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<h2 class="title"><a id="rep"></a>Chapter 12. Berkeley DB Replication </h2>
</div>
</div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="sect1">
<a href="rep.html#rep_intro">Replication introduction</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_id.html">Replication environment IDs</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_pri.html">Replication environment priorities</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_app.html">Building replicated applications</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_mgr_meth.html">Replication Manager methods</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_base_meth.html">Base API methods</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_comm.html">Building the communications infrastructure</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_newsite.html">Connecting to a new site</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="group_membership.html">Managing Replication Manager
group membership</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_add">Adding sites to a replication
group</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_remove">Removing sites from a
replication group</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_primordialstartup">Primordial startups</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="group_membership.html#group_mem_upgrade">Upgrading groups</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_partview.html">Replication views</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_filename.html">Managing replication directories and files</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_filename.html#rep_dir">Replication database directory
considerations</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_filename.html#rep_files">Managing replication internal
files</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_mgrmulti.html">Running Replication Manager in
multiple processes</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_mgrmulti.html#idp1913552">One replication process and multiple subordinate
processes</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mgrmulti.html#idp1910720">Persistence of local site network address
configuration</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mgrmulti.html#idp1905672">Programming considerations</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mgrmulti.html#idp1902040">Handling failure</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mgrmulti.html#idp1877896">Other miscellaneous rules</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_replicate.html">Running replication using the
db_replicate utility</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_replicate.html#idp1926936">One replication process and multiple subordinate processes</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_replicate.html#idp1906152">Common use case</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_replicate.html#idp1939624">Avoiding rollback</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_replicate.html#idp1937856">When to consider an integrated replication application</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_mgr_ack.html">Choosing a Replication Manager acknowledgement policy</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_elect.html">Elections</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_mastersync.html">Synchronizing with a
master</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_mastersync.html#rep_delay_sync">Delaying client synchronization</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mastersync.html#rep_c2c_sync">Client-to-client synchronization</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mastersync.html#idp1985136">Blocked client operations</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_mastersync.html#idp2008240">Clients too far out-of-date to synchronize</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_init.html">Initializing a new site</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_bulk.html">Bulk transfer</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_trans.html">Transactional guarantees</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_lease.html">Master leases</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_lease.html#masterlease_change_groupsize">Changing group
size</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_ryw.html">Read your writes consistency</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_ryw.html#gettoken">Getting a token</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_ryw.html#tokenhandling">Token handling</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_ryw.html#usingtoken">Using a token to check or wait for a transaction</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_clock_skew.html">Clock skew</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="repmgr_channels.html">Using Replication Manager message channels</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="repmgr_channels.html#dbchannel_class">DB_CHANNEL</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="repmgr_channels.html#dbchannel_send">Sending messages over a message channel</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="repmgr_channels.html#dbchannel_receive">Receiving messages</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_twosite.html">Special considerations for two-site replication groups</a>
</span>
</dt>
<dd>
<dl>
<dt>
<span class="sect2">
<a href="rep_twosite.html#twosite_strict">Two-site strict configuration</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_twosite.html#twosite_prefmas">Preferred master mode</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="rep_twosite.html#twosite_other">Other alternatives</a>
</span>
</dt>
</dl>
</dd>
<dt>
<span class="sect1">
<a href="rep_partition.html">Network partitions</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_faq.html">Replication FAQ</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_ex.html">Ex_rep: a replication example</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_ex_comm.html">Ex_rep_base: a TCP/IP based communication infrastructure</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_ex_rq.html">Ex_rep_base: putting it all
together</a>
</span>
</dt>
<dt>
<span class="sect1">
<a href="rep_ex_chan.html">Ex_rep_chan: a Replication Manager channel example</a>
</span>
</dt>
</dl>
</div>
<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="rep_intro"></a>Replication introduction</h2>
</div>
</div>
</div>
<p>
Berkeley DB includes support for building highly available
applications based on replication. Berkeley DB replication
groups consist of some number of independently configured
database environments. There is a single
<span class="emphasis"><em>master</em></span> database environment and one
or more <span class="emphasis"><em>client</em></span> database environments.
Master environments support both database reads and writes;
client environments support only database reads. If the master
environment fails, applications may upgrade a client to be the
new master. The database environments might be on separate
computers, on separate hardware partitions in a non-uniform
memory access (NUMA) system, or on separate disks in a single
server. As always with Berkeley DB environments, any number of
concurrent processes or threads may access a database
environment. In the case of a master environment, any number
of threads of control may read and write the environment, and
in the case of a client environment, any number of threads of
control may read the environment.
</p>
<p>
Applications may be written to provide various degrees of
consistency between the master and clients. The system can be
run synchronously such that replicas are guaranteed to be
up-to-date with all committed transactions, but doing so may
incur a significant performance penalty. Higher performance
solutions sacrifice total consistency, allowing the clients to
be out of date for an application-controlled amount of
time.
</p>
<p>
There are two ways to build replicated applications. The
simpler way is to use the Berkeley DB Replication Manager. The
Replication Manager provides a standard communications
infrastructure, and it creates and manages the background
threads needed for processing replication messages.
</p>
<p>
The Replication Manager implementation is based on TCP/IP
sockets, and uses POSIX 1003.1 style networking and thread
support. (On Windows systems, it uses standard Windows thread
support.) As a result, it is not as portable as the rest of
the Berkeley DB library itself.
</p>
<p>
The alternative is to use the lower-level replication "Base
APIs". This approach affords more flexibility, but requires
the application to provide some critical components:
</p>
<div class="orderedlist">
<ol type="1">
<li>
A communication infrastructure. Applications may use
whatever wire protocol is appropriate for their
application (for example, RPC, TCP/IP, UDP, VI or
message-passing over the backplane).
</li>
<li>
The application is responsible for naming. Berkeley
DB refers to the members of a replication group using an
application-provided ID, and applications must map that ID
to a particular database environment or communication
channel.
</li>
<li>
The application is responsible for monitoring the
status of the master and clients, and identifying any
unavailable database environments.
</li>
<li>
The application must provide whatever security
policies are needed. For example, the application may
choose to encrypt data, use a secure socket layer, or do
nothing at all. The level of security is left to the sole
discretion of the application.
</li>
</ol>
</div>
<p>
(Note that Replication Manager does not provide wire
security for replication messages.)
</p>
<p>
The following pages present various programming
considerations, many of which are directly relevant only for
Base API applications. However, even when using Replication
Manager it is important to understand the concepts.
</p>
<p>
Finally, the Berkeley DB replication implementation has one
other additional feature to increase application reliability.
Replication in Berkeley DB is implemented to perform database
updates using a different code path than the standard ones.
This means operations that manage to crash the replication
master due to a software bug will not necessarily also crash
replication clients.
</p>
<p>
For more information on the Replication Manager operations,
see the <a href="../api_reference/C/rep.html#replist" class="olink">Replication
and Related Methods</a> section in the
<em class="citetitle">Berkeley DB C API Reference Guide.</em>
</p>
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