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<h2 class="title" style="clear: both"><a id="apprec_def"></a>Defining application-specific log
records</h2>
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<p>
By convention, log records are described in files named
<code class="filename">XXX.src</code>, where "XXX" is typically a
descriptive name for a subsystem or other logical group of
logging functions. These files contain interface definition
language descriptions for each type of log record that is used
by the subsystem.
</p>
<p>
All blank lines and lines beginning with a hash ("#")
character in the XXX.src files are ignored.
</p>
<p>
The first non-comment line in the file should begin with the
keyword PREFIX, followed by a string that will be prepended to
every generated function name. Frequently, the PREFIX is
either identical or similar to the name of the
<code class="filename">XXX.src</code> file. For example, the
Berkeley DB application-specific recovery example uses the
file <code class="filename">ex_apprec.src</code>, which begins with the
following PREFIX line:
</p>
<pre class="programlisting">PREFIX ex_apprec</pre>
<p>
Following the PREFIX line are the include files required by
the automatically generated functions. The include files
should be listed in order, prefixed by the keyword INCLUDE.
For example, the Berkeley DB application-specific recovery
example lists the following include files:
</p>
<pre class="programlisting">INCLUDE #include "ex_apprec.h"</pre>
<p>
The rest of the XXX.src file consists of log record
descriptions. Each log record description begins with one of
the following lines:
</p>
<pre class="programlisting">BEGIN <span class="emphasis"><em>RECORD_NAME</em></span> <span class="emphasis"><em>DB_VERSION_NUMBER</em></span> <span class="emphasis"><em>RECORD_NUMBER</em></span></pre>
<pre class="programlisting">BEGIN_COMPAT <span class="emphasis"><em>RECORD_NAME</em></span> <span class="emphasis"><em>DB_VERSION_NUMBER</em></span> <span class="emphasis"><em>RECORD_NUMBER</em></span></pre>
<p>
and ends with the line:
</p>
<pre class="programlisting">END</pre>
<p>
The <span class="emphasis"><em>BEGIN</em></span> line should be used for most
record types.
</p>
<p>
The <span class="emphasis"><em>BEGIN_COMPAT</em></span> is used for log record
compatibility to facilitate online upgrades of replication
groups. Records created with this keyword will produce reading
and printing routines, but no logging routines. The recovery
routines are retrieved from older releases, so no recovery
templates will be generated for these records.
</p>
<p>
The <span class="emphasis"><em>DB_VERSION_NUMBER</em></span> variable should
be replaced with the current major and minor version of
Berkeley DB, with all punctuation removed. For example,
Berkeley DB version 4.2 should be 42, version 4.5 should be
45.
</p>
<p>
The <span class="emphasis"><em>RECORD_NAME</em></span> variable should be
replaced with a record name for this log record. The
<span class="emphasis"><em>RECORD_NUMBER</em></span> variable should be
replaced with a record number.
</p>
<p>
The combination of PREFIX name and
<span class="emphasis"><em>RECORD_NAME</em></span>, and the
<span class="emphasis"><em>RECORD_NUMBER</em></span> must be unique for the
application, that is, values for application-specific and
Berkeley DB log records may not overlap. Further, because
record numbers are stored in log files, which are usually
portable across application and Berkeley DB releases, any
change to the record numbers or log record format or should be
handled as described in the section on log format changes in
the <a href="../upgrading/upgrade_process.html" class="olink">Upgrading Berkeley DB installations</a> chapter of the Berkeley DB Installation and Build Guide. The record
number space below 10,000 is reserved for Berkeley DB itself;
applications should choose record number values equal to or
greater than 10,000.
</p>
<p>
Between the BEGIN and END keywords there should be one
optional <span class="emphasis"><em>DUPLICATE</em></span> line and one line for
each data item logged as part of this log record.</p>
<p>
The <span class="emphasis"><em>DUPLICATE</em></span> line is of the
form:
</p>
<pre class="programlisting">DUPLICATE <span class="emphasis"><em>RECORD_NAME</em></span> <span class="emphasis"><em>DB_VERSION_NUMBER</em></span> <span class="emphasis"><em>RECORD_NUMBER</em></span></pre>
<p>
The <span class="emphasis"><em>DUPLICATE</em></span> specifier should be used
when creating a record that requires its own record number but
can use the argument structure, reading and printing routines
from another record. In this case, we will create a new log
record type, but use the enclosing log record type for the
argument structure and the log reading and printing
routines.
</p>
<p>
The format of lines for each data item logged is as
follows:
</p>
<pre class="programlisting">ARG | DBT | POINTER <span class="emphasis"><em>variable_name</em></span> <span class="emphasis"><em>variable_type</em></span> <span class="emphasis"><em>printf_format</em></span></pre>
<p>
The keyword ARG indicates that the argument is a simple
parameter of the type specified. For example, a file ID might
be logged as:
</p>
<pre class="programlisting">ARG fileID int d</pre>
<p>
The keyword DBT indicates that the argument is a Berkeley DB
DBT structure, containing a length and pointer to a byte
string. The keyword POINTER indicates that the argument is a
pointer to the data type specified (of course the data type,
not the pointer, is what is logged).
</p>
<p>
The <span class="emphasis"><em>variable_name</em></span> is the field name
within the structure that will be used to refer to this item.
The <span class="emphasis"><em>variable_type</em></span> is the C-language type
of the variable, and the printf format is the C-language
format string, without the leading percent ("%") character,
that should be used to display the contents of the field (for
example, "s" for string, "d" for signed integral type, "u" for
unsigned integral type, "ld" for signed long integral type,
"lu" for long unsigned integral type, and so on).
</p>
<p>
For example, ex_apprec.src defines a single log record type,
used to log a directory name that has been stored in a
DBT:
</p>
<pre class="programlisting">BEGIN mkdir 10000
DBT dirname DBT s
END</pre>
<p>
As the name suggests, this example of an application-defined
log record will be used to log the creation of a directory.
There are many more examples of XXX.src files in the Berkeley
DB distribution. For example, the file btree/btree.src
contains the definitions for the log records supported by the
Berkeley DB Btree access method.
</p>
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