Hi All, sentPtr reported by WAL sender should usually never jump back, it should always increase. I observed a strange behaviour with the WAL sender where sentPtr jumps back at the beginning. From code examination it looks like the following behaviour is culprit.
The WAL sender reads WAL from restart_lsn which is what is set in reader->EndRecPtr in XLogBeginRead. So reader->EndRecPtr starts with restart_lsn sentPtr starts with MyReplicationSlot->data.confirmed_flush in StartLogicalReplication(). Usually there will be some or other concurrent transaction happening, so confirmed_flush is higher than restart_lsn. After the first loop over send_data in WalSndLoop(), it gets set to reader->EndRecPtr. So when the first WAL record is read it jumps back to the end of the first record starting at restart_lsn. Eventually it will catch up to confirmed_lsn when the WAL sender reads WAL. This seems to be harmless but the logical receiver may get confused if it receives an LSN lesser than confirmed_flush. -- Best Wishes, Ashutosh Bapat
