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.
--
Michael
