Re: Postgres Synchronous replication

On Thu, May 21, 2015 at 4:10 PM, Ravi Krishna <sravikrishna3(at)gmail(dot)com>
wrote:

>
> AFAIK the commit on master happens only after it receives ack from the
>> slave. This is how synchronous replication ensures that the slave is'in
>> sync'.
>>
>
>
> If that is the case , then why does PG find it impossible to sync back
> with the primary after a crash.
> Other products offering similar technology do not have this issue.
>
> In my opinion this is quite a serious limitation with PG replication.
> Every time the primary crashes and the business continues with the
> promotion of standby as the new primary, the crashed server has to be
> reinitialized for the set up of the replication.
>
>
>
>>
>> On Thu, May 21, 2015 at 3:56 PM, Ravi Krishna <sravikrishna3(at)gmail(dot)com>
>> wrote:
>>
>>> I want to understand how PG sync replication works. This is what I know
>>> (assuming two node sync replication)
>>>
>>> 1 - Application issues commit.
>>> 2 - PG commits the transaction locally on the primary server.
>>> 3 - At this stage the application has not got the commit indication back.
>>> 4 - PG transmits the transaction from the local to the remote server.
>>> 5 - Remote server sends back acknowledgement
>>> 6 - The app gets commit ack back.
>>>
>>> So this means, between step 2 and step 6, the app is not aware that the
>>> transaction has already been committed.
>>> This is the reason why, in the event of server crashing between step 2
>>> and step 6, and the remote takes over as the
>>> new primary, the crashed server can not restart as standby and the only
>>> option is to recreate the db from the remote
>>> server (which is now acting as the primary).
>>>
>>> Am I correct in the understanding?
>>>
>>> One more question: In Step 5, does the remote harden the transaction on
>>> the disk, or merely receives the transaction in the log buffer and it sends
>>> back ACK to the local server.
>>>
>>> Thanks
>>>
>>
>>
>

This issue has been address with pg_rewind in the upcoming 9.5 major
version release

http://hlinnaka.iki.fi/2015/03/23/pg_rewind-in-postgresql-9-5/