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: 2019-05-23T17:38:55Z
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 Thu, 23 May 2019 at 21:29, Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2019-05-23 17:39:21 +0530, Amit Khandekar wrote:
> > On Tue, 21 May 2019 at 21:49, Andres Freund <andres@anarazel.de> wrote:
> > Yeah, I agree we should add such checks to minimize the possibility of
> > reading logical records from a master that has insufficient wal_level.
> > So to summarize :
> > a. CheckLogicalDecodingRequirements() : Add Controlfile wal_level checks
> > b. Call this function call in CreateInitDecodingContext() as well.
> > c. While decoding XLOG_PARAMETER_CHANGE record, emit recovery conflict
> > error if there is an existing logical slot.
> >
> > This made me think more of the race conditions. For instance, in
> > pg_create_logical_replication_slot(), just after
> > CheckLogicalDecodingRequirements and before actually creating the
> > slot, suppose concurrently Controlfile->wal_level is changed from
> > logical to replica.  So suppose a new slot does get created. Later the
> > slot is read, so in pg_logical_slot_get_changes_guts(),
> > CheckLogicalDecodingRequirements() is called where it checks
> > ControlFile->wal_level value. But just before it does that,
> > ControlFile->wal_level concurrently changes back to logical, because
> > of replay of another param-change record. So this logical reader will
> > think that the wal_level is sufficient, and will proceed to read the
> > records, but those records are *before* the wal_level change, so these
> > records don't have logical data.
>
> I don't think that's an actual problem, because there's no decoding
> before the slot exists and CreateInitDecodingContext() has determined
> the start LSN. And by that point the slot exists, slo
> XLOG_PARAMETER_CHANGE replay can error out.

So between the start lsn and the lsn for
parameter-change(logical=>replica) record, there can be some records ,
and these don't have logical data. So the slot created will read from
the start lsn, and proceed to read these records, before reading the
parameter-change record.

Can you re-write the below phrase please ? I suspect there is some
letters missing there :
"And by that point the slot exists, slo XLOG_PARAMETER_CHANGE replay
can error out"

Are you saying we want to error out when the postgres replays the
param change record and there is existing logical slot ? I thought you
were suggesting earlier that it's the decoder.c code which should
error out when reading the param-change record.



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