# Archive Modules

PostgreSQL provides infrastructure to create custom modules for continuous archiving (see continuous-archiving). While archiving via a shell command (i.e., guc-archive-command) is much simpler, a custom archive module will often be considerably more robust and performant.

When a custom guc-archive-library is configured, PostgreSQL will submit completed WAL files to the module, and the server will avoid recycling or removing these WAL files until the module indicates that the files were successfully archived. It is ultimately up to the module to decide what to do with each WAL file, but many recommendations are listed at backup-archiving-wal.

Archiving modules must at least consist of an initialization function (see archive-module-init) and the required callbacks (see archive-module-callbacks). However, archive modules are also permitted to do much more (e.g., declare GUCs and register background workers).

The `contrib/basic_archive` module contains a working example, which demonstrates some useful techniques.

## Initialization Functions

An archive library is loaded by dynamically loading a shared library with the guc-archive-library's name as the library base name. The normal library search path is used to locate the library. To provide the required archive module callbacks and to indicate that the library is actually an archive module, it needs to provide a function named `_PG_archive_module_init`. The result of the function must be a pointer to a struct of type ArchiveModuleCallbacks, which contains everything that the core code needs to know to make use of the archive module. The return value needs to be of server lifetime, which is typically achieved by defining it as a `static const` variable in global scope.

    typedef struct ArchiveModuleCallbacks
    {
        ArchiveStartupCB startup_cb;
        ArchiveCheckConfiguredCB check_configured_cb;
        ArchiveFileCB archive_file_cb;
        ArchiveShutdownCB shutdown_cb;
    } ArchiveModuleCallbacks;
    typedef const ArchiveModuleCallbacks *(*ArchiveModuleInit) (void);

Only the `archive_file_cb` callback is required. The others are optional.

## Archive Module Callbacks

The archive callbacks define the actual archiving behavior of the module. The server will call them as required to process each individual WAL file.

### Startup Callback

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);

### Check Callback

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.

### Archive Callback

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.

### Shutdown Callback

The `shutdown_cb` callback is called when the archiver process exits (e.g., after an error) or the value of guc-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);