On 27 September 2016 at 22:24, Alan Conway <[email protected]> wrote: > On Tue, 2016-09-27 at 15:37 -0400, Alan Conway wrote: >> I want to clarify and document the meaning of these terms for our >> APIs, >> presently I can't find anywhere where they are documented clearly. >> >> The AMQP spec says: "Each peer has its own (independent) idle >> timeout. >> At connection open each peer communicates the maximum >> period between activity (frames) on the connection that it desires >> from >> its partner.The open frame carries the idletime-out >> field for this purpose. To avoid spurious timeouts, the value in >> idle- >> time-out SHOULD be half the peer’s >> actual timeout threshold." >> >> In other words: if I send you an "open" frame with idle-time-out=N >> that >> means *you* should not wait for longer than N milliseconds to send a >> frame to me. It does not mean *I* will close the connection after N >> milliseconds, I SHOULD be more patient and wait for N*2 ms to avoid >> closing prematurely due to minor timing wobbles. >> >> I think the choice of name is slightly ambiguous but the spec is >> clear >> on the semantics, so it's important to document it to remove the >> ambiguity. >> >> Anybody disagree? >> > > Sigh. Sadly proton-C interprets "idle-timeout" differently depending on > which end of the connection you are on: > > // as per the recommendation in the spec, advertise half our > // actual timeout to the remote > const pn_millis_t idle_timeout = transport->local_idle_timeout > ? (transport->local_idle_timeout/2) > : 0; > > So in proton, pn_set_idle_timeout does NOT mean set the AMQP idle- > timeout value, it means set the local "receive timeout" value and send > half that as the AMQP "send timeout" for the peer. > > I'm tempted to use a new term in the Go API: "heartbeat". To me that > clearly means the "send timeout" (hearts beat, they don't listen for > beats) so it coincides with the meaning of the AMQP "idle-timeout", but > without the ambiguity that is exacerbated by proton interpreting it > both ways. > >
Proton may seem to behave differently on each end, but I don't think its necessarily a bad thing that it does, and it is also I think largely just reflecting an annoying bit in the spec around this where different behaviours are allowed for, whereas it would be easier if it had less wiggle room. The transport setter/getter for the local timeout takes the 'actual timeout' and then sends half of it as the advertised value in the Open sent. This makes a certain amount of sense since it ensures that appropriate behaviour is actually satisfied, rather than expecting the user to ensure they only give half the value they really want for their actual timeout. The getter for the remote timeout value on the other hand returns the advertised value from the Open that is received. I expect it does that since it cant actually ever return the remotes 'actual timeout' without making an assumption, i.e that they did in fact advertise half (or less) of their actual timeout, which the spec only says that they SHOULD do. Yes the local setter taking the advertised value may have been better for method consistency with the remote getter. On the other hand, sending of necessary heartbeats is handled directly by the transport during the tick process, so users may not necessarily even use the getter themselves, and proton uses that remote value internally by pessimistically halfing it to account for the case that folks on the other end did not advertise half their actual timeout (since the spec doesnt require that they do). Side note: proton could arguably be less pessimistic here and go for say a percentage much nearer the full advertised value, but then you'd probably need to start guaging how close is too close. I think ensuring the doccumentation on the methods is clear what they do is sufficient enough here. I actually prefer idle-timeout as an name rather than heartbeat due to the way this all works. Since you only tell the other side [half] your timeout, you dont actually have direct control over when they send any needed empty frames to satisfy it (as the above shows, we might send them more often than they require) and 'heartbeat' might seem to imply that you do, and possibly even that they need be sent at that period all the time even despite regular traffic, which is not the case. Robbie --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
