Sorry Bruno, I think I missed the end of your message with the numberOfAliveStreamThreads() proposal. I agree, that would be better than the alternatives I listed. That said:
> They rather suggest that the method returns a list of handles to the stream threads. I hadn't thought of that originally, but now that you mention it, this might be a good idea. I don't think we should return actual handles on the threads, but maybe a list of the thread names rather than a single number of currently alive threads. Since we seem to think it would be difficult if not impossible to keep track of the number of running stream threads, we should apply the same reasoning to the names and not assume the user can/will keep track of every thread returned by addStreamThread() or removeStreamThread(). Users should generally take any required action immediately after adding/removing the thread -- eg deregistering the thread metrics -- but it might still be useful to provide a convenience method listing all of the current threads And of course you could still get the number of threads easily by invoking size() on the returned list (or ordered set?). On Tue, Sep 8, 2020 at 12:16 PM Bruno Cadonna <br...@confluent.io> wrote: > Thank you again for the feedback Sophie! > > As I tried to point out in my previous e-mail, removing a stream thread > from a Kafka Streams client that does not have alive stream threads is > nothing exceptional for the client per se. However, it can become > exceptional within the context of the user. For example, if users want > to remove a stream thread from a client without alive stream threads > because one if their metrics say so, then this is exceptional in the > context of that user metric not in the context of the Kafka Streams > client. In that case, users should throw an exception and handle it. > > Regarding returning null, I do not like to return null because from a > development point of view there is no distinction between returning null > because we have a bug in the code or returning null because there are no > alive stream threads. Additionally, Optional<String> makes it more > explicit that the result could also be empty. > > Thank you for the alternative method names! However, with the names you > propose it is not immediately clear that the method returns an amount of > stream threads. They rather suggest that the method returns a list of > handles to the stream threads. I chose to use "aliveStreamThreads" to be > consistent with the client-level metric "alive-stream-threads" which > reports the same number of stream threads that > numberOfAliveStreamThreads() should report. If others also think that > the proposed name in the KIP is too clumsy, I am open to rename it, though. > > Best, > Bruno > > > On 08.09.20 20:12, Sophie Blee-Goldman wrote: > >> it's never a good sign when the discussion moves into the vote thread > > > > Hah, sorry, the gmail consolidation of [VOTE] and [DISCUSS] threads > strikes > > again. > > Thanks for redirecting me Bruno > > > > I suppose it's unfair to expect the callers to keep perfect track of the > > current > > number of stream threads, but it also seems like you shouldn't be > calling > > removeStreamThread() when there are no threads left. Either you're just > > haphazardly removing threads and could unintentionally slip into a state > of > > no > > running threads without realizing it, or more realistically, you're > > carefully > > removing threads based on some metric(s) that convey whether the system > is > > over or under-provisioned. If your metrics say you're over-provisioned > but > > there's > > not one thread running, well, that certainly sounds exceptional to me. Or > > you might > > be right in that the cluster is over-provisioned but have just been > > directing the > > removeStreamThread() and addStreamThread() calls to instances at random, > and > > end up with one massive instance and one with no threads at all. Again, > > this > > probably merits some human intervention (or system redesign) > > > > That said, I don't think there's any real harm to just returning null in > > this case, but I hope > > that users would pay attention to this since it seems likely to indicate > > something has gone > > seriously wrong. I suppose Optional<String> would be a reasonable > > compromise. > > > > As for the method name, what about activeStreamThreads() or > > liveStreamThreads() ? > > > > On Mon, Sep 7, 2020 at 1:45 AM Bruno Cadonna <br...@confluent.io> wrote: > > > >> Hi John, > >> > >> I agree with you except for checking null. I would rather prefer to use > >> Optional<String> as the return type to both methods. > >> > >> I changed the subject from [VOTE] to [DISCUSS] so that we can follow up > >> in the discussion thread. > >> > >> Best, > >> Bruno > >> > >> On 04.09.20 23:12, John Roesler wrote: > >>> Hi Sophie, > >>> > >>> Uh, oh, it's never a good sign when the discussion moves > >>> into the vote thread :) > >>> > >>> I agree with you, it seems like a good touch for > >>> removeStreamThread() to return the name of the thread that > >>> got removed, rather than a boolean flag. Maybe the return > >>> value would be `null` if there is no thread to remove. > >>> > >>> If we go that way, I'd suggest that addStreamThread() also > >>> return the name of the newly created thread, or null if no > >>> thread can be created right now. > >>> > >>> I'm not completely sure if I think that callers of this > >>> method would know exactly how many threads there are. Sure, > >>> if a human being is sitting there looking at the metrics or > >>> logs and decides to call the method, it would work out, but > >>> I'd expect this kind of method to find its way into > >>> automated tooling that reacts to things like current system > >>> load or resource saturation. Those kinds of toolchains often > >>> are part of a distributed system, and it's probably not that > >>> easy to guarantee that the thread count they observe is > >>> fully consistent with the number of threads that are > >>> actually running. Therefore, an in-situ `int > >>> numStreamThreads()` method might not be a bad idea. Then > >>> again, it seems sort of optional. A caller can catch an > >>> exception or react to a `null` return value just the same > >>> either way. Having both add/remove methods behave similarly > >>> is probably more valuable. > >>> > >>> Thanks, > >>> -John > >>> > >>> > >>> On Thu, 2020-09-03 at 12:15 -0700, Sophie Blee-Goldman > >>> wrote: > >>>> Hey, sorry for the late reply, I just have one minor suggestion. Since > >> we > >>>> don't > >>>> make any guarantees about which thread gets removed or allow the user > to > >>>> specify, I think we should return either the index or full name of the > >>>> thread > >>>> that does get removed by removeThread(). > >>>> > >>>> I know you just updated the KIP to return true/false if there > >> are/aren't any > >>>> threads to be removed, but I think this would be more appropriate as > an > >>>> exception than as a return type. I think it's reasonable to expect > >> users to > >>>> have some sense to how many threads are remaining, and not try to > remove > >>>> a thread when there is none left. To me, that indicates something > wrong > >>>> with the user application code and should be treated as an exceptional > >> case. > >>>> I don't think the same code clarify argument applies here as to the > >>>> addStreamThread() case, as there's no reason for an application to be > >>>> looping and retrying removeStreamThread() since if that fails, it's > >> because > >>>> there are no threads left and thus it will continue to always fail. > And > >> if > >>>> the > >>>> user actually wants to shut down all threads, they should just close > the > >>>> whole application rather than call removeStreamThread() in a loop. > >>>> > >>>> While I generally think it should be straightforward for users to > track > >> how > >>>> many stream threads they have running, maybe it would be nice to add > >>>> a small utility method that does this for them. Something like > >>>> > >>>> // Returns the number of currently alive threads > >>>> boolean runningStreamThreads(); > >>>> > >>>> On Thu, Sep 3, 2020 at 7:41 AM Matthias J. Sax <mj...@apache.org> > >> wrote: > >>>> > >>>>> +1 (binding) > >>>>> > >>>>> On 9/3/20 6:16 AM, Bruno Cadonna wrote: > >>>>>> Hi, > >>>>>> > >>>>>> I would like to start the voting on KIP-663 that proposes to add > >> methods > >>>>>> to the Kafka Streams client to add and remove stream threads during > >>>>>> execution. > >>>>>> > >>>>>> > >>>>> > >> > https://cwiki.apache.org/confluence/display/KAFKA/KIP-663%3A+API+to+Start+and+Shut+Down+Stream+Threads > >>>>>> > >>>>>> Best, > >>>>>> Bruno > >>> > >> > > >
