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<!-- doc/src/sgml/custom-rmgr.sgml -->
<chapter id="custom-rmgr">
<title>Custom WAL Resource Managers</title>
<para>
This chapter explains the interface between the core
<productname>PostgreSQL</productname> system and custom WAL resource
managers, which enable extensions to integrate directly with the <link
linkend="wal"><acronym>WAL</acronym></link>.
</para>
<para>
An extension, especially a <link linkend="tableam">Table Access
Method</link> or <link linkend="indexam">Index Access Method</link>, may
need to use WAL for recovery, replication, and/or <link
linkend="logicaldecoding">Logical Decoding</link>. Custom resource managers
are a more flexible alternative to <link linkend="generic-wal">Generic
WAL</link> (which does not support logical decoding), but more complex for
an extension to implement.
</para>
<para>
To create a new custom WAL resource manager, first define an
<structname>RmgrData</structname> structure with implementations for the
resource manager methods. Refer to
<filename>src/backend/access/transam/README</filename> and
<filename>src/include/access/xlog_internal.h</filename> in the
<productname>PostgreSQL</productname> source.
<programlisting>
/*
* Method table for resource managers.
*
* This struct must be kept in sync with the PG_RMGR definition in
* rmgr.c.
*
* rm_identify must return a name for the record based on xl_info (without
* reference to the rmid). For example, XLOG_BTREE_VACUUM would be named
* "VACUUM". rm_desc can then be called to obtain additional detail for the
* record, if available (e.g. the last block).
*
* rm_mask takes as input a page modified by the resource manager and masks
* out bits that shouldn't be flagged by wal_consistency_checking.
*
* RmgrTable[] is indexed by RmgrId values (see rmgrlist.h). If rm_name is
* NULL, the corresponding RmgrTable entry is considered invalid.
*/
typedef struct RmgrData
{
const char *rm_name;
void (*rm_redo) (XLogReaderState *record);
void (*rm_desc) (StringInfo buf, XLogReaderState *record);
const char *(*rm_identify) (uint8 info);
void (*rm_startup) (void);
void (*rm_cleanup) (void);
void (*rm_mask) (char *pagedata, BlockNumber blkno);
void (*rm_decode) (struct LogicalDecodingContext *ctx,
struct XLogRecordBuffer *buf);
} RmgrData;
</programlisting>
</para>
<para>
The <filename>src/test/modules/test_custom_rmgrs</filename> module
contains a working example, which demonstrates usage of custom WAL
resource managers.
</para>
<para>
Then, register your new resource
manager.
<programlisting>
/*
* Register a new custom WAL resource manager.
*
* Resource manager IDs must be globally unique across all extensions. Refer
* to https://wiki.postgresql.org/wiki/CustomWALResourceManagers to reserve a
* unique RmgrId for your extension, to avoid conflicts with other extension
* developers. During development, use RM_EXPERIMENTAL_ID to avoid needlessly
* reserving a new ID.
*/
extern void RegisterCustomRmgr(RmgrId rmid, const RmgrData *rmgr);
</programlisting>
<function>RegisterCustomRmgr</function> must be called from the
extension module's <link linkend="xfunc-c-dynload">_PG_init</link> function.
While developing a new extension, use <literal>RM_EXPERIMENTAL_ID</literal>
for <parameter>rmid</parameter>. When you are ready to release the extension
to users, reserve a new resource manager ID at the <ulink
url="https://wiki.postgresql.org/wiki/CustomWALResourceManagers">Custom WAL
Resource Manager</ulink> page.
</para>
<para>
Place the extension module implementing the custom resource manager in <xref
linkend="guc-shared-preload-libraries"/> so that it will be loaded early
during <productname>PostgreSQL</productname> startup.
</para>
<note>
<para>
The extension must remain in shared_preload_libraries as long as any
custom WAL records may exist in the system. Otherwise
<productname>PostgreSQL</productname> will not be able to apply or decode
the custom WAL records, which may prevent the server from starting.
</para>
</note>
</chapter>
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