Re: Replication with 9.4

On Tue, Oct 6, 2015 at 12:27 PM, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com
> wrote:

> On Sun, Oct 4, 2015 at 11:47 PM, Michael Paquier
> <michael(dot)paquier(at)gmail(dot)com> wrote:
> > (Seems like you forgot to push the Reply-all button)
> >
> > On Sun, Oct 4, 2015 at 7:01 PM, Madovsky wrote:
> >> On 10/3/2015 3:30 PM, Michael Paquier wrote:
> >>> and no reason is given to justify *why* this would be needed in your
> case
> >> reason for a choice can be often an issue for other :D
> >>
> >> I thought that postgresql 9.4 user could change on the fly with
> >> synchronous_commit from local to on for ex
> >> which hotstandby would become in sync and which in async to avoid a big
> >> latency in case of let's say 100 hot standby.
> >> it was an idea, a concept to let the master write and update the nodes,
> like
> >> a queen bee ;)
> >> but I'm afraid it's not possible, so maybe future version of pg will do
> it,
> >> for now read from the master is my only solution.
> >
> > Well, Thomas Munro (adding him in CC) has sent for integration with
> > 9.6 a patch that would cover your need, by adding to
> > synchronous_commit a mode called 'apply', in which case a master would
> > wait for the transaction to be applied on standby before committing
> > locally:
> >
> http://www.postgresql.org/message-id/CAEepm=1fqkivL4V-OTPHwSgw4aF9HcoGiMrCW-yBtjipX9gsag@mail.gmail.com
> > Perhaps you could help with the review of the patch, this has stalled
> > a bit lately.
>
> That patch (or something more sophisticated long those lines) is a
> small piece of a bigger puzzle, though it might be enough if you only
> have one standby, are prepared to block until manual intervention if
> that standby fails, and don't mind potentially lumpy apply
> performance. See also the work being done to separate wal writing
> from wal applying for smoother performance[1], and handle multiple
> synchronous standbys[2]. But there is another piece of the puzzle
> IMHO: how to know reliably that the standby that you are talking to
> guarantees causal consistency, while also allowing standbys to
> fail/drop out gracefully, and I'm currently working on an idea for
> that.
>

FYI I posted the resulting proposal and patch over on the -hackers list.
Feedback, ideas, flames welcome as always.

http://www.postgresql.org/message-id/flat/CAEepm=0n_OxB2_pNntXND6aD85v5PvADeUY8eZjv9CBLk=zNXA(at)mail(dot)gmail(dot)com

--
Thomas Munro
http://www.enterprisedb.com