Re: Logical decoding on standby

From: Craig Ringer <craig(at)2ndquadrant(dot)com>
To: Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com>
Cc: Thom Brown <thom(at)linux(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Logical decoding on standby
Date: 2017-03-13 02:56:28
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On 7 March 2017 at 21:08, Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com> wrote:

> Patch 4 committed. Few others need rebase.

Since this patch series and initial data copy for logical replication
both add a facility for suppressing initial snapshot export on a
logical slot, I've dropped patch 0003 (make snapshot export on logical
slot creation) in favour of Petr's similar patch.

I will duplicate it in this patch series for ease of application. (The
version here is slightly extended over Petr's so I'll re-post the
modified version on the logical replication initial data copy thread

The main thing I want to direct attention to for Simon, as committer,
is the xlog'ing of VACUUM's xid threshold before we advance it and
start removing tuples. This is necessary for the standby to know
whether a given replication slot is safe to use and fail with conflict
with recovery if it is not, or if it becomes unsafe due to master
vacuum activity. Note that we can _not_ use the various vacuum records
for this because we don't know which are catalogs and which aren't;
we'd have to add a separate "is catalog" field to each vacuum xlog
record, which is undesirable. The downstream can't look up whether
it's a catalog or not because it doesn't have relcache/syscache access
during decoding.

This change might look a bit similar to the vac_truncate_clog change
in the txid_status patch, but it isn't really related. The txid_status
change is about knowing when we can safely look up xids in clog and
preventing a race with clog truncation. This change is about knowing
when we can know all catalog tuples for a given xid will still be in
the heap, not vacuumed away. Both are about making sure standbys know
more about the state of the system in a low-cost way, though.

WaitForMasterCatalogXminReservation(...) in logical.c is also worth
looking more closely at.

Craig Ringer
PostgreSQL Development, 24x7 Support, Training & Services

Attachment Content-Type Size
0001-Add-option-to-control-snapshot-export-to-CREATE_REPL.patch text/x-patch 15.7 KB
0002-Add-a-pg_recvlogical-wrapper-to-PostgresNode.patch text/x-patch 5.2 KB
0003-Follow-timeline-switches-in-logical-decoding.patch text/x-patch 20.9 KB
0004-Logical-decoding-on-standby.patch text/x-patch 100.5 KB

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