Horton

Re: logical replication empty transactions

Ajin Cherian <itsajin@gmail.com>

View thread View on postgr.es
From: Ajin Cherian <itsajin@gmail.com>
To: Peter Smith <smithpb2250@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>, Rahila Syed <rahila.syed@2ndquadrant.com>, Euler Taveira <euler@timbira.com.br>, Amit Kapila <amit.kapila16@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Jeff Janes <jeff.janes@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, Craig Ringer <craig@2ndquadrant.com>
Date: 2021-04-23T05:57:34Z
Lists: pgsql-hackers 
An earlier comment from Anders:
> We could e.g. have a new LogicalDecodingContext callback that is
> called whenever WalSndWaitForWal() would wait. That'd check if there's
> a pending "need" to send out a 'empty transaction'/feedback request
> message. The "need" flag would get cleared whenever we send out data
> bearing an LSN for other reasons.
>

I think the current Keep Alive messages already achieve this by
sending the current LSN as part of the Keep Alive messages.
    /* construct the message... */
    resetStringInfo(&output_message);
    pq_sendbyte(&output_message, 'k');
    pq_sendint64(&output_message, sentPtr); <=== Last sent WAL LSN
    pq_sendint64(&output_message, GetCurrentTimestamp());
    pq_sendbyte(&output_message, requestReply ? 1 : 0);

I'm not sure if anything more is required to keep empty transactions
updated as part of synchronous replicas. If my understanding on this
is not correct, let me know.

regards,
Ajin Cherian
Fujitsu Australia

Commits

  1. Skip empty transactions for logical replication.

  2. Add decoding of sequences to built-in replication

  3. Allow specifying row filters for logical replication of tables.