I'm trying to figure out the logical replication protocol. My attention was drawn to the message "standby status update" (byte('r')). Unfortunately, the documentation does not seem to describe the expected server behavior for this command.

If I send an lsn from the past, will the server resend transactions that were in the future regarding this lsn? Or, as far as I could find out in a practical way, this message will only affect the meta data of the replication slot (from pg_replication_slots table)?

CodePudding user response：

That information is used to move the replication slot ahead and to provide the data that can be seen in pg_stat_replication (sent_lsn, write_lsn, flush_lsn and replay_lsn). With synchronous replication, it also provides the data that the primary has to wait for before it can report a transaction as committed.

Sending old, incorrect log sequence numbers will not change the data that the primary streams to the standby, but it might make the primary retail old WAL, because the replication slot is not moved ahead. It will also confuse monitoring. With synchronous replication, it will lead to hanging DML transactions.