Re: How to check for in-progress transactions

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?

[1]
postgres=# select * from pg_stat_activity where backend_type = 'client
backend' and wait_event = 'SyncRep';
-[ RECORD 1 ]----+------------------------------
datid            | 5
datname          | postgres
pid              | 4187907
leader_pid       |
usesysid         | 10
usename          | ubuntu
application_name | psql
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 2023-03-16 05:16:56.917124+00
xact_start       | 2023-03-16 05:17:09.472092+00
query_start      | 2023-03-16 05:17:09.472092+00
state_change     | 2023-03-16 05:17:09.472095+00
wait_event_type  | IPC
wait_event       | SyncRep
state            | active
backend_xid      | 731
backend_xmin     | 731
query_id         |
query            | create table foo(col1 int);
backend_type     | client backend

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com