On 28/10/2024 17:51, Peter Eisentraut wrote:
This is something I hacked together on the way back from pgconf.eu. It's
highly experimental.
The idea is to do the equivalent of pg_wal_replay_wait() on the protocol
level, so that it is ideally fully transparent to the application code.
The application just issues queries, and they might be serviced by a
primary or a standby, but there is always a correct ordering of reads
after writes.
Additionally, I'm exploring whether this is an idea for a protocol
extension that might be a bit more complex than, say, longer cancel
keys, something we could have a discussion around protocol versioning
around.
The patch adds a protocol extension called _pq_.wait_for_lsn as well as
a libpq connection option wait_for_lsn to activate the same. (Use e.g.,
psql -d 'wait_for_lsn=1'.)
With this protocol extension, two things are changed:
- The ReadyForQuery message sends back the current LSN.
+1
- The Query message sends an LSN to wait for. (This doesn't handle the
extended query protocol yet.)
I'd suggest adding a new message type for this, so that it works the
same with simple and extended query. Or if you just want to wait without
issuing any query.
--
Heikki Linnakangas
Neon (https://neon.tech)
