Hi, I'm thinking if there's a way in core postgres to achieve $subject. In reality, the sync/async standbys can either be closer/farther (which means sync/async standbys can receive WAL at different times) to primary, especially in cloud HA environments with primary in one Availability Zone(AZ)/Region and standbys in different AZs/Regions. $subject may not be possible on dev systems (say, for testing some HA features) unless we can inject a delay in WAL senders before sending WAL.
How about having two developer-only GUCs {async, sync}_wal_sender_delay? When set, the async and sync WAL senders will delay sending WAL by {async, sync}_wal_sender_delay milliseconds/seconds? Although, I can't think of any immediate use, it will be useful someday IMO, say for features like [1], if it gets in. With this set of GUCs, one can even add core regression tests for HA features. Thoughts? [1] https://www.postgresql.org/message-id/CALj2ACWCj60g6TzYMbEO07ZhnBGbdCveCrD413udqbRM0O59RA%40mail.gmail.com Regards, Bharath Rupireddy.