Horton

Re: Minimal logical decoding on standbys

Andres Freund <andres@anarazel.de>

View thread
From: Andres Freund <andres@anarazel.de>
To: "Drouvot, Bertrand" <bdrouvot@amazon.com>
Cc: fabriziomello@gmail.com, Alvaro Herrera <alvherre@2ndquadrant.com>, tushar <tushar.ahuja@enterprisedb.com>, "[pgdg] Robert Haas" <robertmhaas@gmail.com>, Amit Khandekar <amitdkhan.pg@gmail.com>, Rahila Syed <rahila.syed@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-04-07T20:32:18Z
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 2021-04-07 10:09:54 -0700, Andres Freund wrote:
> There's also no test for a recovery conflict due to row removal. Despite
> that being a substantial part of the patchset.

Another aspect that wasn't tested *at all*: Whether logical decoding
actually produces useful and correct results.


> I'm tempted to throw out 024 - all of its tests seem fragile and prove
> little. And then add a few more tests to 025 (and renaming it).

While working on this I found a, somewhat substantial, issue:

When the primary is idle, on the standby logical decoding via walsender
will typically not process the records until further WAL writes come in
from the primary, or until a 10s lapsed.

The problem is that WalSndWaitForWal() waits for the *replay* LSN to
increase, but gets woken up by walreceiver when new WAL has been
flushed. Which means that typically walsenders will get woken up at the
same time that the startup process will be - which means that by the
time the logical walsender checks GetXLogReplayRecPtr() it's unlikely
that the startup process already replayed the record and updated
XLogCtl->lastReplayedEndRecPtr.

I think fixing this would require too invasive changes at this point. I
think we might be able to live with 10s delay issue for one release, but
it sure is ugly :(.

Greetings,

Andres Freund