Horton

Re: Minimal logical decoding on standbys

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
To: Jeff Davis <pgsql@j-davis.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: 2023-03-03T16:23:36Z
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.

Hi,

On 3/2/23 8:45 PM, Jeff Davis wrote:
> 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?
> 

Yes I think so: loc is when we started waiting initially
and RecentFlushPtr is >= to when the broadcast has been sent.

>> - 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:
> WalSndWaitForWal
>   * 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.
> 

What I think is that, in this particular case, we are sure that
the loop exit condition is met as we know that loc <= RecentFlushPtr.

>>
>> 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.
> 

What I meant is that of course I might be wrong.

If we do not agree that the new API (in this particular case) is not needed then
I agree that building the new API is the way to go ;-) (+ it offers the advantage to
be able to be more precise while reporting the wait event).

Regards,

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