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Message-ID: <2c686eeeff7049e7500303248502aaebd876c649.camel@j-davis.com>
Subject: Re: Replication protocol pipelining edge case
From: Jeff Davis <pgsql@j-davis.com>
To: pgsql-hackers@postgresql.org
Date: Tue, 05 Jan 2021 15:23:34 -0800
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Precedence: bulk

On Tue, 2021-01-05 at 09:17 -0800, Jeff Davis wrote:
> The reason I ask is because it makes clients that do pipelining more
> complex. If the server always enters copy mode, then the client can
> send a CopyDone and then pipeline more requests behind it. But if the
> server may or may not enter copy mode, then the client needs to wait
> until it sees a CopyBothResponse before any new requests can be
> pipelined.

In an offline conversation with Andres, he pointed out that we can
never be sure that we'll enter COPY mode, because the START_REPLICATION
command could throw an error. Therefore we need to always wait for the
CopyBothResponse, and can't pipeline any new commands until we get it.

Regards,
	Jeff Davis