Horton

Re: Minimal logical decoding on standbys

Amit Khandekar <amitdkhan.pg@gmail.com>

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From: Amit Khandekar <amitdkhan.pg@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: tushar <tushar.ahuja@enterprisedb.com>, Petr Jelinek <petr.jelinek@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Craig Ringer <craig@2ndquadrant.com>, Petr Jelinek <petr@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2019-06-04T10:21:01Z
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 Fri, 31 May 2019 at 17:31, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
>
> On Fri, 31 May 2019 at 11:08, Amit Khandekar <amitdkhan.pg@gmail.com> wrote:
> >
> > On Thu, 30 May 2019 at 20:13, Andres Freund <andres@anarazel.de> wrote:
> > >
> > > Hi,
> > >
> > > On 2019-05-30 19:46:26 +0530, Amit Khandekar wrote:
> > > > @@ -1042,7 +1042,8 @@ ReplicationSlotReserveWal(void)
> > > >  if (!RecoveryInProgress() && SlotIsLogical(slot))
> > > >  {
> > > >     ....
> > > >  }
> > > >  else
> > > >  {
> > > > -   restart_lsn = GetRedoRecPtr();
> > > > +   restart_lsn = SlotIsLogical(slot) ?
> > > > +                        GetXLogReplayRecPtr(&ThisTimeLineID) : GetRedoRecPtr();
> > > >
> > > > But then when I do pg_create_logical_replication_slot(), it hangs in
> > > > DecodingContextFindStartpoint(), waiting to find new records
> > > > (XLogReadRecord).
> > >
> > > But just till the primary has logged the necessary WAL records? If you
> > > just do CHECKPOINT; or such on the primary, it should succeed quickly?
> >
> > Yes, it waits until there is a commit record, or (just tried) until a
> > checkpoint command.
>
> Is XLOG_RUNNING_XACTS record essential for the logical decoding to
> build a consistent snapshot ?
> Since the restart_lsn is now ReplayRecPtr, there is no
> XLOG_RUNNING_XACTS record, and so the snapshot state is not yet
> SNAPBUILD_CONSISTENT. And so
> DecodingContextFindStartpoint()=>DecodingContextReady() never returns
> true, and hence DecodingContextFindStartpoint() goes in an infinite
> loop, until it gets XLOG_RUNNING_XACTS.

After giving more thought on this, I think it might make sense to
arrange for the xl_running_xact record to be sent from master to the
standby, when a logical slot is to be created on standby. How about
standby sending a new message type to the master, requesting for
xl_running_xact record ? Then on master, ProcessStandbyMessage() will
process this new message type and call LogStandbySnapshot().


--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company