Horton

Re: Switching XLog source from archive to streaming when primary available

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, cary.huang@highgo.ca, pgsql-hackers@lists.postgresql.org, satyanarlapuram@gmail.com
Date: 2022-10-08T21:52:21Z
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. Add basic TAP tests for the low-level backup method, take two

  2. Add a failover option to subscriptions.

  3. Make all Perl warnings fatal

  4. Add PostgreSQL::Test::Cluster::advance_wal

  5. Rename wal_keep_segments to wal_keep_size.

On Mon, Sep 19, 2022 at 07:49:21PM +0530, Bharath Rupireddy wrote:
> SwitchFromArchiveToStreamEnabled() seemed better at this point. I'm
> attaching the v7 patch with that change. Please review it further.

As I mentioned upthread [0], I'm still a little concerned that this patch
will cause the state machine to go straight from archive recovery to
streaming replication, skipping recovery from pg_wal.  I wonder if this
could be resolved by moving the standby to the pg_wal phase instead.
Concretely, this line

+				if (switchSource)
+					break;

would instead change currentSource from XLOG_FROM_ARCHIVE to
XLOG_FROM_PG_WAL before the call to XLogFileReadAnyTLI().  I suspect the
behavior would be basically the same, but it would maintain the existing
ordering.

However, I do see the following note elsewhere in xlogrecovery.c:

 * The segment can be fetched via restore_command, or via walreceiver having
 * streamed the record, or it can already be present in pg_wal. Checking
 * pg_wal is mainly for crash recovery, but it will be polled in standby mode
 * too, in case someone copies a new segment directly to pg_wal. That is not
 * documented or recommended, though.

Given this information, the present behavior might not be too important,
but I don't see a point in changing it without good reason.

[0] https://postgr.es/m/20220906215704.GA2084086%40nathanxps13

-- 
Nathan Bossart
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