Re: Stopping logical replication protocol

From: Oleksandr Shulgin <oleksandr(dot)shulgin(at)zalando(dot)de>
To: Vladimir Gordiychuk <folyga(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Subject: Re: Stopping logical replication protocol
Date: 2016-05-06 16:45:31
Message-ID: CACACo5Q2nKimxm6JPMGBJOu2fpcPdaR6APptT9XyH1WvJyEJHA@mail.gmail.com
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On Fri, May 6, 2016 at 5:23 PM, Vladimir Gordiychuk <folyga(at)gmail(dot)com>
wrote:

> Hi all,
>
> During implementing logical replication protocol for pgjdbc
> https://github.com/pgjdbc/pgjdbc/pull/550 I faced with strange behavior
> of the *walcender.c*:
>
> 1. When WAL consumer catchup master and change his state to streaming,
> not available normally complete replication by send CopyDone message until
> will not generate/create new WAL record. It occurs because logical decoding
> located in *WalSndWaitForWal* until will not available next WAL
> record, and it method receive message from consumer even reply on CopyDone
> with CopyDone but ignore exit from loop and we can wait many times waiting
> CommandStatus & ReadyForQuery packages on consumer.
> 2. Logical decoding ignore message from consumer during decoding and
> writing transaction in socket(*WalSndWriteData*). It affect long
> transactions with many changes. Because for example if we start decoding
> transaction that insert 1 million records and after consume 1% of it date
> we
> decide stop replication, it will be not available until whole million
> record will not send to consumer.
>
> How exactly are you stopping the replication? If you just stop reading
you'll likely to hit some problems, but what if you also close the
connection? I don't think there is any other supported way to do it.

I was working last year on adding support for replication protocol to the
Python driver: https://github.com/psycopg/psycopg2/pull/322 It would be
nice if you could skim through this implementation, since it didn't receive
a whole lot of review.

Cheers.
--
Alex

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