Question

Why does my sync sometimes take 25 minutes to extract nothing?

Environment

Connector: Postgres source connector

Answer

This is expected behavior when there are 0 records in the replication slot. It's necessary to have this timeout because some slots are slow to respond if the WAL gets large. From a WAL consumer (Fivetran) side, a hung replication slot and an idle database appears the same.

If a single change comes through, the sync will finish quickly.

Note: If there are never any writes seen in the process and load bars of the connector, the LSN is likely hung. See this article for more information.