keep-alive to backend + non-idempotent requests = race condition?

Emiel Mols emiel.mols at
Tue Sep 6 14:08:22 UTC 2016


On Thu, Aug 25, 2016 at 3:44 PM Emiel Mols <emiel.mols at> wrote:

> Hey,
> I've been haunted by this for quite some time, seen it in different
> deployments, and think might make for some good ol' mailing list discussion.
> When
> - using keep-alive connections to a backend service (eg php, rails, python)
> - this backend needs to be updatable (it is not okay to have lingering
> workers for hours or days)
> - requests are often not idem-potent (can't repeat them)
> current deployments need to close the kept-alive connection from the
> backend-side, always opening up a race condition where nginx has just sent
> a request and the connection gets closed. This leaves nginx in limbo not
> knowing if the request has been executed and can be repeated.
> When using keep-alive connections the only reliable way of closing them is
> from the client-side (in this case: nginx). I would therefor expect either
> - a feature to signal nginx to close all connections to the backend after
> having deployed new backend code.
> - an upstream keepAliveIdleTimeout config value that guarantees that
> kept-alive connections are not left lingering indefinitely long. If nginx
> guarantees it closes idle connections after 5 seconds, we can be sure that
> 5s+max_request_time after a new backend is deployed all old workers are
> gone.
> - (variant on the previous) support for a http header from the backend to
> indicate such a timeout value. It's funny that this header kind-of already
> exists in the spec <
> >, but in practice is implemented by no-one.
> The 2nd and/or 3rd options seem most elegant to me. I wouldn't mind
> implementing myself if someone versed in the architecture would give some
> pointers.
> Best regards,
> - Emiel
> BTW: a similar issue should exist between browsers and web servers. Since
> latency is a lot higher on these links, I can only assume it to happen a
> lot.
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