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Re: New replication mode: write

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: New replication mode: write
Date: 2012-01-23 09:28:22
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Lists: pgsql-hackers
On Mon, Jan 23, 2012 at 9:02 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Mon, Jan 23, 2012 at 4:58 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> On Mon, Jan 16, 2012 at 12:45 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>>>>> Please add the Apply mode.
>>>> OK, will do.
>>> Done. Attached is the updated version of the patch.
>> I notice that the Apply mode isn't fully implemented. I had in mind
>> that you would add the latch required to respond more quickly when
>> only the Apply pointer has changed.
>> Is there a reason not to use WaitLatchOrSocket() in WALReceiver? Or
>> was there another reason for not implementing that?
> I agree that the feature you pointed is useful for the Apply mode. But
> I'm afraid that implementing that feature is not easy and would make
> the patch big and complicated, so I didn't implement the Apply mode first.
> To make the walreceiver call WaitLatchOrSocket(), we would need to
> merge it and libpq_select() into one function. But the former is the backend
> function and the latter is the frontend one. Now I have no good idea to
> merge them cleanly.

We can wait on the socket wherever it comes from. poll/select doesn't
care how we got the socket.

So we just need a common handler that calls either
walreceiver/libpqwalreceiver function as required to handle the

> If we send back the reply as soon as the Apply pointer is changed, I'm
> afraid quite lots of reply messages are sent frequently, which might
> cause performance problem. This is also one of the reasons why I didn't
> implement the quick-response feature. To address this problem, we might
> need to change the master so that it sends the Wait pointer to the standby,
> and change the standby so that it replies whenever the Apply pointer
> catches up with the Wait one. This can reduce the number of useless
> reply from the standby about the Apply pointer.

We send back one reply per incoming message. The incoming messages
don't know request state and checking that has a cost which I don't
think is an appropriate payment since we only need this info when the
link goes quiet.

When the link goes quiet we still need to send replies if we have
apply mode, but we only need to send apply messages if the lsn has
changed because of a commit. That will considerably reduce the
messages sent so I don't see a problem.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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