Re: Inconsistent DB data in Streaming Replication

On Wed, Apr 17, 2013 at 7:49 PM, Florian Pflug <fgp(at)phlo(dot)org> wrote:
> On Apr17, 2013, at 12:22 , Amit Kapila <amit(dot)kapila(at)huawei(dot)com> wrote:
>> Do you mean to say that as an error has occurred, so it would not be able to
>> flush received WAL, which could result in loss of WAL?
>> I think even if error occurs, it will call flush in WalRcvDie(), before
>> terminating WALReceiver.
>
> Hm, true, but for that to prevent the problem the inner processing
> loop needs to always read up to EOF before it exits and we attempt
> to send a reply. Which I don't think it necessarily does. Assume,
> that the master sends a chunk of data, waits a bit, and finally
> sends the shutdown record and exits. The slave might then receive
> the first chunk, and it might trigger sending a reply. At the time
> the reply is sent, the master has already sent the shutdown record
> and closed the connection, and we'll thus fail to reply and abort.
> Since the shutdown record has never been read from the socket,
> XLogWalRcvFlush won't flush it, and the slave ends up behind the
> master.
>
> Also, since XLogWalRcvProcessMsg responds to keep-alives messages,
> we might also error out of the inner processing loop if the server
> closes the socket after sending a keepalive but before we attempt
> to respond.
>
> Fixing this on the receive side alone seems quite messy and fragile.
> So instead, I think we should let the master send a shutdown message
> after it has sent everything it wants to send, and wait for the client
> to acknowledge it before shutting down the socket.

Agreed. I've tried to fix this problem on only the walreceiver side, but
that failed. I agree that we should change walsender so that it waits
for the replay from the standby before closing the connection.

Regards,

--
Fujii Masao