency, <c>NObjects</c> is
          a recommendation only and the result can contain anything
          from an empty list to all available results.</p>
      </desc>
    </func>
    <func>
      <name name="select" arity="1" since=""/>
      <fsummary>Continues selecting objects.</fsummary>
      <desc>
        <p>Selects more objects with the match specification initiated
          by <c>mnesia:select/4</c>.</p>
        <p>Notice that any modifying operations, that is,
          <c>mnesia:write</c> or <c>mnesia:delete</c>, that are done
          between the <c>mnesia:select/4</c> and <c>mnesia:select/1</c>
          calls are not visible in the result.</p>
      </desc>
    </func>
    <func>
      <name name="set_debug_level" arity="1" since=""/>
      <fsummary>Changes the internal debug level of Mnesia.</fsummary>
      <desc>
      <marker id="set_debug_level"></marker>
        <p>Changes the internal debug level of Mnesia.
          For details, see
          <seeerl marker="#configuration_parameters">Section
          Configuration Parameters</seeerl>.</p>
      </desc>
    </func>
    <func>
      <name name="set_master_nodes" arity="1" since=""/>
      <fsummary>Sets the master nodes for all tables.</fsummary>
      <desc>
      <marker id="set_master_nodes_1"></marker>
        <p>For each table Mnesia determines its replica nodes
          (<c>TabNodes</c>) and starts
          <c>mnesia:set_master_nodes(Tab, TabMasterNodes)</c>. where
          <c>TabMasterNodes</c> is the intersection of
          <c>MasterNodes</c> and <c>TabNodes</c>. For semantics, see
          <c>mnesia:set_master_nodes/2</c>.</p>
      </desc>
    </func>
    <func>
      <name name="set_master_nodes" arity="2" since=""/>
      <fsummary>Sets the master nodes for a table.</fsummary>
      <desc>
      <marker id="set_master_nodes_2"></marker>
        <p>If the application detects a
          communication failure (in a potentially partitioned network)
          that can have caused an inconsistent database, it can use the
          function <c>mnesia:set_master_nodes(Tab, MasterNodes)</c> to
          define from which nodes each table is to be loaded.
          At startup, the Mnesia normal table load algorithm is
          bypassed and the table is loaded from one of the master nodes
          defined for the table, regardless of when and if Mnesia
          terminated on other nodes. <c>MasterNodes</c> can only
          contain nodes where the table has a replica. If the
          <c>MasterNodes</c> list is empty, the master node recovery
          mechanism for the particular table is reset, and the
          normal load mechanism is used at the next restart.</p>
        <p>The master node setting is always local. It can be
          changed regardless if Mnesia is started or not.</p>
        <p>The database can also become inconsistent if
          configuration parameter <c>max_wait_for_decision</c> is used
          or if <c>mnesia:force_load_table/1</c> is used.</p>
      </desc>
    </func>
    <func>
      <name name="snmp_close_table" arity="1" since=""/>
      <fsummary>Removes the possibility for SNMP to manipulate the table.</fsummary>
      <desc>
        <p>Removes the possibility for SNMP to manipulate the table.</p>
      </desc>
    </func>
    <func>
      <name name="snmp_get_mnesia_key" arity="2" since=""/>
      <fsummary>Gets the corresponding Mnesia key from an SNMP index.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>RowIndex ::= [integer()]</v>
        <v>Key ::= key() | {key(), key(), ...}</v>
        <v>key() ::= integer() | string() | [integer()]</v>
      </type>
      <desc>
        <p>Transforms an SNMP index to the corresponding Mnesia
          key. If the SNMP table has multiple keys, the key is a tuple
          of the key columns.</p>
      </desc>
    </func>
    <func>
      <name name="snmp_get_next_index" arity="2" since=""/>
      <fsummary>Gets the index of the next lexicographical row.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>RowIndex ::= [integer()]</v>
        <v>NextIndex ::= [integer()]</v>
      </type>
      <desc>
        <p><c>RowIndex</c> can specify a non-existing row.
          Specifically, it can be the empty list. Returns the index
          of the next lexicographical row. If <c>RowIndex</c> is the
          empty list, this function returns the index of the first row
          in the table.</p>
      </desc>
    </func>
    <func>
      <name name="snmp_get_row" arity="2" since=""/>
      <fsummary>Retrieves a row indexed by an SNMP index.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>RowIndex ::= [integer()]</v>
        <v>Row ::= record(Tab)</v>
      </type>
      <desc>
        <p>Reads a row by its SNMP index. This index is specified as
          an SNMP Object Identifier, a list of integers.</p>
      </desc>
    </func>
    <func>
      <name name="snmp_open_table" arity="2" since=""/>
      <fsummary>Organizes a Mnesia table as an SNMP table.</fsummary>
      <type>
        <v>Tab ::= atom()</v>
        <v>SnmpStruct ::= [{key, type()}]</v>
        <v>type() ::= type_spec() | {type_spec(), type_spec(), ...}</v>
        <v>type_spec() ::= fix_string | string | integer</v>
      </type>
      <desc>
        <p>A direct one-to-one mapping can be established between
          Mnesia tables and SNMP tables. Many telecommunication
          applications are controlled and monitored by the SNMP
          protocol. This connection between Mnesia and SNMP
          makes it simple and convenient to achieve this mapping.</p>
        <p>Argument <c>SnmpStruct</c> is a list of SNMP
          information. Currently, the only information needed is
          information about the key types in the table. Multiple
          keys cannot be handled in Mnesia, but many SNMP
          tables have multiple keys. Therefore, the following
          convention is used: if a table has multiple keys, these must
          always be stored as a tuple of the keys. Information about
          the key types is specified as a tuple of atoms describing
          the types. The only significant type is <c>fix_string</c>.
          This means that a string has a fixed size.</p>
        <p>For example, the following causes table <c>person</c>
          to be ordered as an SNMP table:</p>
        <code type="none">
mnesia:snmp_open_table(person, [{key, string}])</code>
        <p>Consider the following schema for a table of company
          employees. Each employee is identified by department number
          and name. The other table column stores the telephone
          number:</p>
        <code type="none">
mnesia:create_table(employee,
    [{snmp, [{key, {integer, string}}]},
     {attributes, record_info(fields, employees)}]),</code>
        <p>The corresponding SNMP table would have three columns:
          <c>department</c>, <c>name</c>, and <c>telno</c>.</p>
        <p>An option is to have table columns that are not visible
          through the SNMP protocol. These columns must be the last
          columns of the table. In the previous example, the SNMP
          table could have columns <c>department</c> and <c>name</c>
          only. The application could then use column <c>telno</c>
          internally, but it would not be visible to the SNMP
          managers.</p>
        <p>In a table monitored by SNMP, all elements must be
          integers, strings, or lists of integers.</p>
        <p>When a table is SNMP ordered, modifications are more
          expensive than usual, O(logN). Also, more memory is used.</p>
        <p>Notice that only the lexicographical SNMP ordering is
          implemented in Mnesia, not the actual SNMP monitoring.</p>
      </desc>
    </func>
    <func>
      <name name="start" arity="0" since=""/>
      <fsummary>Starts a local Mnesia system.</fsummary>
      <desc>
      <marker id="start"></marker>
        <p>Mnesia startup is asynchronous. The function call
	        <c>mnesia:start()</c> returns the atom <c>ok</c> and then
          starts to initialize the different tables. Depending on the
          size of the database, this can take some time, and the
          application programmer must wait for the tables that the
          application needs before they can be used. This is achieved
          by using the function <c>mnesia:wait_for_tables/2</c>.</p>
        <p>The startup procedure for a set of Mnesia nodes is a
          fairly complicated operation. A Mnesia system consists
          of a set of nodes, with Mnesia started locally on all
          participating nodes. Normally, each node has a directory where
          all the Mnesia files are written. This directory is
          referred to as the Mnesia directory. Mnesia can
          also be started on disc-less nodes. For more information
          about disc-less nodes, see <c>mnesia:create_schema/1</c>
          and the User's Guide.</p>
        <p>The set of nodes that makes up a Mnesia system is kept
          in a schema. Mnesia nodes can be added to or removed
          from the schema. The initial schema is normally created on
          disc with the function <c>mnesia:create_schema/1</c>. On
          disc-less nodes, a tiny default schema is generated each time
          Mnesia is started. During the startup procedure,
          Mnesia exchanges schema information between the nodes
          to verify that the table definitions are compatible.</p>
        <p>Each schema has a unique cookie, which can be regarded as a
          unique schema identifier. The cookie must be the same on all
          nodes where Mnesia is supposed to run. For details,
          see the User's Guide.</p>
        <p>The schema file and all other files that Mnesia
          needs are kept in the Mnesia directory. The
          command-line option <c>-mnesia dir Dir</c> can be used to
          specify the location of this directory to the Mnesia
          system. If no such command-line option is found, the name
          of the directory defaults to <c>Mnesia.Node</c>.</p>
        <p><c>application:start(mnesia)</c> can also be used.</p>
      </desc>
    </func>
    <func>
      <name name="stop" arity="0" since=""/>
      <fsummary>Stops Mnesia locally.</fsummary>
      <desc>
      <marker id="stop"></marker>
        <p>Stops Mnesia locally on the current node.</p>
        <p><c>application:stop(mnesia)</c> can also be used.</p>
      </desc>
    </func>
    <func>
      <name name="subscribe" arity="1" since=""/>
      <fsummary>Subscribes to events of type <c>EventCategory</c>.</fsummary>
      <desc>
      <marker id="subscribe"></marker>
        <p>Ensures that a copy of all events of type
          <c>EventCategory</c> is sent to the caller. The available
          event types are described in the <seeguide marker="Mnesia_chap5#event_handling">User's Guide</seeguide>.</p>
      </desc>
    </func>
    <func>
      <name name="sync_dirty" arity="1" since=""/>
      <name name="sync_dirty" arity="2" since=""/>
      <fsummary>Calls the <c>Fun</c> in a context that is not protected by a transaction.</fsummary>
      <desc>
      <marker id="sync_dirty"></marker>
        <p>Calls the <c>Fun</c> in a context that is not protected by
          a transaction. The Mnesia function calls performed in
          the <c>Fun</c> are mapped to the corresponding dirty functions.
          It is performed in almost the same context as
          <c>mnesia:async_dirty/1,2</c>. The difference is that the
          operations are performed synchronously. The caller waits for
          the updates to be performed on all active replicas before
          the <c>Fun</c> returns. For details, see
          <c>mnesia:activity/4</c> and the User's Guide.</p>
      </desc>
    </func>
    <func>
      <name name="sync_log" arity="0" since="OTP 17.0"/>
      <fsummary>Performs a file sync of the local log file.</fsummary>
      <desc>
        <p>Ensures that the local transaction log file is synced to disk.
          On a single node system, data written to disk tables since the
          last dump can be lost if there is a power outage.
          See <seemfa marker="#dump_log/0">dump_log/0</seemfa>.</p>
      </desc>
    </func>
    <func>
      <name name="sync_transaction" arity="1" since=""/>
      <name name="sync_transaction" arity="2" clause_i="1" since=""/>
      <name name="sync_transaction" arity="2" clause_i="2" since=""/>
      <name name="sync_transaction" arity="3" since=""/>
      <fsummary>Synchronously executes a transaction.</fsummary>
      <desc>
      <marker id="sync_transaction"></marker>
        <p>Waits until data have been committed and
          logged to disk (if disk is used) on every involved node before
          it returns, otherwise it behaves as
          <c>mnesia:transaction/[1,2,3]</c>.</p>
        <p>This functionality can be used to avoid that one process
          overloads a database on another node.</p>
      </desc>
    </func>
    <func>
      <name name="system_info" arity="1" since=""/>
      <fsummary>Returns information about the Mnesia system.</fsummary>
      <desc>
      <marker id="system_info"></marker>
        <p>Returns information about the Mnesia system, such as
          transaction statistics, <c>db_nodes</c>, and configuration
          parameters. The valid keys are as follows:</p>
        <list type="bulleted">
          <item>
            <p><c>all</c>. Returns a list of all local system
              information. Each element is a <c>{InfoKey, InfoVal}</c>
              tuple.</p>
            <p>New <c>InfoKey</c>s can be added and old
              undocumented <c>InfoKey</c>s can be removed without
              notice.</p>
          </item>
          <item>
            <p><c>access_module</c>. Returns the name of module that is
              configured to be the activity access callback module.</p>
          </item>
          <item>
            <p><c>auto_repair</c>. Returns <c>true</c> or <c>false</c>
              to indicate if Mnesia is configured to start the
              auto-repair facility on corrupted disc files.</p>
          </item>
          <item>
            <p><c>backup_module</c>. Returns the name of the module
              that is configured to be the backup callback module.</p>
          </item>
          <item>
            <p><c>checkpoints</c>. Returns a list of the names of the
              checkpoints currently active on this node.</p>
          </item>
          <item>
            <p><c>event_module</c>. Returns the name of the module
              that is the event handler callback module.</p>
          </item>
          <item>
            <p><c>db_nodes</c>. Returns the nodes that make up the
              persistent database. Disc-less nodes are only included
              in the list of nodes if they explicitly have been added
              to the schema, for example, with
              <c>mnesia:add_table_copy/3</c>. The function can be
              started even if Mnesia is not yet running.</p>
          </item>
          <item>
            <p><c>debug</c>. Returns the current debug level of
              Mnesia.</p>
          </item>
          <item>
            <p><c>directory</c>. Returns the name of the Mnesia
              directory. It can be called even if Mnesia is
              not yet running.</p>
          </item>
          <item>
            <p><c>dump_log_load_regulation</c>. Returns a boolean that
              tells if Mnesia is configured to regulate the
              dumper process load.</p>
            <p>This feature is temporary and will be removed in future
              releases.</p>
          </item>
          <item>
            <p><c>dump_log_time_threshold</c>. Returns the time
              threshold for transaction log dumps in milliseconds.</p>
          </item>
          <item>
            <p><c>dump_log_update_in_place</c>. Returns a boolean that
              tells if Mnesia is configured to perform the
              updates in the Dets files directly, or if the
              updates are to be performed in a copy of the Dets
              files.</p>
          </item>
          <item>
            <p><c>dump_log_write_threshold</c>.
              Returns the write threshold for transaction log dumps as
              the number of writes to the transaction log.</p>
          </item>
          <item>
            <p><c>extra_db_nodes</c>. Returns a list
              of extra <c>db_nodes</c> to be contacted at startup.</p>
          </item>
          <item>
            <p><c>fallback_activated</c>. Returns <c>true</c>
              if a fallback is activated, otherwise <c>false</c>.</p>
          </item>
          <item>
            <p><c>held_locks</c>. Returns a list of all
              locks held by the local Mnesia lock manager.</p>
          </item>
          <item>
            <p><c>is_running</c>. Returns <c>yes</c> or <c>no</c> to
              indicate if Mnesia is running. It can
              also return <c>starting</c> or <c>stopping</c>. Can be
              called even if Mnesia is not yet running.</p>
          </item>
          <item>
            <p><c>local_tables</c>. Returns a list
              of all tables that are configured to reside locally.</p>
          </item>
          <item>
            <p><c>lock_queue</c>. Returns a list of
              all transactions that are queued for execution by the
              local lock manager.</p>
          </item>
          <item>
            <p><c>log_version</c>. Returns the version
              number of the Mnesia transaction log format.</p>
          </item>
          <item>
            <p><c>master_node_tables</c>. Returns a
              list of all tables with at least one master node.</p>
          </item>
          <item>
            <p><c>protocol_version</c>. Returns the version number of
              the Mnesia inter-process communication protocol.</p>
          </item>
          <item>
            <p><c>running_db_nodes</c>. Returns a list of nodes where
              Mnesia currently is running. This function can be
              called even if Mnesia is not yet running, but it
              then has slightly different semantics.</p>
            <p>If Mnesia is down on the local node, the function
              returns those other <c>db_nodes</c> and
              <c>extra_db_nodes</c> that for the moment are
              operational.</p>
            <p>If Mnesia is started, the function returns
              those nodes that Mnesia on the local node is fully
              connected to. Only those nodes that Mnesia has
              exchanged schema information with are included as
              <c>running_db_nodes</c>. After the merge of schemas, the
              local Mnesia system is fully operable and
              applications can perform access of remote replicas.
              Before the schema merge, Mnesia only operates
              locally. Sometimes there are more nodes included in the
              <c>running_db_nodes</c> list than all <c>db_nodes</c>
              and <c>extra_db_nodes</c> together.</p>
          </item>
          <item>
            <p><c>schema_location</c>. Returns the
              initial schema location.</p>
          </item>
          <item>
            <p><c>subscribers</c>. Returns a list of
              local processes currently subscribing to system events.</p>
          </item>
          <item>
            <p><c>tables</c>. Returns a list of all
              locally known tables.</p>
          </item>
          <item>
            <p><c>transactions</c>. Returns a list
              of all currently active local transactions.</p>
          </item>
          <item>
            <p><c>transaction_failures</c>. Returns a
              number that indicates how many transactions have
              failed since Mnesia was started.</p>
          </item>
          <item>
            <p><c>transaction_commits</c>. Returns a
              number that indicates how many transactions have
              terminated successfully since Mnesia was started.</p>
          </item>
          <item>
            <p><c>transaction_restarts</c>. Returns a
              number that indicates how many transactions have been
              restarted since Mnesia was started.</p>
          </item>
          <item>
            <p><c>transaction_log_writes</c>.
              Returns a number that indicates how many write
              operations that have been performed to the transaction
              log since startup.</p>
          </item>
          <item>
            <p><c>use_dir</c>. Returns a boolean that indicates if
              the Mnesia directory is used or not. Can be
              started even if Mnesia is not yet running.</p>
          </item>
          <item>
            <p><c>version</c>. Returns the current
              version number of Mnesia.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="table" arity="1" since=""/>
      <name name="table" arity="2" since=""/>
      <fsummary>Return a QLC query handle.</fsummary>
      <desc>
      <marker id="table"></marker>
        <p>Returns a Query List Comprehension (QLC) query handle,
          see the <seeerl marker="stdlib:qlc">qlc(3)</seeerl>
          manual page in STDLIB. The module <c>qlc</c>
          implements a query language that can use Mnesia
          tables as sources of data. Calling
          <c>mnesia:table/1,2</c> is the means to make the
          <c>mnesia</c> table <c>Tab</c> usable to QLC.</p>
        <p><c>Option</c> can contain Mnesia
          options or QLC options. Mnesia recognizes the
          following options (any other option is forwarded to
          QLC).</p>
        <list type="bulleted">
          <item><c>{lock, Lock}</c>, where <c>lock</c> can be
            <c>read</c> or <c>write</c>. Default is <c>read</c>.
          </item>
          <item><c>{n_objects,Number}</c>, where <c>n_objects</c>
            specifies (roughly) the number of objects returned
            from Mnesia to QLC. Queries to remote tables
            can need a larger chunk to reduce network overhead.
            By default, <c>100</c> objects at a time are returned.
          </item>
          <item><c>{traverse, SelectMethod}</c>, where
            <c>traverse</c> determines the method to traverse
            the whole table (if needed). The default method is
          <c>select</c>.
          </item>
        </list>
        <p>There are two alternatives for <c>select</c>:</p>
        <list type="bulleted">
          <item>
            <p><c>select</c>. The table is traversed by calling
              <c>mnesia:select/4</c> and <c>mnesia:select/1</c>.
              The match specification (the second argument of
              <c>select/3</c>) is assembled by QLC: simple filters
              are translated into equivalent match specifications.
              More complicated filters need to be applied to all
              objects returned by <c>select/3</c> given a match
              specification that matches all objects.</p>
          </item>
          <item>
            <p><c>{select, MatchSpec}</c>. As for <c>select</c>,
              the table is traversed by calling <c>mnesia:select/3</c>
              and <c>mnesia:select/1</c>. The difference is that the
              match specification is explicitly given. This is how to
              state match specifications that cannot easily be
              expressed within the syntax provided by QLC.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="table_info" arity="2" since=""/>
      <fsummary>Returns local information about table.</fsummary>
      <desc>
      <marker id="table_info"></marker>
        <p>The <c>table_info/2</c> function takes two arguments.
          The first is the name of a Mnesia table.
          The second is one of the following keys:</p>
        <list type="bulleted">
          <item>
            <p><c>all</c>. Returns a list of all local table
              information. Each element is a
              <c>{InfoKey, ItemVal}</c> tuple.</p>
            <p>New <c>InfoItem</c>s can be added and old undocumented
              <c>InfoItem</c>s can be removed without notice.</p>
          </item>
          <item>
            <p><c>access_mode</c>. Returns the
              access mode of the table. The access mode can be
              <c>read_only</c> or <c>read_write</c>.</p>
          </item>
          <item>
            <p><c>arity</c>. Returns the arity of
              records in the table as specified in the schema.</p>
          </item>
          <item>
            <p><c>attributes</c>. Returns the table
              attribute names that are specified in the schema.</p>
          </item>
          <item>
            <p><c>checkpoints</c>. Returns the names
              of the currently active checkpoints, which involve this
              table on this node.</p>
          </item>
          <item>
            <p><c>cookie</c>. Returns a table cookie,
              which is a unique system-generated identifier for the
              table. The cookie is used internally to ensure that two
              different table definitions using the same table name
              cannot accidentally be intermixed. The cookie is
              generated when the table is created initially.</p>
          </item>
          <item>
            <p><c>disc_copies</c>. Returns the nodes where a
              <c>disc_copy</c> of the table resides according to the
              schema.</p>
          </item>
          <item>
            <p><c>disc_only_copies</c>. Returns the nodes where a
              <c>disc_only_copy</c> of the table resides
              according to the schema.</p>
          </item>
          <item>
            <p><c>index</c>. Returns the list of
              index position integers for the table.</p>
          </item>
          <item>
            <p><c>load_node</c>. Returns the name of
              the node that Mnesia loaded the table from. The
              structure of the returned value is unspecified, but
              can be useful for debugging purposes.</p>
          </item>
          <item>
            <p><c>load_order</c>. Returns the load
              order priority of the table. It is an integer and
              defaults to <c>0</c> (zero).</p>
          </item>
          <item>
            <p><c>load_reason</c>. Returns the
              reason of why Mnesia decided to load the table.
              The structure of the returned value is unspecified, but
              can be useful for debugging purposes.</p>
          </item>
          <item>
            <p><c>local_content</c>. Returns <c>true</c> or
              <c>false</c> to indicate if the table is configured to
              have locally unique content on each node.</p>
          </item>
          <item>
            <p><c>master_nodes</c>. Returns the master nodes of a
              table.</p>
          </item>
          <item>
            <p><c>memory</c>. Returns for <c>ram_copies</c> and
            <c>disc_copies</c> tables the number of words allocated in
            memory to the table on this node.
	    For <c>disc_only_copies</c> tables the number of bytes stored
	    on disc is returned.
	    </p>
          </item>
          <item>
            <p><c>ram_copies</c>. Returns the nodes where a
              <c>ram_copy</c> of the table resides according to the
              schema.</p>
          </item>
          <item>
            <p><c>record_name</c>. Returns the
              record name, common for all records in the table.</p>
          </item>
          <item>
            <p><c>size</c>. Returns the number of
              records inserted in the table.</p>
          </item>
          <item>
            <p><c>snmp</c>. Returns the SNMP struct. <c>[]</c> means
              that the table currently has no SNMP properties.</p>
          </item>
          <item>
            <p><c>storage_type</c>. Returns the local
              storage type of the table. It can be <c>disc_copies</c>,
              <c>ram_copies</c>, <c>disc_only_copies</c>, or the atom
              <c>unknown</c>. <c>unknown</c> is returned for all
              tables that only reside remotely.</p>
          </item>
          <item>
            <p><c>subscribers</c>. Returns a list
              of local processes currently subscribing to local table
              events that involve this table on this node.</p>
          </item>
          <item>
            <p><c>type</c>. Returns the table type, which is
              <c>bag</c>, <c>set</c>, or <c>ordered_set</c>.</p>
          </item>
          <item>
            <p><c>user_properties</c>. Returns the
              user-associated table properties of the table. It is a
              list of the stored property records.</p>
          </item>
          <item>
            <p><c>version</c>. Returns the current
              version of the table definition. The table version is
              incremented when the table definition is changed. The
              table definition can be incremented directly when it
              has been changed in a schema transaction, or
              when a committed table definition is merged with
              table definitions from other nodes during startup.</p>
          </item>
          <item>
            <p><c>where_to_read</c>. Returns the node
              where the table can be read. If value <c>nowhere</c>
              is returned, either the table is not loaded or it
              resides at a remote node that is not running.</p>
          </item>
          <item>
            <p><c>where_to_write</c>. Returns a list of the nodes
              that currently hold an active replica of the table.</p>
          </item>
          <item>
            <p><c>wild_pattern</c>. Returns a
              structure that can be given to the various match
              functions for a certain table. A record tuple is where
              all record fields have value <c>'_'</c>.</p>
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="transaction" arity="1" since=""/>
      <name name="transaction" arity="2" clause_i="1" since=""/>
      <name name="transaction" arity="2" clause_i="2" since=""/>
      <name name="transaction" arity="3" since=""/>

      <fsummary>Executes a transaction.</fsummary>
      <desc>
      <marker id="transaction"></marker>
        <p>Executes the functional object <c>Fun</c>
          with arguments <c>Args</c> as a transaction.</p>
        <p>The code that executes inside the transaction
          can consist of a series of table manipulation functions.
          If something goes wrong inside the transaction as a result
          of a user error or a certain table not being available, the
          entire transaction is terminated and the function
          <c>transaction/1</c> returns the tuple
          <c>{aborted, Reason}</c>.</p>
        <p>If all is going well, <c>{atomic, ResultOfFun}</c> is
          returned, where <c>ResultOfFun</c> is the value of the
          last expression in <c>Fun</c>.</p>
        <p>A function that adds a family to the database can be
          written as follows if there is a structure
          <c>{family, Father, Mother, ChildrenList}</c>:</p>
        <code type="none">
add_family({family, F, M, Children}) ->
    ChildOids = lists:map(fun oid/1, Children),
    Trans = fun() ->
        mnesia:write(F#person{children = ChildOids}),
        mnesia:write(M#person{children = ChildOids}),
        Write = fun(Child) -> mnesia:write(Child) end,
        lists:foreach(Write, Children)
    end,
    mnesia:transaction(Trans).

oid(Rec) -> {element(1, Rec), element(2, Rec)}.</code>
        <p>This code adds a set of people to the database. Running
          this code within one transaction ensures that either the whole
          family is added to the database, or the whole transaction
          terminates. For example, if the last child is badly formatted,
          or the executing process terminates because of an
          <c>'EXIT'</c> signal while executing the family code, the
          transaction terminates. Thus, the situation where half a
          family is added can never occur.</p>
        <p>It is also useful to update the database within a transaction
          if several processes concurrently update the same records.
          For example, the function <c>raise(Name, Amount)</c>, which
          adds <c>Amount</c> to the salary field of a person, is to
          be implemented as follows:</p>
        <code type="none">
raise(Name, Amount) ->
    mnesia:transaction(fun() ->
        case mnesia:wread({person, Name}) of
            [P] ->
                Salary = Amount + P#person.salary,
                P2 = P#person{salary = Salary},
                mnesia:write(P2);
            _ ->
                mnesia:abort("No such person")
        end
    end).</code>
        <p>When this function executes within a transaction,
          several processes running on different nodes can concurrently
          execute the function <c>raise/2</c> without interfering
          with each other.</p>
        <p>Since Mnesia detects deadlocks, a transaction can be
          restarted any number of times. This function attempts a
          restart as specified in <c>Retries</c>. <c>Retries</c> must
          be an integer greater than 0 or the atom <c>infinity</c>.
          Default is <c>infinity</c>.</p>
      </desc>
    </func>
    <func>
      <name name="transform_table" arity="4" since=""/>
      <fsummary>Changes format on all records in table <c>Tab</c>.</fsummary>
      <desc>
      <marker id="transform_table_4"></marker>
        <p>Applies argument <c>Fun</c> to all
          records in the table. <c>Fun</c> is a function that takes a
          record of the old type and returns a transformed record of
          the new type. Argument <c>Fun</c> can also be the atom
          <c>ignore</c>, which indicates that only the metadata
          about the table is updated. Use of
          <c>ignore</c> is not recommended, but included
          as a possibility for the user do to an own transformation.</p>
        <p><c>NewAttributeList</c> and <c>NewRecordName</c>
          specify the attributes and the new record type of the
          converted table. Table name always remains unchanged. If
          <c>record_name</c> is changed, only the Mnesia
          functions that use table identifiers work, for example,
          <c>mnesia:write/3</c> works, but not <c>mnesia:write/1</c>.</p>
      </desc>
    </func>
    <func>
      <name name="transform_table" arity="3" since=""/>
      <fsummary>Changes format on all records in table <c>Tab</c>.</fsummary>
      <desc>
        <p>Calls <c>mnesia:transform_table(Tab, Fun,
          NewAttributeList, RecName)</c>, where <c>RecName</c> is
          <c>mnesia:table_info(Tab, record_name)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="traverse_backup" arity="4" since=""/>
      <name name="traverse_backup" arity="6" since=""/>
      <fsummary>Traversal of a backup.</fsummary>
      <desc>
      <marker id="traverse_backup"></marker>
        <p>Iterates over a backup, either to transform it into a
          new backup, or read it. The arguments are explained briefly
          here. For details, see the User's Guide.</p>
        <list type="bulleted">
          <item><c>SourceMod</c> and <c>TargetMod</c> are the names of
           the modules that actually access the backup media.
          </item>
          <item><c>Source</c> and <c>Target</c> are opaque data used
           exclusively by modules <c>SourceMod</c> and <c>TargetMod</c>
           to initialize the backup media.
          </item>
          <item><c>Acc</c> is an initial accumulator value.
          </item>
          <item><c>Fun(BackupItems, Acc)</c> is applied to each item in
           the backup. The <c>Fun</c> must return a tuple
          <c>{BackupItems,NewAcc}</c>, where <c>BackupItems</c> is
           a list of valid backup items, and <c>NewAcc</c> is a new
           accumulator value. The returned backup items are written
           in the target backup.
          </item>
          <item><c>LastAcc</c> is the last accumulator value. This is
           the last <c>NewAcc</c> value that was returned by <c>Fun</c>.
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="uninstall_fallback" arity="0" since=""/>
      <fsummary>Uninstalls a fallback.</fsummary>
      <desc>
      <marker id="uninstall_fallback_0"></marker>
        <p>Calls the function
          <c>mnesia:uninstall_fallback([{scope, global}])</c>.</p>
      </desc>
    </func>
    <func>
      <name name="uninstall_fallback" arity="1" since=""/>
      <fsummary>Uninstalls a fallback.</fsummary>
      <desc>
        <p>Deinstalls a fallback before it
          has been used to restore the database. This is normally a
          distributed operation that is either performed on all
          nodes with disc resident schema, or none. Uninstallation of
          fallbacks requires Erlang to be operational on all
          involved nodes, but it does not matter if Mnesia is
          running or not. Which nodes that are considered as
          disc-resident nodes is determined from the schema
          information in the local fallback.</p>
        <p><c>Args</c> is a list of the following tuples:</p>
        <list type="bulleted">
          <item><c>{module, BackupMod}</c>. For semantics,
           see <c>mnesia:install_fallback/2</c>.
          </item>
          <item><c>{scope, Scope}</c>. For semantics,
           see <c>mnesia:install_fallback/2</c>.
          </item>
          <item><c>{mnesia_dir, AlternateDir}</c>. For semantics,
           see <c>mnesia:install_fallback/2</c>.
          </item>
        </list>
      </desc>
    </func>
    <func>
      <name name="unsubscribe" arity="1" since=""/>
      <fsummary>Subscribes to events of type <c>EventCategory</c>.</fsummary>
      <desc>
      <marker id="unsubscribe"></marker>
        <p>Stops sending events of type
          <c>EventCategory</c> to the caller.</p>
        <p><c>Node</c> is the local node.</p>
      </desc>
    </func>
    <func>
      <name name="wait_for_tables" arity="2" since=""/>
      <fsummary>Waits for tables to be accessible.</fsummary>
      <desc>
      <marker id="wait_for_tables"></marker>
        <p>Some applications need to wait for certain tables to be
          accessible to do useful work. <c>mnesia:wait_for_tables/2</c>
          either hangs until all tables in <c>TabList</c> are accessible,
          or until <c>timeout</c> is reached.</p>
      </desc>
    </func>
    <func>
      <name name="wread" arity="1" since=""/>
      <fsummary>Reads records with given key.</fsummary>
      <desc>
      <marker id="wread"></marker>
        <p>Calls the function <c>mnesia:read(Tab, Key, write)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="write" arity="1" since=""/>
      <fsummary>Writes a record into the database.</fsummary>
      <desc>
      <marker id="write_1"></marker>
        <p>Calls the function <c>mnesia:write(Tab, Record, write)</c>,
           where <c>Tab</c> is <c>element(1, Record)</c>.</p>
      </desc>
    </func>
    <func>
      <name name="write" arity="3" since=""/>
      <fsummary>Writes a record into the database.</fsummary>
      <desc>
      <marker id="write_3"></marker>
        <p>Writes record <c>Record</c> to table <c>Tab</c>.</p>
        <p>The function returns <c>ok</c>, or terminates if an error
          occurs. For example, the transaction terminates if no
          <c>person</c> table exists.</p>
        <p>The semantics of this function is context-sensitive. For
          details, see <c>mnesia:activity/4</c>. In
          transaction-context, it acquires a lock of type
          <c>LockKind</c>. The lock types <c>write</c> and
          <c>sticky_write</c> are supported.</p>
      </desc>
    </func>
    <func>
      <name name="write_lock_table" arity="1" since=""/>
      <fsummary>Sets write lock on an entire table.</fsummary>
      <desc>
      <marker id="write_lock_table"></marker>
        <p>Calls the function
          <c>mnesia:lock({table, Tab}, write)</c>.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Configuration Parameters</title>
    <marker id="configuration_parameters"></marker>
    <p>Mnesia reads the following application configuration
      parameters:</p>
    <list type="bulleted">
      <item>
        <p><c>-mnesia access_module Module</c>. The name of the
          Mnesia activity access callback module. Default is
          <c>mnesia</c>.</p>
      </item>
      <item>
        <p><c>-mnesia auto_repair true | false</c>. This flag
          controls if Mnesia automatically tries to repair
          files that have not been properly closed. Default is
          <c>true</c>.</p>
      </item>
      <item>
        <p><c>-mnesia backup_module Module</c>. The name of the
          Mnesia backup callback module. Default is
          <c>mnesia_backup</c>.</p>
      </item>
      <item>
        <p><c>-mnesia debug Level</c>. Controls the debug level
          of Mnesia. The possible values are as follows:</p>
        <taglist>
          <tag><c>none</c></tag>
          <item>
            <p>No trace outputs. This is the default.</p>
          </item>
          <tag><c>verbose</c></tag>
          <item>
            <p>Activates tracing of important debug events. These
              events generate <c>{mnesia_info, Format, Args}</c>
              system events. Processes can subscribe to these events with
              <c>mnesia:subscribe/1</c>. The events are always sent to
              the Mnesia event handler.</p>
          </item>
          <tag><c>debug</c></tag>
          <item>
            <p>Activates all events at the verbose level plus full
              trace of all debug events. These debug events generate
              <c>{mnesia_info, Format, Args}</c> system events.
              Processes can subscribe to these events with
              <c>mnesia:subscribe/1</c>. The events are always sent to
              the Mnesia event handler. On this debug level,
              the Mnesia event handler starts subscribing to
              updates in the schema table.</p>
          </item>
          <tag><c>trace</c></tag>
          <item>
            <p>Activates all events at the debug level. On this
              level, the Mnesia event handler starts subscribing
              to updates on all Mnesia tables. This level is
              intended only for debugging small toy systems, as many
              large events can be generated.</p>
          </item>
          <tag><c>false</c></tag>
          <item>An alias for none.
          </item>
          <tag><c>true</c></tag>
          <item>An alias for debug.
          </item>
        </taglist>
      </item>
      <item>
        <p><c>-mnesia core_dir Directory</c>. The name of the
          directory where Mnesia core files is stored, or
          false. Setting it implies that also RAM-only nodes
          generate a core file if a crash occurs.</p>
      </item>
      <item>
        <p><c>-mnesia dc_dump_limit Number</c>. Controls how often
          <c>disc_copies</c> tables are dumped from memory.
          Tables are dumped when
          <c>filesize(Log) > (filesize(Tab)/Dc_dump_limit)</c>.
          Lower values reduce CPU overhead but increase disk space
          and startup times. Default is 4.</p>
      </item>
      <item>
        <p><c>-mnesia dir Directory</c>. The name of the directory
          where all Mnesia data is stored. The directory name
          must be unique for the current node. Two nodes must never
          share the the same Mnesia directory. The results
          are unpredictable.</p>
      </item>
      <item>
        <p><c>-mnesia dump_disc_copies_at_startup true | false</c>.
          If set to false, this disables the dumping of <c>disc_copies</c>
          tables during startup while tables are being loaded. The default
          is true.</p>
      </item>
      <item>
        <p><c>-mnesia dump_log_load_regulation true | false</c>.
          Controls if log dumps are to be performed as fast as
          possible, or if the dumper is to do its own load
          regulation. Default is <c>false</c>.</p>
        <p>This feature is temporary and will be removed in a
          future release</p>
      </item>
      <item>
        <p><c>-mnesia dump_log_update_in_place true | false</c>.
          Controls if log dumps are performed on a copy of the
          original data file, or if the log dump is performed
          on the original data file. Default is <c>true</c></p>
      </item>
      <item>
        <marker id="dump_log_write_threshold"></marker>
        <p><c>-mnesia dump_log_write_threshold Max</c>.
          <c>Max</c> is an integer that specifies the maximum
          number of writes allowed to the transaction log before
          a new dump of the log is performed. Default is <c>1000</c>
          log writes.</p>
      </item>
      <item>
        <marker id="dump_log_time_threshold"></marker>
        <p><c>-mnesia dump_log_time_threshold Max</c>.
          <c>Max</c> is an integer that specifies the dump log
          interval in milliseconds. Default is 3 minutes. If a
          dump has not been performed within
          <c>dump_log_time_threshold</c> milliseconds, a new dump
          is performed regardless of the number of writes
          performed.</p>
      </item>
      <item>
        <p><c>-mnesia event_module Module</c>. The name of the
          Mnesia event handler callback module. Default is
          <c>mnesia_event</c>.</p>
      </item>
      <item>
        <p><c>-mnesia extra_db_nodes Nodes</c> specifies a list of
          nodes, in addition to the ones found in the schema, with
          which Mnesia is also to establish contact. Default
          is <c>[]</c> (empty list).</p>
      </item>
      <item>
        <p><c>-mnesia fallback_error_function {UserModule, UserFunc}</c>.
          Specifies a user-supplied callback function, which is
          called if a fallback is installed and Mnesia goes
          down on another node. Mnesia calls the function
          with one argument, the name of the dying node, for example,
          <c>UserModule:UserFunc(DyingNode)</c>. Mnesia must
          be restarted, otherwise the database can be inconsistent.
          The default behavior is to terminate Mnesia.</p>
      </item>
      <item>
        <p><c>-mnesia max_wait_for_decision Timeout</c>. Specifies
          how long Mnesia waits for other nodes to share their
          knowledge about the outcome of an unclear transaction. By
          default, <c>Timeout</c> is set to the atom <c>infinity</c>.
          This implies that if Mnesia upon startup detects
          a "heavyweight transaction" whose outcome is unclear, the
          local Mnesia waits until Mnesia is started
          on some (in the worst case all) of the other nodes that were
          involved in the interrupted transaction. This is a rare
          situation, but if it occurs, Mnesia does not guess if
          the transaction on the other nodes was committed or
          terminated. Mnesia waits until it knows the outcome
          and then acts accordingly.</p>
        <p>If <c>Timeout</c> is set to an integer value in
          milliseconds, Mnesia forces "heavyweight transactions"
          to be finished, even if the outcome of the transaction for
          the moment is unclear. After <c>Timeout</c> milliseconds,
          Mnesia commits or terminates the transaction and
          continues with the startup. This can lead to a situation
          where the transaction is committed on some nodes and
          terminated on other nodes. If the transaction is a
          schema transaction, the inconsistency can be fatal.</p>
      </item>
      <item>
        <p><c>-mnesia no_table_loaders NUMBER</c>. Specifies the number
          of parallel table loaders during start. More loaders can be
          good if the network latency is high or if many tables
          contain few records. Default is <c>2</c>.</p>
      </item>
      <item>
        <p><c>-mnesia send_compressed Level</c>. Specifies the level of
          compression to be used when copying a table from the local
          node to another one. Default is <c>0</c>.</p>
        <p><c>Level</c> must be an integer in the interval
          <c>[0, 9]</c>, where <c>0</c> means no compression and
          <c>9</c> means maximum compression. Before setting it to a
          non-zero value, ensure that the remote nodes
          understand this configuration.</p>
      </item>
      <item>
        <p><c>-mnesia max_transfer_size Number</c>. Specifies the estimated size
          in bytes of a single packet of data to be used when copying a table from the local
          node to another one. Default is <c>64000</c>.</p>
      </item>
      <item>
        <p><c>-mnesia schema_location Loc</c>. Controls where
          Mnesia looks for its schema. Parameter
          <c>Loc</c> can be one of the following atoms:</p>
        <taglist>
          <tag><c>disc</c></tag>
          <item>
            <p>Mandatory disc. The schema is assumed to be located
              in the Mnesia directory. If the schema cannot
              be found, Mnesia refuses to start. This is the
              old behavior.</p>
          </item>
          <tag><c>ram</c></tag>
          <item>
            <p>Mandatory RAM. The schema resides in RAM
              only. At startup, a tiny new schema is generated. This
              default schema only contains the definition of the schema
              table and only resides on the local node. Since no other
              nodes are found in the default schema, configuration
              parameter <c>extra_db_nodes</c> must be used to let the
              node share its table definitions with other nodes.</p>
            <p>Parameter <c>extra_db_nodes</c> can also be
              used on disc based nodes.</p>
          </item>
          <tag><c>opt_disc</c></tag>
          <item>
            <p>Optional disc. The schema can reside on disc or in
              RAM. If the schema is found on disc, Mnesia
              starts as a disc-based node and the storage type of
              the schema table is <c>disc_copies</c>. If no schema is
              found on disc, Mnesia starts as a disc-less node
              and the storage type of the schema table is
              <c>ram_copies</c>. Default value for the application
              parameter is <c>opt_disc</c>.</p>
          </item>
        </taglist>
      </item>
    </list>
    <p>First, the SASL application parameters are checked,
      then the command-line flags are checked, and finally, the
      default value is chosen.</p>
  </section>

  <section>
    <title>See Also</title>
    <p><seeerl marker="kernel:application">application(3)</seeerl>,
      <seeerl marker="stdlib:dets">dets(3)</seeerl>,
      <seeerl marker="kernel:disk_log">disk_log(3)</seeerl>,
      <seeerl marker="stdlib:ets">ets(3)</seeerl>,
      <seeerl marker="mnesia:mnesia_registry">mnesia_registry(3)</seeerl>,
      <seeerl marker="stdlib:qlc">qlc(3)</seeerl></p>
  </section>

</erlref>