Subject: Re: Sync Rep for 2011CF1
From: Simon Riggs <simon@2ndQuadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Aidan Van Dyk <aidan@highrise.ca>, Tom Lane <tgl@sss.pgh.pa.us>,
	Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>,
	pgsql-hackers@postgresql.org
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Date: Sat, 22 Jan 2011 13:31:14 +0000
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On Fri, 2011-01-21 at 13:32 -0500, Robert Haas wrote:

> One idea might be to wait both before and after commit.  If
> allow_standalone_primary is off, and a commit is attempted, we check
> whether there's a slave connected, and if not, wait for one to
> connect.  Then, we write and sync the commit WAL record.  Next, we
> wait for the WAL to be ack'd.  Of course, the standby might disappear
> between the first check and the second, but it would greatly reduce
> the possibility of the master being ahead of the standby after a
> crash, which might be useful for some people.

I like this idea.

I think it would be too invasive to make a check before we insert each
WAL record, as Aidan suggests. Even if we did that, you aren't protected
when a standby goes down because you'll still have written half a
transaction and still be waiting.

So I propose that 

if (!allow_standalone_primary)
    ConfirmSyncRepAvailable();

before PreCommit_Notify(). That puts transaction into a wait state that
lasts until a sync rep standby is available. Note that it is before the
actual commit, so if we decide we need to we can cancel those
transactions and have them properly abort.

I won't add that code yet, in case better ideas emerge.

There is no support for preventing connections at startup, so I will
remove that completely, now.

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
 PostgreSQL Development, 24x7 Support, Training and Services