Horton

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

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Kartyshov Ivan <i.kartyshov@postgrespro.ru>, 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: 2024-04-01T10:27:09Z
Lists: pgsql-hackers

Attachments

On Mon, Apr 1, 2024 at 5:25 AM Bharath Rupireddy <
bharath.rupireddyforpostgres@gmail.com> wrote:

> On Mon, Apr 1, 2024 at 5:54 AM Alexander Korotkov <aekorotkov@gmail.com>
> wrote:
> >
> > > 9. To me the following query blocks even though I didn't mention
> timeout.
> > > CALL pg_wal_replay_wait('0/fffffff');
> >
> > If your primary server is freshly initialized, you need to do quite
> > data modifications to reach this LSN.
>
> Right, but why pg_wal_replay_wait  blocks without a timeout? It must
> return an error saying it can't reach the target LSN, no?
>

How can replica know this?  It doesn't look feasible to distinguish this
situation from the situation when connection between primary and replica
became slow.  I'd keep this simple.  We have pg_sleep_for() which waits for
the specified time whatever it is.  And we have pg_wal_replay_wait() waits
till replay lsn grows to the target whatever it is.  It's up to the user to
specify the correct value.


> Did you forget to attach the new patch?
>

Yes, here it is.

------
Regards,
Alexander Korotkov

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