[pgpool-general: 3920] Re: "replication" mode inconsistencies

[Chris Pacejo]

Thank you.  I've confirmed that if only *one* of the two servers is
unreachable, pgpool behaves as expected (waits for the server to be
manually reattached).

Although I wonder also, even if pgpool *did* correctly refuse to send
traffic if both servers were "down" in pgpool_status on restart, how
should we know in which direction to recover data (from A to B or B to
A)?  Pgpool does not record in pgpool_status which "down" server was
the last to go down (and is thus authoritative).  As a workaround I
think it would work to write a failover/failback_command which records
this information.

On Wed, Aug 5, 2015 at 6:59 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:
> Pgpool should recognize that both A and B are in down status, but
> actually not. Let me investigate...
>
> Best regards,
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese:http://www.sraoss.co.jp
>
>> Consider the following sequence, starting from a healthy system of two
>> PG servers (A and B) joined in "replication" mode:
>>
>> 1) Server A loses connectivity.
>> 2) A write comes in, which pgpool commits to server B.
>> 3) Server B loses connectivity.
>> 4) Server A regains connectivity.
>> 5) pgpool restarts (due to either sysadmin action or failure).
>>
>> At this point, pgpool happily directs all traffic to server A, which
>> does *not* have the most recent commit to server B.  This is very bad
>> since I have now lost data consistency.
>>
>> Rather, I would expect that pgpool remembers that it has written data
>> to B but not to A, and would refuse incoming connections until A has
>> been recovered from B.
>>
>> Even to workaround, if before restarting pgpool, I had some tool which
>> checked the state in which pgpool left the two servers and then
>> rectified them, that would suffice.  However since pgpool doesn't seem
>> to track at all the fact that it had written some data only to B but
>> not to A, that information is not available (e.g. from pgpool_status).
>>
>> What am I missing?  How is it that others use pgpool in "replication"
>> mode without encountering data inconsistencies when nodes fail?
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