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<!-- doc/src/sgml/archive-modules.sgml -->
<chapter id="archive-modules">
<title>Archive Modules</title>
<indexterm zone="archive-modules">
<primary>Archive Modules</primary>
</indexterm>
<para>
PostgreSQL provides infrastructure to create custom modules for continuous
archiving (see <xref linkend="continuous-archiving"/>). While archiving via
a shell command (i.e., <xref linkend="guc-archive-command"/>) is much
simpler, a custom archive module will often be considerably more robust and
performant.
</para>
<para>
When a custom <xref linkend="guc-archive-library"/> is configured, PostgreSQL
will submit completed WAL files to the module, and the server will avoid
recycling or removing these WAL files until the module indicates that the files
were successfully archived. It is ultimately up to the module to decide what
to do with each WAL file, but many recommendations are listed at
<xref linkend="backup-archiving-wal"/>.
</para>
<para>
Archiving modules must at least consist of an initialization function (see
<xref linkend="archive-module-init"/>) and the required callbacks (see
<xref linkend="archive-module-callbacks"/>). However, archive modules are
also permitted to do much more (e.g., declare GUCs and register background
workers).
</para>
<para>
The <filename>contrib/basic_archive</filename> module contains a working
example, which demonstrates some useful techniques.
</para>
<sect1 id="archive-module-init">
<title>Initialization Functions</title>
<indexterm zone="archive-module-init">
<primary>_PG_archive_module_init</primary>
</indexterm>
<para>
An archive library is loaded by dynamically loading a shared library with the
<xref linkend="guc-archive-library"/>'s name as the library base name. The
normal library search path is used to locate the library. To provide the
required archive module callbacks and to indicate that the library is
actually an archive module, it needs to provide a function named
<function>_PG_archive_module_init</function>. The result of the function
must be a pointer to a struct of type
<structname>ArchiveModuleCallbacks</structname>, which contains everything
that the core code needs to know to make use of the archive module. The
return value needs to be of server lifetime, which is typically achieved by
defining it as a <literal>static const</literal> variable in global scope.
<programlisting>
typedef struct ArchiveModuleCallbacks
{
ArchiveStartupCB startup_cb;
ArchiveCheckConfiguredCB check_configured_cb;
ArchiveFileCB archive_file_cb;
ArchiveShutdownCB shutdown_cb;
} ArchiveModuleCallbacks;
typedef const ArchiveModuleCallbacks *(*ArchiveModuleInit) (void);
</programlisting>
Only the <function>archive_file_cb</function> callback is required. The
others are optional.
</para>
</sect1>
<sect1 id="archive-module-callbacks">
<title>Archive Module Callbacks</title>
<para>
The archive callbacks define the actual archiving behavior of the module.
The server will call them as required to process each individual WAL file.
</para>
<sect2 id="archive-module-startup">
<title>Startup Callback</title>
<para>
The <function>startup_cb</function> callback is called shortly after the
module is loaded. This callback can be used to perform any additional
initialization required. If the archive module has any state, it can use
<structfield>state->private_data</structfield> to store it.
<programlisting>
typedef void (*ArchiveStartupCB) (ArchiveModuleState *state);
</programlisting>
</para>
</sect2>
<sect2 id="archive-module-check">
<title>Check Callback</title>
<para>
The <function>check_configured_cb</function> callback is called to determine
whether the module is fully configured and ready to accept WAL files (e.g.,
its configuration parameters are set to valid values). If no
<function>check_configured_cb</function> is defined, the server always
assumes the module is configured.
<programlisting>
typedef bool (*ArchiveCheckConfiguredCB) (ArchiveModuleState *state);
</programlisting>
If <literal>true</literal> is returned, the server will proceed with
archiving the file by calling the <function>archive_file_cb</function>
callback. If <literal>false</literal> is returned, archiving will not
proceed, and the archiver will emit the following message to the server log:
<screen>
WARNING: archive_mode enabled, yet archiving is not configured
</screen>
In the latter case, the server will periodically call this function, and
archiving will proceed only when it returns <literal>true</literal>.
</para>
</sect2>
<sect2 id="archive-module-archive">
<title>Archive Callback</title>
<para>
The <function>archive_file_cb</function> callback is called to archive a
single WAL file.
<programlisting>
typedef bool (*ArchiveFileCB) (ArchiveModuleState *state, const char *file, const char *path);
</programlisting>
If <literal>true</literal> is returned, the server proceeds as if the file
was successfully archived, which may include recycling or removing the
original WAL file. If <literal>false</literal> is returned, the server will
keep the original WAL file and retry archiving later.
<replaceable>file</replaceable> will contain just the file name of the WAL
file to archive, while <replaceable>path</replaceable> contains the full
path of the WAL file (including the file name).
</para>
</sect2>
<sect2 id="archive-module-shutdown">
<title>Shutdown Callback</title>
<para>
The <function>shutdown_cb</function> callback is called when the archiver
process exits (e.g., after an error) or the value of
<xref linkend="guc-archive-library"/> changes. If no
<function>shutdown_cb</function> is defined, no special action is taken in
these situations. If the archive module has any state, this callback should
free it to avoid leaks.
<programlisting>
typedef void (*ArchiveShutdownCB) (ArchiveModuleState *state);
</programlisting>
</para>
</sect2>
</sect1>
</chapter>
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