Re: Replication with 9.4

On 10/3/2015 4:48 AM, Michael Paquier wrote:
> On Sat, Oct 3, 2015 at 8:09 PM, Madovsky <infos(at)madovsky(dot)org> wrote:
>> I would like to fix a issue I'm facing of with the version 9.4 streaming
>> replication.
>> is it possible to set on the fly the synchronous commit on the master (or
>> standby?)
>> which only sync commit the hot standby node used by the client who has a
>> read only sql session on?
> By referring to the docs:
> http://www.postgresql.org/docs/devel/static/warm-standby.html#SYNCHRONOUS-REPLICATION
> Synchronous replication gives the insurance that a transaction has
> been flushed to the disk of the standby which is in sync, aka the one
> with the lowest priority depending on the nodes currently connected.
> This does not ensure that the transaction has been *replayed* on the
> standby. You are sure that the transaction data is available. Hence if
> you wish to know that a transaction in a standby is running a
> transaction with enough data replayed, you should make the WAL
> position of the master necessary for the transaction of the standby
> something that your application is aware of.

I really well understood Michael thanks,
the docs doesn't cover if the sync priorities can be changed
so one node can be considered fully sync and the other only async
thus to minimize sync request overhead...
usually a client connect to a node would like to see the results
on the node where he has a session on.
I just wanted to avoid a SELECT request to the master and
stay on the HOT STANDBY for all read requests.
my script open 2 session, on on the master and one on the hot standby
in case of block transactions.