Horton

RE: speed up a logical replica setup

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: 'Amit Kapila' <amit.kapila16@gmail.com>
Cc: Euler Taveira <euler@eulerto.com>, Alexander Lakhin <exclusion@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Date: 2024-07-17T11:58:47Z
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. pg_createsubscriber: Remove obsolete comment

  2. pg_createsubscriber: Fix an unpredictable recovery wait time.

  3. Fix unstable test in 040_pg_createsubscriber.

  4. Fix the testcase introduced in commit 81d20fbf7a.

  5. Further weaken new pg_createsubscriber test on Windows.

  6. Temporarily(?) weaken new pg_createsubscriber test on Windows.

  7. Make pg_createsubscriber warn if publisher has two-phase commit enabled.

  8. Make pg_createsubscriber more wary about quoting connection parameters.

  9. pg_createsubscriber: Remove failover replication slots on subscriber

  10. pg_createsubscriber: Remove replication slot check on primary

  11. pg_createsubscriber: Only --recovery-timeout controls the end of recovery process

  12. pg_createsubscriber: creates a new logical replica from a standby server

  13. Add some const decorations

  14. Add option force_initdb to PostgreSQL::Test::Cluster:init()

  15. Remove MSVC scripts

Dear Amit,

> Your analysis sounds correct to me.

Okay, so we could have a same picture...

> > IIUC, the root cause is that pg_create_logical_replication_slot() returns a LSN
> > which is not generated yet. So, I think both mine [1] and Euler's approach [2]
> > can solve the issue. My proposal was to add an extra WAL record after the final
> > slot creation, and Euler's one was to use a restart_lsn as the
> recovery_target_lsn.
> >
> 
> I don't think it is correct to set restart_lsn as consistent_lsn point
> because the same is used to set replication origin progress. Later
> when we start the subscriber, the system will use that LSN as a
> start_decoding_at point which is the point after which all the commits
> will be replicated. So, we will end up incorrectly using restart_lsn
> (LSN from where we start reading the WAL) as start_decoding_at point.
> How could that be correct?

I didn't say we could use restart_lsn as consistent point of logical replication,
but I could agree the approach has issues.

> Now, even if we use restart_lsn as recovery_target_lsn and the LSN
> returned by pg_create_logical_replication_slot() as consistent LSN to
> set replication progress, that also could lead to data loss because
> the subscriber may never get data between restart_lsn value and
> consistent LSN value.

You considered the case, e.g., tuples were inserted just after the restart_lsn
but before the RUNNING_XACT record? In this case, yes, the streaming replication
finishes before replicating tuples but logical replication will skip them.
Euler's approach cannot be used as-is.

Best regards,
Hayato Kuroda
FUJITSU LIMITED