Postgresql 中文操作指南
Chapter 66. Custom WAL Resource Managers
本章介绍了核心 PostgreSQL 系统与自定义 WAL 资源管理器之间的界面，该界面使扩展能够直接与 WAL 集成。
扩展（尤其是 Table Access Method 或 Index Access Method ）可能需要使用 WAL 进行恢复、复制和/或 Logical Decoding 。自定义资源管理器是 Generic WAL （不支持逻辑解码）的更灵活的替代方案，但对于扩展而言，其实现更复杂。
要想创建自定义 WAL 资源管理器，首先定义一个 RmgrData 结构，其中包含对资源管理器方法的实现。请参考 PostgreSQL 源中的 src/backend/access/transam/README 和 src/include/access/xlog_internal.h 。
/*
* 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;src/test/modules/test_custom_rmgrs 模块中包含一个可运行的示例，用以展示自定义 WAL 资源管理器的工作方式。
然后，注册您的新资源管理器。
/*
* 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);必须从扩展模块的 RegisterCustomRmgr PG_init function. While developing a new extension, use _RM_EXPERIMENTAL_ID 中调用 rmid 。在准备好向用户发布扩展时，请在 Custom WAL Resource Manager 页面中预留一个新资源管理器 ID。
将实现自定义资源管理器的扩展模块置于 shared_preload_libraries 中，以便在 PostgreSQL 启动期间尽早加载。