The archive callbacks define the actual archiving behavior of the module. The server will call them as required to process each individual WAL file.
The startup_cb 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 state->private_data to store it.
typedef void (*ArchiveStartupCB) (ArchiveModuleState *state);
The check_configured_cb 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 check_configured_cb is defined, the server always assumes the module is configured.
typedef bool (*ArchiveCheckConfiguredCB) (ArchiveModuleState *state);
If true is returned, the server will proceed with archiving the file by calling the archive_file_cb callback. If false is returned, archiving will not proceed, and the archiver will emit the following message to the server log:
WARNING: archive_mode enabled, yet archiving is not configured
In the latter case, the server will periodically call this function, and archiving will proceed only when it returns true.
When returning false, it may be useful to append some additional information to the generic warning message. To do that, provide a message to the arch_module_check_errdetail macro before returning false. Like errdetail(), this macro accepts a format string followed by an optional list of arguments. The resulting string will be emitted as the DETAIL line of the warning message.
The archive_file_cb callback is called to archive a single WAL file.
typedef bool (*ArchiveFileCB) (ArchiveModuleState *state, const char *file, const char *path);
If true is returned, the server proceeds as if the file was successfully archived, which may include recycling or removing the original WAL file. If false is returned or an error is thrown, the server will keep the original WAL file and retry archiving later. file will contain just the file name of the WAL file to archive, while path contains the full path of the WAL file (including the file name).
The archive_file_cb 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 startup_cb callback.
The shutdown_cb callback is called when the archiver process exits (e.g., after an error) or the value of archive_library changes. If no shutdown_cb 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.
typedef void (*ArchiveShutdownCB) (ArchiveModuleState *state);
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