177 lines
6.9 KiB
Plaintext
177 lines
6.9 KiB
Plaintext
<!-- 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>
|
|
|
|
<note>
|
|
<para>
|
|
When returning <literal>false</literal>, it may be useful to append some
|
|
additional information to the generic warning message. To do that, provide
|
|
a message to the <function>arch_module_check_errdetail</function> macro
|
|
before returning <literal>false</literal>. Like
|
|
<function>errdetail()</function>, this macro accepts a format string
|
|
followed by an optional list of arguments. The resulting string will be
|
|
emitted as the <literal>DETAIL</literal> line of the warning message.
|
|
</para>
|
|
</note>
|
|
</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 or an error is thrown, 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>
|
|
|
|
<note>
|
|
<para>
|
|
The <function>archive_file_cb</function> callback is called in a
|
|
short-lived memory context that will be reset between invocations. If you
|
|
need longer-lived storage, create a memory context in the module's
|
|
<function>startup_cb</function> callback.
|
|
</para>
|
|
</note>
|
|
</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>
|