> On Thu, May 21, 2015 at 4:10 PM, Ravi Krishna <sravikrishna3@xxxxxxxxx> > 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@xxxxxxxxx> >>> 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/ pg_rewind is used 9.3 onwards Saludos, Gilberto Castillo ETECSA, La Habana, Cuba
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