64.2. Custom WAL Resource Managers
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## 64.2. Custom WAL Resource Managers #

This section 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.

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.

### Note

The extension must remain in `shared_preload_libraries` as long as any custom WAL records may exist in the system. Otherwise PostgreSQL™ will not be able to apply or decode the custom WAL records, which may prevent the server from starting.

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