Horton

Re: Minimal logical decoding on standbys

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, fabriziomello@gmail.com, tushar <tushar.ahuja@enterprisedb.com>, Rahila Syed <rahila.syed@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-03-02T19:45:51Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Reduce the log level in 035_standby_logical_decoding.pl.

  2. 035_standby_logical_decoding: Add missing waits for replication

  3. For cascading replication, wake physical and logical walsenders separately

  4. Handle logical slot conflicts on standby

  5. Support invalidating replication slots due to horizon and wal_level

  6. Prevent use of invalidated logical slot in CreateDecodingContext()

  7. Replace replication slot's invalidated_at LSN with an enum

  8. Pass down table relation into more index relation functions

  9. Assert only valid flag bits are passed to visibilitymap_set()

  10. Remove unused _bt_delitems_delete() argument.

  11. Add xl_btree_delete optimization.

On Thu, 2023-03-02 at 10:20 +0100, Drouvot, Bertrand wrote:
> Right, but in our case, right after the wakeup (the one due to the CV
> broadcast,
> aka the one that will remove it from the wait queue) we'll exit the
> loop due to:
> 
> "
>          /* check whether we're done */
>          if (loc <= RecentFlushPtr)
>              break;
> "
> 
> as the CV broadcast means that a flush/replay occurred.

But does it mean that the flush/replay advanced *enough* to be greater
than or equal to loc?

> - If it is awakened due to the CV broadcast, then we'll right after
> exit the loop (see above)

...

> I think that's not needed as we'd exit the loop right after we are
> awakened by a CV broadcast.

See the comment here:

 * If this process has been taken out of the wait list, then we know  
 * that it has been signaled by ConditionVariableSignal (or           
 * ConditionVariableBroadcast), so we should return to the caller. But
 * that doesn't guarantee that the exit condition is met, only that we
 * ought to check it.

You seem to be arguing that in this case, it doesn't matter; that
walreceiver knows what walsender is waiting for, and will never wake it
up before it's ready. I don't think that's true, and even if it is, it
needs explanation.

> 
> I agree that's a good idea and that it should/would work too. I just
> wanted to highlight that in this particular
> case that might not be necessary to build this new API.

In this case it looks easier to add the right API than to be sure about
whether it's needed or not.

Regards,
	Jeff Davis