Thanks a lot for your reply. It looks like this is exactly what I need. For my use case, I'm trying to get read-only transactions to wait for the replication of prior writes.
On Thu, Mar 16, 2023 at 1:18 AM Tejasvi Kashi <mail@tejasvi.dev> wrote: > > For my use case, I'm trying to ascertain if there are any in-flight transactions that are yet to be replicated to synchronous standbys (in a synchronous streaming replication setting) > > I've been looking at sent_lsn, write_lsn, flush_lsn etc., of the walsender, but with no success. Considering the visibility change added above, is there a way for me to check for transactions that have been committed locally but are waiting for replication?
I think you can look for SyncRep wait_event from pg_stat_activity, something like [1]. The backends will wait indefinitely until latch is set (postmaster death or an ack is received from sync standbys) in SyncRepWaitForLSN(). backend_xid is your locally-committed-but-not-yet-replicated txn id. Will this help?
Well, if you're planning to know all locally-committed-but-not-yet-replicated txns from an extension or any other source code, you may run the full query [1] or if running a query seems costly, you can look at what pg_stat_get_activity() does to get each backend's wait_event_info and have your code do that.
BTW, what exactly is the use-case that'd want locally-committed-but-not-yet-replicated txns info?
