Re: Sync Rep v17

On Tue, Mar 1, 2011 at 3:39 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> PREPARE TRANSACTION and ROLLBACK PREPARED should wait for
>> replication as well as COMMIT PREPARED?
>
> PREPARE - Yes
> ROLLBACK - No
>
> Further discussion welcome

If we don't make ROLLBACK PREPARED wait for replication, we might need to
issue ROLLBACK PREPARED to new master again after failover, even if we've
already received a success indication of ROLLBACK PREPARED from old master.
This looks strange to me because, OTOH, in simple COMMIT/ROLLBACK case,
we don't need to issue that to new master again after failover.

>> What if fast shutdown is requested while RecordTransactionCommit
>> is waiting in SyncRepWaitForLSN? ISTM fast shutdown cannot complete
>> until replication has been successfully done (i.e., until at least one
>> synchronous standby has connected to the master especially if
>> allow_standalone_primary is disabled). Is this OK?
>
> A "behaviour" - important, though needs further discussion.

One of the scenarios which I'm concerned is:

1. The primary is running with allow_standalone_primary = on.
2. While some backends are waiting for replication, the user requests
fast shutdown.
3. Since the timeout expires, those backends stop waiting and return the success
indication to the client (but not replicated to the standby).
4. Since there is no backend waiting for replication, fast shutdown completes.
5. The clusterware like pacemaker detects the death of the primary and
triggers the
failover.
6. New primary doesn't have some transactions committed to the client, i.e.,
transaction lost happens!!

To avoid such a transaction lost, we should prevent the primary from
returning the
success indication to the client while fast shutdown is being executed, even if
allow_standalone_primary is enabled, I think. Thought?

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center