Thanks for your email John. I agree that it seems to be an anti-pattern to write code that makes assumptions if poll() returns data or not. Thus, we should fix-forward the system test from my point of view.
>From my understanding, the impact of KIP-695 is that we might return early from poll() (ie, before the timeout passed) with no data, only if an empty fetch request comes back and there is no other fetch request that did return data. Thus, for most cases, poll() should still return early and provide data. -- Thus, I have no concerns with the slight behavior change. Would be good to get input from others about this question though. -Matthias On 2/3/21 10:06 AM, John Roesler wrote: > Hello again all, > > I'm resurrecting this thread to discuss an issue that has > come up after merging the code for this KIP. > > The issue is that some of the system tests need to be > updated in the same way that this integration test needed to > be updated: > https://github.com/apache/kafka/pull/9836/files#diff-735dcc2179315ebd78a7c75fd21b70b0ae81b90f3d5ec761740bc80abeae891fR1875-R1888 > > This issue was reported here: > https://issues.apache.org/jira/browse/KAFKA-12268 > and there is some preliminary discussion here: > https://github.com/apache/kafka/pull/10022 > > First, let me offer my apologies for failing to catch this > before the merge. I'm sorry that it became Rajini's work to > track down the cause of the failure, when it was my > responsibility to ensure the feature was merged safely. > > To recap the situation: > Consumer#poll(Duration) will now return before the duration > expires even if there are no records returned if there is > some returned metadata. > > This behavior was important for KIP-695. In the situation > where we get no records back for some partition, Streams > needs to have the freshest possible information about > whether there are no new records on the broker, or whether > there are records on the broker that we still need to fetch. > If that's not clear, the KIP contains the full story. > > It's definitely a behavior change, but our rationale was > that it's an acceptable behavior change. Our big alternative > is to add a _new_ method to Consumer to > pollForRecordsOrMetadata(Duration) or something. > > It seems unreliable to expect the broker to return a > particular record within a particular timeout in general, > which is what these tests are doing. The broker can decide > for several reasons not to return data for a partition, but > return data for another partition instead. > > It seems like the only case where you might reasonably try > to rely on that is in a test, where you first write a record > to a partition, then you assign only that one partition to a > consumer, then you poll on the consumer, expecting it to > return the data you just wrote. > > So the $10 question here is whether we should support this > apparently artificial (testing-only) use case to the point > where it's worth adding a whole new method to the Consumer > interface. > > Thanks all, > John > > On Thu, 2020-12-17 at 13:18 -0600, John Roesler wrote: >> Thanks Jason, >> >> We would only return the metadata for the latest fetches. >> So, if someone wanted to use this to lazily maintain a >> client-side metadata map for all partitions, they'd have to >> store it separately and merge in new updates as they arrive. >> >> This way: >> 1. We don't need to increase the complexity of the client by >> storing that metadata >> 2. Users will be able to treat all returned metadata as >> "fresh" without having to reason about the timestamps. >> 3. All parts of the returned ConsumerRecords object have the >> same lifecycle: all the data and metadata are the results of >> the most recent round of fetch responses that had not been >> previously polled. >> >> Does that seem sensible to you? I'll update the KIP to >> clarify this. >> >> Thanks, >> -John >> >> On Wed, 2020-12-16 at 10:29 -0800, Jason Gustafson wrote: >>> Hi John, >>> >>> Just one question. It wasn't very clear to me exactly when the metadata >>> would be returned in `ConsumerRecords`. Would we /always/ include the >>> metadata for all partitions that are assigned, or would it be based on the >>> latest fetches? >>> >>> Thanks, >>> Jason >>> >>> On Fri, Dec 11, 2020 at 4:07 PM John Roesler <vvcep...@apache.org> wrote: >>> >>>> Thanks, Guozhang! >>>> >>>> All of your feedback sounds good to me. I’ll update the KIP when I am able. >>>> >>>> 3) I believe it is the position after the fetch, but I will confirm. I >>>> think omitting position may render beginning and end offsets useless as >>>> well, which leaves only lag. That would be fine with me, but it also seems >>>> nice to supply this extra metadata since it is well defined and probably >>>> handy for others. Therefore, I’d go the route of specifying the exact >>>> semantics and keeping it. >>>> >>>> Thanks for the review, >>>> John >>>> >>>> On Fri, Dec 11, 2020, at 17:36, Guozhang Wang wrote: >>>>> Hello John, >>>>> >>>>> Thanks for the updates! I've made a pass on the KIP and also the POC PR, >>>>> here are some minor comments: >>>>> >>>>> 1) nit: "receivedTimestamp" -> it seems the metadata keep getting >>>> updated, >>>>> and we do not create a new object but just update the values in-place, so >>>>> maybe calling it `lastUpdateTimstamp` is better? >>>>> >>>>> 2) It will be great to verify in javadocs that the new API >>>>> "ConsumerRecords#metadata(): Map<TopicPartition, Metadata>" may return a >>>>> superset of TopicPartitions than the existing API that returns the data >>>> by >>>>> partitions, in case users assume their map key-entries would always be >>>> the >>>>> same. >>>>> >>>>> 3) The "position()" API of the call needs better clarification: is it the >>>>> current position AFTER the records are returned, or is it BEFORE the >>>>> records are returned? Personally I'd suggest we do not include it if it >>>> is >>>>> not used anywhere yet just to avoid possible misuage, but I'm fine if you >>>>> like to keep it still; in that case just clarify its semantics. >>>>> >>>>> >>>>> Other than that，I'm +1 on the KIP as well ! >>>>> >>>>> >>>>> Guozhang >>>>> >>>>> >>>>> On Fri, Dec 11, 2020 at 8:15 AM Walker Carlson <wcarl...@confluent.io> >>>>> wrote: >>>>> >>>>>> Thanks for the KIP! >>>>>> >>>>>> +1 (non-binding) >>>>>> >>>>>> walker >>>>>> >>>>>> On Wed, Dec 9, 2020 at 11:40 AM Bruno Cadonna <br...@confluent.io> >>>> wrote: >>>>>> >>>>>>> Thanks for the KIP, John! >>>>>>> >>>>>>> +1 (non-binding) >>>>>>> >>>>>>> Best, >>>>>>> Bruno >>>>>>> >>>>>>> On 08.12.20 18:03, John Roesler wrote: >>>>>>>> Hello all, >>>>>>>> >>>>>>>> There hasn't been much discussion on KIP-695 so far, so I'd >>>>>>>> like to go ahead and call for a vote. >>>>>>>> >>>>>>>> As a reminder, the purpose of KIP-695 to improve on the >>>>>>>> "task idling" feature we introduced in KIP-353. This KIP >>>>>>>> will allow Streams to offer deterministic time semantics in >>>>>>>> join-type topologies. For example, it makes sure that >>>>>>>> when you join two topics, that we collate the topics by >>>>>>>> timestamp. That was always the intent with task idling (KIP- >>>>>>>> 353), but it turns out the previous mechanism couldn't >>>>>>>> provide the desired semantics. >>>>>>>> >>>>>>>> The details are here: >>>>>>>> https://cwiki.apache.org/confluence/x/JSXZCQ >>>>>>>> >>>>>>>> Thanks, >>>>>>>> -John >>>>>>>> >>>>>>> >>>>>> >>>>> >>>>> >>>>> -- >>>>> -- Guozhang >>>>> >>>> >> >> > >
