Re: logical replication empty transactions

Hi,

On 2020-03-06 13:53:02 +0800, Craig Ringer wrote:
> On Mon, 2 Mar 2020 at 19:26, Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> > One thing that is not clear to me is how will we advance restart_lsn
> > if we don't send any empty xact in a system where there are many such
> > xacts?
> 
> Same way we already do it for writes that are not replicated over
> logical replication, like vacuum work etc. The upstream sends feedback
> with reply-requested. The downstream replies. The upstream advances
> confirmed_flush_lsn, and that lazily updates restart_lsn.

It'll still delay it a bit.


> The bigger issue here is that if you don't send empty txns on logical
> replication you don't get an eager, timely response from the
> replica(s), which delays synchronous replication. You need to send
> empty txns when synchronous replication is enabled, or instead poke
> the walsender to force immediate feedback with reply requested.

Somewhat independent from the issue at hand: It'd be really good if we
could evolve the syncrep framework to support per-database waiting... It
shouldn't be that hard, and the current situation sucks quite a bit (and
yes, I'm to blame).

I'm not quite sure what you mean by "poke the walsender"? Kinda sounds
like sending a signal, but decoding happens inside after the walsender,
so there's no need for that. Do you just mean somehow requesting that
walsender sends a feedback message?

To address the volume we could:

1a) Introduce a pgoutput message type to indicate that the LSN has
  advanced, without needing separate BEGIN/COMMIT. Right now BEGIN is
  21 bytes, COMMIT is 26. But we really don't need that much here. A
  single message should do the trick.

1b) Add a LogicalOutputPluginWriterUpdateProgress parameter (and
  possibly rename) that indicates that we are intentionally "ignoring"
  WAL. For walsender that callback then could check if it could just
  forward the position of the client (if it was entirely caught up
  before), or if it should send a feedback request (if syncrep is
  enabled, or distance is big).

2) Reduce the rate of 'empty transaction'/feedback request messages. If
  we know that we're not going to be blocked waiting for more WAL, or
  blocked sending messages out to the network, we don't immediately need
  to send out the messages. Instead we could continue decoding until
  there's actual data, or until we're going to get blocked.

  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.

Greetings,

Andres Freund