Re: protocol-level wait-for-LSN

On Wed, 30 Oct 2024 at 12:53, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
> We might have made a mistake by calling this mechanism "protocol
> extensions", because it makes people think of user-defined extensions.

I think this is a real problem, that's probably worth fixing. I
created a separate thread to address this[1]

> So yes, each protocol extension needs to know about all the other
> protocol extensions that it can be used with. In practice we'll avoid
> doing crazy stuff so that the protocol extensions are orthogonal

Just as an example, let's say we add a server-side query time to the
protocol (which honestly seems like a pretty useful feature). So that
ReadyForQuery now returns the query time if the query_time protocol.
For clients it isn't difficult at all to support any combination of
query_time & wait_for_lsn options. As long as we define that the
wait_for_lsn field is before the query_time field if both exist, then
two simple if statements like this would do the trick:

if (wait_for_lsn_enabled) {
// interpret next field as LSN
}
if (query_time_enabled) {
// interpret next field as query time
}

[1]: https://www.postgresql.org/message-id/CAGECzQQoc%2BV94TrF-5cMikCMaf-uUnU52euwSCtQBeDYqXnXyA%40mail.gmail.com