This chapter explains the interface between the core PostgreSQL system and custom WAL resource managers, which enable extensions to integrate directly with the WAL.

An extension, especially a Table Access Method or Index Access Method, may need to use WAL for recovery, replication, and/or Logical Decoding. Custom resource managers are a more flexible alternative to Generic WAL (which does not support logical decoding), but more complex for an extension to implement.

To create a new custom WAL resource manager, first define an RmgrData structure with implementations for the resource manager methods. Refer to src/backend/access/transam/README and src/include/access/xlog_internal.h in the PostgreSQL source.

/*
 * 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;

The src/test/modules/test_custom_rmgrs module contains a working example, which demonstrates usage of custom WAL resource managers.

Then, register your new resource manager.

/*
 * 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);

RegisterCustomRmgr must be called from the extension module's _PG_init function. While developing a new extension, use RM_EXPERIMENTAL_ID for rmid. When you are ready to release the extension to users, reserve a new resource manager ID at the Custom WAL Resource Manager page.

Place the extension module implementing the custom resource manager in shared_preload_libraries so that it will be loaded early during PostgreSQL startup.