To initiate streaming replication, the frontend sends the replication parameter in the startup message. This tells the backend to go into walsender mode, wherein a small set of replication commands can be issued instead of SQL statements. Only the simple query protocol can be used in walsender mode. The commands accepted in walsender mode are:
Requests the server to identify itself. Server replies with a result set of a single row, containing two fields:
The unique system identifier identifying the cluster. This can be used to check that the base backup used to initialize the standby came from the same cluster.
Current TimelineID. Also useful to check that the standby is consistent with the master.
Instructs server to start streaming WAL, starting at WAL position XXX/XXX. The server can reply with an error, e.g. if the requested section of WAL has already been recycled. On success, server responds with a CopyOutResponse message, and then starts to stream WAL to the frontend. WAL will continue to be streamed until the connection is broken; no further commands will be accepted.
WAL data is sent as a series of CopyData messages. (This allows other information to be intermixed; in particular the server can send an ErrorResponse message if it encounters a failure after beginning to stream.) The payload in each CopyData message follows this format:
Identifies the message as WAL data.
The starting point of the WAL data in this message, given in XLogRecPtr format.
The current end of WAL on the server, given in XLogRecPtr format.
The server's system clock at the time of transmission, given in TimestampTz format.
A section of the WAL data stream.
A single WAL record is never split across two CopyData messages. When a WAL record crosses a WAL page boundary, and is therefore already split using continuation records, it can be split at the page boundary. In other words, the first main WAL record and its continuation records can be sent in different CopyData messages.
Note that all fields within the WAL data and the above-described header will be in the sending server's native format. Endianness, and the format for the timestamp, are unpredictable unless the receiver has verified that the sender's system identifier matches its own pg_control contents.
If the WAL sender process is terminated normally (during postmaster shutdown), it will send a CommandComplete message before exiting. This might not happen during an abnormal shutdown, of course.