On Thu, Apr 13, 2017 at 01:17:07PM +0200, Christopher Faulet wrote: > The hello-handshake is done only once, when a new connection with a SPOA is > opened. But we can improve the SPOP by adding a new frame type to handle > admin tasks (like graceful stop). This way, for a specific connection, it > would be possible to wait for last ACK frames without sending new frames to > the SPOA. Then stopping the SPOA listeners to let the SPOP health check > failed should do the trick, I guess.
For this you need a "closing" state for your connections, where you know they are not eligible to new requests. That's pretty much similar to what we do by setting a server weight to zero in HTTP load balancing. But such transient states for connections can quickly become tricky to handle. The first thing is that you want to be sure that the replacement process is ready or the agent has reported a zero weight before starting to notify about the closing state, otherwise you'll end up with single- request connections that quickly pile up and can even take CPU if they're made over SSL. And it's very important not to conflate the agent state with the connection state and to avoid reporting the agent state inside existing connections. The problem when doing this precisely is during reloads where both old and new connections coexist and report conflicting information. That's why health checks and/or agent checks are much better to get the active SPOA health. For example the agent's health and willingness to accept new connections could be reported during the handshake. This way a health check can be developped for this and there's no risk that such information could be misreported later and cause trouble. Willy
