Horton

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

Ivan Kartyshov <i.kartyshov@postgrespro.ru>

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From: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
To: Andy Fan <zhihuifan1213@163.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Euler Taveira <euler@eulerto.com>, Thomas Munro <thomas.munro@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, Amit Kapila <amit.kapila16@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Peter Smith <smithpb2250@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2024-04-02T10:11:27Z
Lists: pgsql-hackers 
On 2024-04-02 11:14, Andy Fan wrote:
> If so, when users call pg_wal_replay_wait, they can get informed when
> the wal is replaied to the target_lsn, but they can't know how long 
> time
> it waits unless they check it in application side, I think such
> information will be useful for monitor purpose sometimes.
> 
> select pg_wal_replay_wait(lsn, 1000);  may just take 1ms in fact, in
> this case, I want this function return 1.

Hi Andy, to get timing we can use \time in psql.
Here is an example.
postgres=# \timing
Timing is on.
postgres=# select 1;
  ?column?
----------
         1
(1 row)

Time: 0.536 ms


>        <returnvalue>void</returnvalue>
And returning VOID is the best option, rather than returning TRUE|FALSE
or timing. It left the logic of the procedure very simple, we get an
error if LSN is not reached.

8 years, we tried to add this feature, and now we suggest the best way
for this feature is to commit the minimal version first.

Let's discuss further improvements in future versions.

-- 
Ivan Kartyshov
Postgres Professional: www.postgrespro.com

Commits

  1. Ensure standby promotion point in 043_wal_replay_wait.pl

  2. Minor cleanup related to pg_wal_replay_wait() procedure

  3. Adjust pg_wal_replay_wait() procedure behavior on promoted standby

  4. pg_wal_replay_wait(): Fix typo in the doc

  5. Implement pg_wal_replay_wait() stored procedure

  6. Revert: Implement pg_wal_replay_wait() stored procedure

  7. Call WaitLSNCleanup() in AbortTransaction()

  8. Clarify what is protected by WaitLSNLock

  9. Use an LWLock instead of a spinlock in waitlsn.c

  10. Use the pairing heap instead of a flat array for LSN replay waiters

  11. Minor improvements for waitlsn.c

  12. Make the order of the header file includes consistent