Re: Synchronization levels in SR

On Tue, Sep 7, 2010 at 9:45 AM, Markus Wanner <markus(at)bluegap(dot)ch> wrote:
> On 09/07/2010 02:16 PM, Robert Haas wrote:
>>
>> Right, definitely.  The trouble is that if they happen concurrently,
>> and there's a crash, you have to be prepared for the possibility that
>> either one of the two has completed and the other is not.
>
> Understood.
>
>> In
>> practice, this means that the master and standby need to compare notes
>> on the ending WAL location and whichever one is further advanced needs
>> to stream the intervening records to the other.
>
> Not necessarily, no. Remember that the client didn't get a commit
> confirmation. So reverting might also be a correct solution (i.e. not
> violating the durability constraint).

In theory, that's true, but if we do that, then there's an even bigger
problem: the slave might have replayed WAL ahead of the master
location; therefore the slave is now corrupt and a new base backup
must be taken.

>> This would be an
>> awesome feature, but it's hard, so for a first version, it makes sense
>> to commit on the master first and then on the standby after the master
>> is known done.
>
> The obvious downside of that is that latency adds up, instead of just being
> the max of the two operations. And that for normal operation. While at best
> it saves an un-confirmed transaction in the failure case.
>
> It might be harder to implement, yes.

Yeah, I hope we'll get there eventually.

--
Robert Haas
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