Re: Sync Rep for 2011CF1

On Sat, Jan 22, 2011 at 8:31 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> 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.

Time's running short - do you have an updated patch?

--
Robert Haas
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