Re: pgsql: Add new replication mode synchronous_commit = 'remote_apply'.

On Tue, Mar 29, 2016 at 9:43 PM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> On Wed, Mar 30, 2016 at 10:39 AM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Tue, Mar 29, 2016 at 9:37 PM, Michael Paquier
>> <michael.paquier@gmail.com> wrote:
>>> On Wed, Mar 30, 2016 at 10:31 AM, Robert Haas <rhaas@postgresql.org> wrote:
>>>> Add new replication mode synchronous_commit = 'remote_apply'.
>>>>
>>>> In this mode, the master waits for the transaction to be applied on
>>>> the remote side, not just written to disk.  That means that you can
>>>> count on a transaction started on the standby to see all commits
>>>> previously acknowledged by the master.
>>>>
>>>> To make this work, the standby sends a reply after replaying each
>>>> commit record generated with synchronous_commit >= 'remote_apply'.
>>>> This introduces a small inefficiency: the extra replies will be sent
>>>> even by standbys that aren't the current synchronous standby.  But
>>>> previously-existing synchronous_commit levels make no attempt at all
>>>> to optimize which replies are sent based on what the primary cares
>>>> about, so this is no worse, and at least avoids any extra replies for
>>>> people not using the feature at all.
>>>>
>>>> Thomas Munro, reviewed by Michael Paquier and by me.  Some additional
>>>> tweaks by me.
>>>
>>> The commit message does not directly mention that the spec of
>>> walrcv_receive has been changed in a backward-incompatible way so as
>>> the wait control can be done with a latch directly in walreceiver.c
>>> and not in libpqwalreceiver.c. That's not worth a mention in the
>>> release notes as this is really low-level and compilation on any code
>>> using this hook would simply fail on 9.6, so I am just mentioning it
>>> for the sake of the archives.
>>
>> Yeah, I didn't really think that mattered much.  I'm not really sure
>> what you even mean by backward-incompatible -- AFAIK, that's a private
>> interface which we can whack around whenever we like.
>
> By "Backward-incompatible", I mean that any custom library using this
> walrcv hook is not going to compile anymore and we don't provide a
> pre-9.5 equivalent. I don't think that's worth worrying though, I have
> yet to see this interface being used for something else than
> libpqwalreceiver to be honest.

Yeah, I'd be very surprised if anyone did that.

--
Robert Haas
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