standby.signal is an optional data directory file determining whether the node starts up as a standby.
If the file
standby.signal is present in the data directory, the server will start in standby mode and enter recovery.
standby.signal can be empty, or contain arbitrary data (for example, a replication management tool could write information noting details about when/how the standby was created).
Note that if
standby.signal is present, but neither
restore_command are specified, the server will start up in recovery and wait for WAL files to be placed in the pg_wal directory by an external process:
[2021-05-12 08:36:12 UTC] WARNING: 01000: specified neither primary_conninfo nor restore_command [2021-05-12 08:36:12 UTC] HINT: The database server will regularly poll the pg_wal subdirectory to check for files placed there.
recovery.signal are present in the data directory,
standby.signal takes precedence. This situation is not logged.