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: Amit Kapila <amit.kapila16@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Jeff Davis <pgsql@j-davis.com>, Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.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: 2023-04-05T12:44:09Z
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 4/5/23 12:28 PM, Amit Kapila wrote:
> On Wed, Apr 5, 2023 at 2:41 PM Drouvot, Bertrand
> <bertranddrouvot.pg@gmail.com> wrote:
>> Maybe we could change the doc with something among those lines instead?
>>
>> "
>> Existing logical slots on standby also get invalidated if wal_level on primary is reduced to
>> less than 'logical'. This is done as soon as the standby detects such a change in the WAL stream.
>>
>> It means, that for walsenders that are lagging (if any), some WAL records up to the parameter change on the
>> primary won't be decoded".
>>
>> I don't know whether this is what one would expect but that should be less of a surprise if documented.
>>
>> What do you think?
>>
> 
> Yeah, I think it is better to document to avoid any surprises if
> nobody else sees any problem with it.

Ack.

> BTW, another thought that
> crosses my mind is that let's not invalidate the slots when the
> standby startup process processes parameter_change record and rather
> do it when walsender decodes the parameter_change record, if we think
> that is safe. I have shared this as this crosses my mind while
> thinking about this part of the patch and wanted to validate my
> thoughts, we don't need to change even if the idea is valid.
>

I think this is a valid idea but I think I do prefer the current one (where the
startup process triggers the invalidations) because:

   - I think this is better to invalidate as soon as possible. In case of inactive logical
replication slot (walsenders stopped) it could take time to get "notified". While with the current
approach you'd get notified in the logfile and pg_replication_slots even if walsenders are stopped.

   - This is not a "slot" dependent invalidation (as opposed to the xid invalidations case)

   - This is "somehow" the same behavior as on the primary: if one change the wal_level to be < logical
then the engine will not start (if logical slot in place). Then what has been decoded is until the time
the engine has been stopped. So if there is walsender lag, you'd not see some records.

> minor nitpick:
> +
> + /* Intentional fall through to session cancel */
> + /* FALLTHROUGH */
> 
> Do we need to repeat fall through twice in different ways?
> 

Do you mean, you'd prefer what was done in v52/0002?

Regards,

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