Horton

Re: Skipping logical replication transactions on subscriber side

Peter Eisentraut <peter.eisentraut@enterprisedb.com>

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From: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2021-06-01T15:35:44Z
Lists: pgsql-hackers 
On 01.06.21 06:01, Amit Kapila wrote:
> But, won't that be costly in cases where we have errors in the
> processing of very large transactions? Subscription has to process all
> the data before it gets an error. I think we can even imagine this
> feature to be extended to use commitLSN as a skip candidate in which
> case we can even avoid getting the data of that transaction from the
> publisher. So if this information is persistent, the user can even set
> the skip identifier after the restart before the publisher can send
> all the data.

At least in current practice, skipping parts of the logical replication 
stream on the subscriber is a rare, emergency-level operation when 
something that shouldn't have happened happened.  So it doesn't really 
matter how costly it is.  It's not going to be more costly than the 
error happening in the first place.  All you'd need is one shared memory 
slot per subscription to store a xid to skip.

We will also want some proper conflict handling at some point.  But I 
think what is being discussed here is meant to be a repair tool, not a 
policy tool, and I'm afraid it might get over-engineered.

Commits

  1. Test ALIGNOF_DOUBLE==4 compatibility under ALIGNOF_DOUBLE==8.

  2. Reorder subskiplsn in pg_subscription to avoid alignment issues.

  3. Add ALTER SUBSCRIPTION ... SKIP.

  4. Optionally disable subscriptions on error.

  5. Update docs of logical replication for commit 8d74fc96db.

  6. Respect permissions within logical replication.

  7. Fix regression test failure caused by commit 8d74fc96db.

  8. Add a view to show the stats of subscription workers.

  9. Add logical change details to logical replication worker errcontext.

  10. Rename LOGICAL_REP_MSG_STREAM_END to LOGICAL_REP_MSG_STREAM_STOP.

  11. Fix typo in protocol.sgml.

  12. Remove unused argument in apply_handle_commit_internal().

  13. Fix replication of in-progress transactions in tablesync worker.

  14. Reorder pg_sequence columns to avoid alignment issue