Re: [HACKERS] make async slave to wait for lsn to be replayed

Hello.

At Wed, 18 Nov 2020 15:05:00 +0300, a(dot)pervushina(at)postgrespro(dot)ru wrote in
> I've changed the BEGIN WAIT FOR LSN statement to core functions
> pg_waitlsn, pg_waitlsn_infinite and pg_waitlsn_no_wait.
> Currently the functions work inside repeatable read transactions, but
> waitlsn creates a snapshot if called first in a transaction block,
> which can possibly lead the transaction to working incorrectly, so the
> function gives a warning.

According to the discuttion here, implementing as functions is not
optimal. As a Poc, I made it as a procedure. However I'm not sure it
is the correct implement as a native procedure but it seems working as
expected.

> Usage examples
> ==========
> select pg_waitlsn(‘LSN’, timeout);
> select pg_waitlsn_infinite(‘LSN’);
> select pg_waitlsn_no_wait(‘LSN’);

The first and second usage is coverd by a single procedure. The last
function is equivalent to pg_last_wal_replay_lsn(). As the result, the
following procedure is provided in the attached.

pg_waitlsn(wait_lsn pg_lsn, timeout integer DEFAULT -1)

Any opinions mainly compared to implementation as a command?

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center