Re: Proposal: "Causal reads" mode for load balancing reads without stale data

On Thu, Mar 24, 2016 at 11:20 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Mar 24, 2016 at 2:11 AM, Michael Paquier
> <michael.paquier@gmail.com> wrote:
>> -   SyncRepWaitForLSN(gxact->prepare_end_lsn);
>> +   SyncRepWaitForLSN(gxact->prepare_end_lsn, false);
>> Isn't it important to ensure that a PREPARE LSN is applied as well on
>> the standby with remote_apply? Say if an application prepares a
>> transaction, it would commit locally but its LSN may not be applied on
>> the standby with this patch. That would be a surprising behavior for
>> the user.
>
> You need to wait for COMMIT PREPARED, but I don't see why you need to
> wait for PREPARE itself.

Multi-master conflict resolution. Knowing in-time that a prepared
transaction has been applied is useful on another node for lock
conflicts resolution. Say a PRIMARY KEY insert is prepared, and we
want to know at application level that its prepared state is visible
everywhere.
-- 
Michael