51. Archive Modules
When a custom 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 Section 26.3.1.
contrib/basic_archivemodule contains a working example, which demonstrates some useful techniques.