Horton

Re: Minimal logical decoding on standbys

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Andres Freund <andres@anarazel.de>, 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>, Melanie Plageman <melanieplageman@gmail.com>
Date: 2023-04-05T14:24:14Z
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 Tue, Apr 4, 2023 at 8:33 PM Jeff Davis <pgsql@j-davis.com> wrote:
> Perhaps a commit message like:
>
> "For cascading replication, wake up physical walsenders separately from
> logical walsenders.
>
> Physical walsenders can't send data until it's been flushed; logical
> walsenders can't decode and send data until it's been applied. On the
> standby, the WAL is flushed first, which will only wake up physical
> walsenders; and then applied, which will only wake up logical
> walsenders.
>
> Previously, all walsenders were awakened when the WAL was flushed. That
> was fine for logical walsenders on the primary; but on the standby the
> flushed WAL would not have been applied yet, so logical walsenders were
> awakened too early."

This sounds great. I think it's very clear about what is being changed
and why. I see that Bertrand already pulled this language into v60.

> For comments, I agree that WalSndWakeup() clearly needs a comment
> update. The call site in ApplyWalRecord() could also use a comment. You
> could add a comment at every call site, but I don't think that's
> necessary if there's a good comment over WalSndWakeup().

Right, we don't want to go overboard, but I think putting some of the
text you wrote above for the commit message, or something with a
similar theme, in the comment for WalSndWakeup() would be quite
helpful. We want people to understand why the physical and logical
cases are different.

I agree with you that ApplyWalRecord() is the other place where we
need a good comment. I think the one in v60 needs more word-smithing.
It should probably be a bit more detailed and clear about not only
what we're doing but why we're doing it.

The comment in InitWalSenderSlot() seems like it might be slightly
overdone, but I don't have a big problem with it so if we leave it
as-is that's fine.

Now that I understand what's going on here a bit better, I'm inclined
to think that this patch is basically fine. At least, I don't see any
obvious problem with it.

-- 
Robert Haas
EDB: http://www.enterprisedb.com