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<p>Library Version 12.1.6.1</p>
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<th colspan="3" align="center">Automatically generated
functions</th>
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<tr>
<td width="20%" align="left"><a accesskey="p" href="apprec_def.html">Prev</a> </td>
<th width="60%" align="center">Chapter 14. Application Specific Logging and
Recovery </th>
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<div class="sect1" lang="en" xml:lang="en">
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<div>
<div>
<h2 class="title" style="clear: both"><a id="apprec_auto"></a>Automatically generated
functions</h2>
</div>
</div>
</div>
<p>
The XXX.src file is processed using the gen_rec.awk script
included in the dist directory of the Berkeley DB
distribution. This is an awk script that is executed from with
the following command line:
</p>
<pre class="programlisting">awk -f gen_rec.awk \
-v source_file=<span class="emphasis"><em>C_FILE</em></span> \
-v header_file=<span class="emphasis"><em>H_FILE</em></span> \
-v print_file=<span class="emphasis"><em>P_FILE</em></span> \
-v template_file=<span class="emphasis"><em>TMP_FILE</em></span> < XXX.src</pre>
<p>
where <span class="emphasis"><em>C_FILE</em></span> is the name of the file
into which to place the automatically generated C code,
<span class="emphasis"><em>H_FILE</em></span> is the name of the file into
which to place the automatically generated data structures and
declarations, <span class="emphasis"><em>P_FILE</em></span> is the name of the
file into which to place the automatically generated C code
that prints log records, and <span class="emphasis"><em>TMP_FILE</em></span> is
the name of the file into which to place a template for the
recovery routines.
</p>
<p>
Because the gen_rec.awk script uses sources files located
relative to the Berkeley DB dist directory, it must be run
from the dist directory. For example, in building the Berkeley
DB logging and recovery routines for ex_apprec, the following
script is used to rebuild the automatically generated
files:
</p>
<pre class="programlisting">E=../examples/c/ex_apprec
cd ../../dist
awk -f gen_rec.awk \
-v source_file=$E/ex_apprec_auto.c \
-v header_file=$E/ex_apprec_auto.h \
-v print_file=$E/ex_apprec_autop.c \
-v template_file=$E/ex_apprec_template < $E/ex_apprec.src</pre>
<p>
For each log record description found in the XXX.src file,
the following structure declarations and #defines will be
created in the file <span class="emphasis"><em>header_file</em></span>:
</p>
<pre class="programlisting">#define DB_PREFIX_RECORD_TYPE /* Integer ID number */
typedef struct _PREFIX_RECORD_TYPE_args {
/*
* These three fields are generated for every record.
*/
u_int32_t type; /* Record type used for dispatch. */
/*
* Transaction handle that identifies the transaction on whose
* behalf the record is being logged.
*/
DB_TXN *txnid;
/*
* The log sequence number returned by the previous call to log_put
* for this transaction.
*/
DB_LSN *prev_lsn;
/*
* The rest of the structure contains one field for each of
* the entries in the record statement.
*/
};</pre>
<p>
Thus, the auto-generated ex_apprec_mkdir_args structure
looks as follows:
</p>
<pre class="programlisting">typedef struct _ex_apprec_mkdir_args {
u_int32_t type;
DB_TXN *txnid;
DB_LSN prev_lsn;
DBT dirname;
} ex_apprec_mkdir_args;</pre>
<p>
The template_file will contain a template for a recovery
function. The recovery function is called on each record read
from the log during system recovery, transaction abort, or the
application of log records on a replication client, and is
expected to redo or undo the operations described by that
record. The details of the recovery function will be specific
to the record being logged and need to be written manually,
but the template provides a good starting point. (See
ex_apprec_template and ex_apprec_rec.c for an example of both
the template produced and the resulting recovery
function.)
</p>
<p>
The template file should be copied to a source file in the
application (but not the automatically generated source_file,
as that will get overwritten each time gen_rec.awk is run) and
fully developed there. The recovery function takes the
following parameters:
</p>
<div class="blockquote">
<blockquote class="blockquote">
<div class="variablelist">
<dl>
<dt>
<span class="term">dbenv</span>
</dt>
<dd>
The environment in which recovery is
running.
</dd>
<dt>
<span class="term">rec</span>
</dt>
<dd>
The record being recovered.
</dd>
<dt>
<span class="term">lsn</span>
</dt>
<dd>
The log sequence number of the record being
recovered. The prev_lsn field, automatically
included in every auto-generated log record,
should be returned through this argument. The
prev_lsn field is used to chain log records
together to allow transaction aborts; because the
recovery function is the only place that a log
record gets parsed, the responsibility for
returning this value lies with the recovery
function writer.
</dd>
<dt>
<span class="term">op</span>
</dt>
<dd>
A parameter of type db_recops, which
indicates what operation is being run
(<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_ABORT" class="olink">DB_TXN_ABORT</a>, <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_APPLY" class="olink">DB_TXN_APPLY</a>,
<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_BACKWARD_ROLL" class="olink">DB_TXN_BACKWARD_ROLL</a>, <a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_FORWARD_ROLL" class="olink">DB_TXN_FORWARD_ROLL</a> or
<a href="../api_reference/C/envset_app_dispatch.html#set_app_dispatch_DB_TXN_PRINT" class="olink">DB_TXN_PRINT</a>).
</dd>
</dl>
</div>
</blockquote>
</div>
<p>
In addition to the header_file and template_file, a
source_file is created, containing a log, read, recovery, and
print function for each record type.
</p>
<p>
The log function marshalls the parameters into a buffer, and
calls <a href="../api_reference/C/logput.html" class="olink">DB_ENV->log_put()</a> on that buffer returning 0 on success and
non-zero on failure. The log function takes the following
parameters:
</p>
<div class="blockquote">
<blockquote class="blockquote">
<div class="variablelist">
<dl>
<dt>
<span class="term">dbenv</span>
</dt>
<dd>The environment in which recovery is
running.</dd>
<dt>
<span class="term">txnid</span>
</dt>
<dd>
The transaction identifier for the
transaction handle returned by
<a href="../api_reference/C/txnbegin.html" class="olink">DB_ENV->txn_begin()</a>.
</dd>
<dt>
<span class="term">lsnp</span>
</dt>
<dd>
A pointer to storage for a log sequence
number into which the log sequence number of the
new log record will be returned.
</dd>
<dt>
<span class="term">syncflag</span>
</dt>
<dd>
A flag indicating whether the record must be
written synchronously. Valid values are 0 and
<a href="../api_reference/C/logput.html#put_DB_FLUSH" class="olink">DB_FLUSH</a>.
</dd>
<dt>
<span class="term">args</span>
</dt>
<dd>
The remaining parameters to the log message
are the fields described in the XXX.src file, in
order.
</dd>
</dl>
</div>
</blockquote>
</div>
<p>
The read function takes a buffer and unmarshalls its
contents into a structure of the appropriate type. It returns
0 on success and non-zero on error. After the fields of the
structure have been used, the pointer returned from the read
function should be freed. The read function takes the
following parameters:
</p>
<div class="blockquote">
<blockquote class="blockquote">
<div class="variablelist">
<dl>
<dt>
<span class="term">dbenv</span>
</dt>
<dd>
The environment in which recovery is
running.
</dd>
<dt>
<span class="term">recbuf</span>
</dt>
<dd>
A buffer.
</dd>
<dt>
<span class="term">argp</span>
</dt>
<dd>
A pointer to a structure of the appropriate
type.
</dd>
</dl>
</div>
</blockquote>
</div>
<p>
The print function displays the contents of the record. The
print function takes the same parameters as the recovery
function described previously. Although some of the parameters
are unused by the print function, taking the same parameters
allows a single dispatch loop to dispatch to a variety of
functions. The print function takes the following
parameters:
</p>
<div class="blockquote">
<blockquote class="blockquote">
<div class="variablelist">
<dl>
<dt>
<span class="term">dbenv</span>
</dt>
<dd>
The environment in which recovery is
running.
</dd>
<dt>
<span class="term">rec</span>
</dt>
<dd>
The record being recovered.
</dd>
<dt>
<span class="term">lsn</span>
</dt>
<dd>
The log sequence number of the record being
recovered.
</dd>
<dt>
<span class="term">op</span>
</dt>
<dd>
Unused.
</dd>
</dl>
</div>
</blockquote>
</div>
<p>
Finally, the source file will contain a function (named
XXX_init_print, where XXX is replaced by the prefix) which
should be added to the initialization part of the standalone
<a href="../api_reference/C/db_printlog.html" class="olink">db_printlog</a> utility code so that utility can be used to display
application-specific log records.
</p>
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