Hi Guozhang, Your understanding of the rationale is accurate, and what you suggest is completely plausible, however I have a slightly different take on the situation.
When the KIP was originally drafted, making KafkaProducer#send asynchronous was one element of the changes (this is a little more general than (a), but has similar implications). As you're aware, doing so would allow new topics to aggregate since the producer could continue to push new records, whereas today the producer thread is blocked waiting for resolution. However, there were concerns about changing client behavior unexpectedly in this manner, and the change isn't as trivial as one would hope. For example, we'd have to introduce an intermediate queue of records for topics without metadata, and have that play well with the buffer pool which ensures the memory limit isn't exceeded. A side effect is that a producer could hit 'memory full' conditions easier, which could have unintended consequences if, say, the model was setup such that different producer threads produced to a disjoint set of topics. Where one producer thread was blocked waiting for new metadata, it could now push enough data to block all producer threads due to memory limits, so we'd need to be careful here. For case (a) described, another concern would be adding additional a new source of latency (possibly seconds) for new topics. Not a huge issue, but it is new behavior to existing clients and adds to the complexity of verifying no major regressions. It also wouldn't resolve all cases we're interested in. One behavior we're witnessing is the following: a producer generates to a very large number of topics (several thousand), however the period of consecutive records for a topic can often be beyond the current hard-coded expiry of 5 minutes. Therefore, when the producer does submit a request for this topic after 5 minutes, it has to request all of the topic metadata again. While batching new topics in the start-up case would definitely help, here it would likely be lost effort. Being able to increase the metadata eviction for the above case would improve things, but there's no perfect value when consecutive produce times can be modelled as a probability distribution. Rather, the better solution could be what we discussed earlier, where we'd lower the eviction timeout, but make the cost of fetching uncached topic metadata much smaller. The changes in the KIP were meant to improve the general case without affecting external client behavior, and then the plan was to fix the asynchronous send in the next iteration, if necessary. Point (b) is along the lines of the latest revision: only send requests for uncached topics, if they exist, otherwise request the full working set. Piggy-backing was originally included, but removed because its utility was in doubt. So to summarize, you're correct in that asynchronous topic fetching would be a big improvement, and should be an item of future work. However what this KIP proposes should be the safest/easiest set of changes to resolve the current pain points. Please let me know if you agree/disagree with this assessment. Thanks, Brian On Mon, Jan 20, 2020 at 10:52 AM Guozhang Wang <wangg...@gmail.com> wrote: > Hello Brian, > > I looked at the new proposal again, and I'd like to reason about its > rationale from the listed motivations in your wiki: > > 1) more RPCs: we may send metadata requests more frequently than > appropriate. This is especially the case during producer start-up, where > the more topics it needs to send to, the more metadata requests it needs to > send. This the original reported issue as in KAFKA-8904. > > 2) large RPCs: we including all topics in the work set when sending > metadata request. But I think our conjecture (as Colin has pointed out) is > that this alone is fine most of the time, assuming e.g. you are sending > such large RPC only once every 10 minutes. It is only because of 1) where > you are sending large RPC too frequently which is a common issue. > > 3) we want to have a configurable eviction period than hard-coded values. I > consider it as a semi-orthogonal motivation compared with 2) / 3) but we > wanted to piggy-back this fix along with the KIP. > > So from there, 1) and 2) does not contradict to each other since our belief > is that large RPCs is usually okay as long as it is not large-and-frequent > RPCs, and we actually prefer large-infrequent RPC > smaller-frequent RPC > > large-and-frequent RPC (of course). > > The current proposal basically tries to un-tangle 2) from 1), i.e. for the > scenario of KAFKA-8904 it would result in smaller-frequent RPC during > startup than large-and-frequent RPC. But I'm wondering why don't we just do > even better and make it large-infrequent RPC? More specifically, just like > Lucas suggested in the ticket: > > a. when there's new topic with unknown metadata enqueued, instead of > requesting a metadata immediately just delay it for a short period (no more > than seconds) hoping that more unknown topics would be requested in the > period; during this period we would not know which partition it would go to > of course, so we buffer it in a different manner. > > b. when we are about to send metadata, if there are unknown topic(s) -- > consider them "urgent topics" -- just send them without other topics; > otherwise, send the work set in the request. If we want to go even fancier, > we can still piggy-back some non-urgent along with urgent ones but it is > more complicated to reason about the trade-off so a simpler approach is > fine too. > > c. fixing 3) with a new config, which is relatively orthogonal to a) and > b). > > > > Guozhang > > > > > On Tue, Jan 14, 2020 at 10:39 AM Brian Byrne <bby...@confluent.io> wrote: > > > Hello all, > > > > After further offline discussion, I've removed any efforts to control > > metadata RPC sizes. There are now only two items proposed in this KIP: > > > > (1) When encountering a new topic, only issue a metadata request for that > > particular topic. For all other cases, continue as it does today with a > > full working set refresh. > > > > (2) Introduces client configuration flag "metadata.eviction.period.ms" > to > > control cache eviction duration. I've reset the default back to the > current > > (hard-coded) value of 5 minutes since we can identify cases where > changing > > it would cause surprises. > > > > The votes have been cleared. My apologies for continually interrupting > and > > making changes to the KIP, but hopefully this is an agreeable minimum > > solution to move forward. > > > > Thanks, > > Brian > > > > On Mon, Jan 6, 2020 at 5:23 PM Colin McCabe <cmcc...@apache.org> wrote: > > > > > On Mon, Jan 6, 2020, at 14:40, Brian Byrne wrote: > > > > So the performance of a metadata RPC that occurs every once every 10 > > > > seconds should not be measured strictly in CPU cost, but rather the > > > effect > > > > on the 95-99%. The larger the request is, the more opportunity there > is > > > to > > > > put a burst stress on the producer and broker, and the larger the > > > response > > > > payload to push through the control plane socket. Maybe that's not at > > 5k > > > > topics, but there are groups that are 10k+ topics and pushing > further. > > > > > > KAFKA-7019 made reading the metadata lock-free. There is no a priori > > > reason to prefer lots of small requests to a few big requests (within > > > reason!) In fact, it's quite the opposite: when we make lots of small > > > requests, it uses more network bandwidth than when we make a few big > > ones. > > > There are a few reasons for this: the request and response headers > have a > > > fixed overhead, one big array takes less space when serialized than > > several > > > small ones, etc. There is also TCP and IP overhead, etc. > > > > > > The broker can only push a few tens of thousands of metadata requests a > > > second, due to the overhead of message processing. This is why almost > > all > > > of the admin commands support batching. So if you need to create 1,000 > > > topics, you make one request, not 1,000 requests, for example. > > > > > > It's definitely reasonable to limit the number of topics made per > > metadata > > > request. But the reason for this is not improving performance, but > > > preventing certain bad corner cases that happen when RPCs get too big. > > For > > > example, one problem that can happen when a metadata response gets too > > big > > > is that the client could time out before it finishes reading the > > response. > > > Or if the response got way too big, it could even exceed the maximum > > > response size. > > > > > > So I think the limit should be pretty high here. We might also > consider > > > putting the limit in terms of number of partitions rather than number > of > > > topics, since that's what really matters here (this is harder to > > implement, > > > I realize...) If I had to put a rough number on it, I'd say we don't > > want > > > more than like 50 MB of response data. This is vaguely in line with > how > > we > > > do fetch responses as well (although I think the limit there is > higher). > > > > > > We should also keep in mind that anyone with a wildcard subscription is > > > making full metadata requests, which will return back information about > > > every topic in the system. > > > > > > > > > > > There's definitely weight to the metadata RPCs. Looking at a previous > > > > local, non-loaded test I ran, I calculate about 2 microseconds per > > > > partition latency to the producer. At 10,000 topics with 100 > partitions > > > > each, that's a full 2-second bubble in the best case. I can rerun a > > more > > > > targeted performance test, but I feel that's missing the point. > > > > > > > > > > If the metadata is fetched in the background, there should be no impact > > on > > > producer latency, right? > > > > > > It would be good to talk more about the importance of background > metadata > > > fetching in the KIP. The fact that we don't do this is actually a big > > > problem with the current implementation. As I understand it, when the > > > metadata gets too old, we slam on the brakes and wait for a metadata > > fetch > > > to complete, rather than starting the metadata fetch BEFORE we need it. > > > It's just bad scheduling. > > > > > > best, > > > Colin > > > > > > > > > > > Brian > > > > > > > > On Mon, Jan 6, 2020 at 1:31 PM Colin McCabe <cmcc...@apache.org> > > wrote: > > > > > > > > > On Mon, Jan 6, 2020, at 13:07, Brian Byrne wrote: > > > > > > Hi Colin, > > > > > > > > > > > > Thanks again for the feedback! > > > > > > > > > > > > On Mon, Jan 6, 2020 at 12:07 PM Colin McCabe <cmcc...@apache.org > > > > > wrote: > > > > > > > > > > > > > Metadata requests don't (always) go to the controller, right? > We > > > > > should > > > > > > > fix the wording in this section. > > > > > > > > > > > > > > > > > > > You're correct, s/controller/broker(s)/. > > > > > > > > > > > > I feel like "Proposed Changes" should come before "Public > > Interfaces" > > > > > > > here. The new configuration won't make sense to the reader > until > > > he > > > > > or she > > > > > > > has read the "changes" section. Also, it's not clear from the > > name > > > > > that > > > > > > > "metadata evict" refers to a span of time. What do you think > > > about " > > > > > > > metadata.eviction.period.ms" as a configuration name? > > > > > > > > > > > > > > > > > > > Sure, makes sense. Updated order and config name. > > > > > > > > > > > > > > > > > > > Where is the "10 seconds" coming from here? The current > default > > > for > > > > > > > metadata.max.age.ms is 5 * 60 * 1000 ms, which implies that we > > > want to > > > > > > > refresh every 5 minutes. Definitely not every 10 seconds. > > > > > > > > > > > > > > > > > > > The 10 seconds is another arbitrary value to prevent a large > number > > > of > > > > > RPCs > > > > > > if the target batch size were fixed at 20. For example, if > there's > > > 5,000 > > > > > > topics with a 5-minute interval, then instead of issuing an RPC > > every > > > > > > 1.2 seconds with batch size of 20, it would issue an RPC every 10 > > > seconds > > > > > > with batch size of 167. > > > > > > > > > > > > > > > > Hmm. This will lead to many more RPCs compared to the current > > > situation > > > > > of issuing an RPC every 5 minutes with 5,000 topics, right? See > > below > > > for > > > > > more discussion. > > > > > > > > > > > > > > > > > > > > > > > > Stepping back a little bit, it seems like the big problem you > > > > > identified > > > > > > > is the O(N^2) behavior of producing to X, then Y, then Z, etc. > > etc. > > > > > where > > > > > > > each new produce to a fresh topic triggers a metadata request > > with > > > all > > > > > the > > > > > > > preceding topics included. > > > > > > > > > > > > > > Of course we need to send out a metadata request before > producing > > > to X, > > > > > > > then Y, then Z, but that request could just specify X, or just > > > specify > > > > > Y, > > > > > > > etc. etc. In other words, we could decouple decouple the > routine > > > > > metadata > > > > > > > fetch which happens on a 5 minute timer from the need to fetch > > > > > metadata for > > > > > > > something specific right now. > > > > > > > > > > > > > > I guess my question is, do we really need to allow routine > > metadata > > > > > > > fetches to "piggyback" on the emergency metadata fetches? It > > adds > > > a > > > > > lot of > > > > > > > complexity, and we don't have any benchmarks proving that it's > > > better. > > > > > > > Also, as I understand it, whether we piggyback or not, the > number > > > of > > > > > > > metadata fetches is the same, right? > > > > > > > > > > > > > > > > > > > So it's possible to do as you suggest, but I would argue that > it'd > > be > > > > > more > > > > > > complex with how the code is structured and wouldn't add any > extra > > > > > > complexity. The derived metadata class effectively respond to a > > > > > > notification that a metadata RPC is going to be issued, where > they > > > return > > > > > > the metadata request structure with topics to refresh, which is > > > decoupled > > > > > > from what generated the event (new topic, stale metadata, > periodic > > > > > refresh > > > > > > alarm). There is also a strict implementation detail that only > one > > > > > metadata > > > > > > request can be outstanding at any time, which lends itself to > > > consolidate > > > > > > complexity in the base metadata class and have the derived use > the > > > > > "listen > > > > > > for next update" model. > > > > > > > > > > > > By maintaining an ordered list of topics by their last metadata > > > refresh > > > > > > time (0 for new topics), it's a matter of pulling from the front > of > > > the > > > > > > list to see which topics should be included in the next update. > > > Always > > > > > > include all urgent topics, then include non-urgent (i.e. need to > be > > > > > > refreshed soon-ish) up to the target batch size. > > > > > > > > > > > > The number of metadata fetches could be reduced. Assuming a > target > > > batch > > > > > > size of 20, a new topic might also refresh 19 other "refresh > soon" > > > topics > > > > > > in the same RPC, as opposed to those 19 being handled in a > > subsequent > > > > > RPC. > > > > > > > > > > > > Although to counter the above, the batching/piggybacking logic > > isn't > > > > > > necessarily about reducing the total number of RPCs, but rather > to > > > > > > distribute the load more evenly over time. For example, if a > large > > > number > > > > > > of topics need to be refreshed at the approximate same time > (common > > > for > > > > > > startups cases that hit a large number of topics), the updates > are > > > more > > > > > > evenly distributed to avoid a flood. > > > > > > > > > > It wouldn't be a flood in the current case, right? It would just > be > > a > > > > > single metadata request for a lot of topics. > > > > > > > > > > Let's compare the two cases. In the current scenario, we have 1 > > > metadata > > > > > request every 5 minutes. This request is for 5,000 topics (let's > > > say). In > > > > > the new scenario, we have a request every 10 seconds for 167 topics > > > each. > > > > > > > > > > Which do you think will be more expensive? I think the second > > scenario > > > > > certainly will because of the overhead of 30x as many requests send > > > over > > > > > the wire. Metadata accesses are now lockless, so the big metadata > > > request > > > > > just isn't that much of a problem. I bet if you benchmark it, > > sending > > > back > > > > > metadata for 167 topics won't be that much cheaper than sending > back > > > > > metadata for 5k. Certainly not 30x cheaper. There will eventually > > be > > > a > > > > > point where we need to split metadata requests, but it's definitely > > > not at > > > > > 5,000 topics. > > > > > > > > > > regards, > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Jan 6, 2020, at 10:26, Lucas Bradstreet wrote: > > > > > > > > +1 (non-binding) > > > > > > > > > > > > > > > > On Thu, Jan 2, 2020 at 11:15 AM Brian Byrne < > > bby...@confluent.io > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > Hello all, > > > > > > > > > > > > > > > > > > After further discussion and improvements, I'd like to > > > reinstate > > > > > the > > > > > > > voting > > > > > > > > > process. > > > > > > > > > > > > > > > > > > The updated KIP: > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-526 > > > > > > > > > > > %3A+Reduce+Producer+Metadata+Lookups+for+Large+Number+of+Topics > > > > > > > > > < > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-526%3A+Reduce+Producer+Metadata+Lookups+for+Large+Number+of+Topics > > > > > > > > > > > > > > > > > > > > > > > > > > The continued discussion: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://lists.apache.org/thread.html/b2f8f830ef04587144cf0840c7d4811bbf0a14f3c459723dbc5acf9e@%3Cdev.kafka.apache.org%3E > > > > > > > > > > > > > > > > > > I'd be happy to address any further comments/feedback. > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > On Mon, Dec 9, 2019 at 11:02 PM Guozhang Wang < > > > wangg...@gmail.com> > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > With the concluded summary on the other discussion > thread, > > > I'm > > > > > +1 on > > > > > > > the > > > > > > > > > > proposal. > > > > > > > > > > > > > > > > > > > > Thanks Brian! > > > > > > > > > > > > > > > > > > > > On Tue, Nov 19, 2019 at 8:00 PM deng ziming < > > > > > > > dengziming1...@gmail.com> > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > For new (uncached) topics, one problem here is that > we > > > don't > > > > > know > > > > > > > > > which > > > > > > > > > > > > partition to map a record to in the event that it > has a > > > key > > > > > or > > > > > > > custom > > > > > > > > > > > > partitioner, so the RecordAccumulator wouldn't know > > which > > > > > > > > > batch/broker > > > > > > > > > > it > > > > > > > > > > > > belongs. We'd need an intermediate record queue that > > > > > subsequently > > > > > > > > > moved > > > > > > > > > > > the > > > > > > > > > > > > records into RecordAccumulators once metadata > > resolution > > > was > > > > > > > > > complete. > > > > > > > > > > > For > > > > > > > > > > > > known topics, we don't currently block at all in > > > > > waitOnMetadata. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > You are right, I forget this fact, and the intermediate > > > record > > > > > > > queue > > > > > > > > > will > > > > > > > > > > > help, but I have some questions > > > > > > > > > > > > > > > > > > > > > > if we add an intermediate record queue in > KafkaProducer, > > > when > > > > > > > should we > > > > > > > > > > > move the records into RecordAccumulators? > > > > > > > > > > > only NetworkClient is aware of the MetadataResponse, > here > > > is > > > > > the > > > > > > > > > > > hierarchical structure of the related classes: > > > > > > > > > > > KafkaProducer > > > > > > > > > > > Accumulator > > > > > > > > > > > Sender > > > > > > > > > > > NetworkClient > > > > > > > > > > > > > metadataUpdater.handleCompletedMetadataResponse > > > > > > > > > > > > > > > > > > > > > > so > > > > > > > > > > > 1. we should also add a metadataUpdater to > KafkaProducer? > > > > > > > > > > > 2. if the topic really does not exists? the > intermediate > > > record > > > > > > > queue > > > > > > > > > > will > > > > > > > > > > > become too large? > > > > > > > > > > > 3. and should we `block` when the intermediate record > > > queue is > > > > > too > > > > > > > > > large? > > > > > > > > > > > and this will again bring the blocking problem? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Wed, Nov 20, 2019 at 12:40 AM Brian Byrne < > > > > > bby...@confluent.io> > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > Hi Deng, > > > > > > > > > > > > > > > > > > > > > > > > Thanks for the feedback. > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Nov 18, 2019 at 6:56 PM deng ziming < > > > > > > > > > dengziming1...@gmail.com> > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > hi, I reviewed the current code, the > ProduceMetadata > > > > > maintains > > > > > > > an > > > > > > > > > > > expiry > > > > > > > > > > > > > threshold for every topic, every time when we write > > to > > > a > > > > > topic > > > > > > > we > > > > > > > > > > will > > > > > > > > > > > > set > > > > > > > > > > > > > the expiry time to -1 to indicate it should be > > updated, > > > > > this > > > > > > > does > > > > > > > > > > work > > > > > > > > > > > to > > > > > > > > > > > > > reduce the size of the topic working set, but the > > > producer > > > > > will > > > > > > > > > > > continue > > > > > > > > > > > > > fetching metadata for these topics in every > metadata > > > > > request > > > > > > > for > > > > > > > > > the > > > > > > > > > > > full > > > > > > > > > > > > > expiry duration. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Indeed, you are correct, I terribly misread the code > > > here. > > > > > > > > > Fortunately > > > > > > > > > > > this > > > > > > > > > > > > was only a minor optimization in the KIP that's no > > longer > > > > > > > necessary. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > and we can improve the situation by 2 means: > > > > > > > > > > > > > 1. we maintain a refresh threshold for every > > topic > > > > > which > > > > > > > is for > > > > > > > > > > > > example > > > > > > > > > > > > > 0.8 * expiry_threshold, and when we send > > > `MetadataRequest` > > > > > to > > > > > > > > > brokers > > > > > > > > > > > we > > > > > > > > > > > > > just request unknownLeaderTopics + > > > unknownPartitionTopics + > > > > > > > topics > > > > > > > > > > > > > reach refresh threshold. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Right, this is similar to what I suggested, with a > > larger > > > > > window > > > > > > > on > > > > > > > > > the > > > > > > > > > > > > "staleness" that permits for batching to an > appropriate > > > size > > > > > > > (except > > > > > > > > > if > > > > > > > > > > > > there's any unknown topics, you'd want to issue the > > > request > > > > > > > > > > immediately). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > 2. we don't invoke KafkaProducer#waitOnMetadata > > > when we > > > > > > > call > > > > > > > > > > > > > KafkaProducer#send because of we just send data to > > > > > > > > > RecordAccumulator, > > > > > > > > > > > and > > > > > > > > > > > > > before we send data to brokers we will invoke > > > > > > > > > > > RecordAccumulator#ready(), > > > > > > > > > > > > so > > > > > > > > > > > > > we can only invoke waitOnMetadata to block when > > (number > > > > > topics > > > > > > > > > > > > > reach refresh threshold)>(number of all known > > > topics)*0.2. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > For new (uncached) topics, one problem here is that > we > > > don't > > > > > know > > > > > > > > > which > > > > > > > > > > > > partition to map a record to in the event that it > has a > > > key > > > > > or > > > > > > > custom > > > > > > > > > > > > partitioner, so the RecordAccumulator wouldn't know > > which > > > > > > > > > batch/broker > > > > > > > > > > it > > > > > > > > > > > > belongs. We'd need an intermediate record queue that > > > > > subsequently > > > > > > > > > moved > > > > > > > > > > > the > > > > > > > > > > > > records into RecordAccumulators once metadata > > resolution > > > was > > > > > > > > > complete. > > > > > > > > > > > For > > > > > > > > > > > > known topics, we don't currently block at all in > > > > > waitOnMetadata. > > > > > > > > > > > > > > > > > > > > > > > > The last major point of minimizing producer startup > > > metadata > > > > > > > RPCs may > > > > > > > > > > > still > > > > > > > > > > > > need to be improved, but this would be a large > > > improvement > > > > > on the > > > > > > > > > > current > > > > > > > > > > > > situation. > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I think the above 2 ways are enough to solve the > > > current > > > > > > > problem. > > > > > > > > > > > > > > > > > > > > > > > > > > On Tue, Nov 19, 2019 at 3:20 AM Colin McCabe < > > > > > > > cmcc...@apache.org> > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Nov 18, 2019, at 10:05, Brian Byrne > wrote: > > > > > > > > > > > > > > > On Fri, Nov 15, 2019 at 5:08 PM Colin McCabe < > > > > > > > > > cmcc...@apache.org > > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Two seconds doesn't seem like a reasonable > > > amount of > > > > > > > time to > > > > > > > > > > > leave > > > > > > > > > > > > > for > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > metadata fetch. Fetching halfway through the > > > > > expiration > > > > > > > > > period > > > > > > > > > > > > seems > > > > > > > > > > > > > > more > > > > > > > > > > > > > > > > reasonable. It also doesn't require us to > > > create a > > > > > new > > > > > > > > > > > > configuration > > > > > > > > > > > > > > key, > > > > > > > > > > > > > > > > which is nice. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Another option is to just do the metadata > fetch > > > every > > > > > > > > > > > > > > metadata.max.age.ms, > > > > > > > > > > > > > > > > but not expire the topic until we can't fetch > > the > > > > > > > metadata > > > > > > > > > for > > > > > > > > > > 2 > > > > > > > > > > > * > > > > > > > > > > > > > > > > metadata.max.age.ms. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I'd expect two seconds to be reasonable in the > > > common > > > > > case. > > > > > > > > > Keep > > > > > > > > > > in > > > > > > > > > > > > > mind > > > > > > > > > > > > > > > that this doesn't affect correctness, and a > > control > > > > > > > operation > > > > > > > > > > > > returning > > > > > > > > > > > > > > > cached metadata should be on the order of > > > milliseconds. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi Brian, > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks again for the KIP. > > > > > > > > > > > > > > > > > > > > > > > > > > > > I think the issue here is not the common case, > but > > > the > > > > > > > uncommon > > > > > > > > > > case > > > > > > > > > > > > > where > > > > > > > > > > > > > > the metadata fetch takes longer than expected. > In > > > that > > > > > > > case, we > > > > > > > > > > > don't > > > > > > > > > > > > > want > > > > > > > > > > > > > > to be in the position of having our metadata > expire > > > > > because > > > > > > > we > > > > > > > > > > waited > > > > > > > > > > > > too > > > > > > > > > > > > > > long to renew it. > > > > > > > > > > > > > > > > > > > > > > > > > > > > This is one reason why I think that the metadata > > > > > expiration > > > > > > > time > > > > > > > > > > > should > > > > > > > > > > > > > be > > > > > > > > > > > > > > longer than the metadata refresh time. In fact, > it > > > > > might be > > > > > > > > > worth > > > > > > > > > > > > having > > > > > > > > > > > > > > two separate configuration keys for these two > > > values. I > > > > > > > could > > > > > > > > > > > imagine > > > > > > > > > > > > a > > > > > > > > > > > > > > user who is having trouble with metadata > expiration > > > > > wanting > > > > > > > to > > > > > > > > > > > increase > > > > > > > > > > > > > the > > > > > > > > > > > > > > metadata expiration time, but without increasing > > the > > > > > metadata > > > > > > > > > > refresh > > > > > > > > > > > > > > period. In a sense, the metadata expiration time > > is > > > like > > > > > > > the ZK > > > > > > > > > > > > session > > > > > > > > > > > > > > expiration time. You might want to turn it up if > > the > > > > > > > cluster is > > > > > > > > > > > > > > experiencing load spikes. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > But to the general > > > > > > > > > > > > > > > point, defining the algorithm would mean > > enforcing > > > it > > > > > to > > > > > > > fair > > > > > > > > > > > > accuracy, > > > > > > > > > > > > > > > whereas if the suggestion is that it'll be > > > performed > > > > > at a > > > > > > > > > > > reasonable > > > > > > > > > > > > > > time, > > > > > > > > > > > > > > > it allows for batching and other optimizations. > > > > > Perhaps I > > > > > > > > > > shouldn't > > > > > > > > > > > > be > > > > > > > > > > > > > > > regarding what's defined in a KIP to be > > > contractual in > > > > > > > these > > > > > > > > > > cases, > > > > > > > > > > > > but > > > > > > > > > > > > > > you > > > > > > > > > > > > > > > could consider a first implementation to > collect > > > topics > > > > > > > whose > > > > > > > > > > > > metadata > > > > > > > > > > > > > > has > > > > > > > > > > > > > > > exceeded (metadata.max.age.ms / 2), and > sending > > > the > > > > > batch > > > > > > > > > once a > > > > > > > > > > > > > > > constituent topic's metadata is near the > expiry, > > > or a > > > > > > > > > sufficient > > > > > > > > > > > > number > > > > > > > > > > > > > > of > > > > > > > > > > > > > > > topics have been collected (10? 100? 1000?). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I'm concerned that if we change the metadata > > caching > > > > > strategy > > > > > > > > > > without > > > > > > > > > > > > > > discussing it first, it may improve certain > > > workloads but > > > > > > > make > > > > > > > > > > others > > > > > > > > > > > > > > worse. We need to be concrete about what the > > > proposed > > > > > > > strategy > > > > > > > > > is > > > > > > > > > > so > > > > > > > > > > > > > that > > > > > > > > > > > > > > we can really evaluate it. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > We should be specific about what happens if > the > > > > > first few > > > > > > > > > > > metadata > > > > > > > > > > > > > > fetches > > > > > > > > > > > > > > > > fail. Do we use exponential backoff to > decide > > > when > > > > > to > > > > > > > > > resend? > > > > > > > > > > > It > > > > > > > > > > > > > > seems > > > > > > > > > > > > > > > > like we really should, for all the usual > > reasons > > > > > (reduce > > > > > > > the > > > > > > > > > > load > > > > > > > > > > > > on > > > > > > > > > > > > > > > > brokers, ride out temporary service > > disruptions, > > > > > etc.) > > > > > > > Maybe > > > > > > > > > > we > > > > > > > > > > > > > could > > > > > > > > > > > > > > have > > > > > > > > > > > > > > > > an exponential retry backoff for each broker > > (in > > > > > other > > > > > > > words, > > > > > > > > > > we > > > > > > > > > > > > > > should try > > > > > > > > > > > > > > > > to contact a different broker before applying > > the > > > > > > > backoff.) > > > > > > > > > I > > > > > > > > > > > > think > > > > > > > > > > > > > > this > > > > > > > > > > > > > > > > already sort of happens with the disconnect > > > timeout, > > > > > but > > > > > > > we > > > > > > > > > > might > > > > > > > > > > > > > need > > > > > > > > > > > > > > a > > > > > > > > > > > > > > > > more general solution. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I don't plan to change this behavior. Currently > > it > > > > > retries > > > > > > > > > after > > > > > > > > > > a > > > > > > > > > > > > > fixed > > > > > > > > > > > > > > > value of 'retry.backoff.ms' (defaults to 100 > > ms). > > > It's > > > > > > > > > possible > > > > > > > > > > > that > > > > > > > > > > > > > > > different brokers are attempted, but I haven't > > dug > > > > > into it. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I think it's critical to understand what the > > current > > > > > > > behavior is > > > > > > > > > > > before > > > > > > > > > > > > > we > > > > > > > > > > > > > > try to change it. The difference between > retrying > > > the > > > > > same > > > > > > > > > broker > > > > > > > > > > > and > > > > > > > > > > > > > > trying a different one has a large impact it has > on > > > > > cluster > > > > > > > load > > > > > > > > > > and > > > > > > > > > > > > > > latency. For what it's worth, I believe the > > > behavior is > > > > > the > > > > > > > > > second > > > > > > > > > > > > one, > > > > > > > > > > > > > > but it has been a while since I checked. Let's > > > figure > > > > > this > > > > > > > out. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks for the clarification. Fully > > > asynchronous is > > > > > the > > > > > > > way > > > > > > > > > to > > > > > > > > > > > > go, I > > > > > > > > > > > > > > > > agree. I'm having trouble understanding how > > > > > timeouts are > > > > > > > > > > handled > > > > > > > > > > > > in > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > KIP. It seems like if we can't fetch the > > > metadata > > > > > > > within the > > > > > > > > > > > > > > designated > > > > > > > > > > > > > > > > metadata timeout, the future / callback > should > > > > > receive a > > > > > > > > > > > > > > TimeoutException > > > > > > > > > > > > > > > > right? We do not want the send call to be > > > deferred > > > > > > > forever > > > > > > > > > if > > > > > > > > > > > > > metadata > > > > > > > > > > > > > > > > can't be fetched. Eventually it should fail > if > > > it > > > > > can't > > > > > > > be > > > > > > > > > > > > > performed. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I do think this is something that will have > to > > be > > > > > > > mentioned > > > > > > > > > in > > > > > > > > > > > the > > > > > > > > > > > > > > > > compatibility section. There is some code > out > > > there > > > > > > > that is > > > > > > > > > > > > probably > > > > > > > > > > > > > > > > prepared to handle a timeout exception from > the > > > send > > > > > > > > > function, > > > > > > > > > > > > which > > > > > > > > > > > > > > may > > > > > > > > > > > > > > > > need to be updated to check for a timeout > from > > > the > > > > > > > future or > > > > > > > > > > > > > callback. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Correct, a timeout exception would be delivered > > in > > > the > > > > > > > future. > > > > > > > > > > > Sure, > > > > > > > > > > > > I > > > > > > > > > > > > > > can > > > > > > > > > > > > > > > add that note to the KIP. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > > > > > > best, > > > > > > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It seems like this is an existing problem. > You > > > may > > > > > fire > > > > > > > off > > > > > > > > > a > > > > > > > > > > > lot > > > > > > > > > > > > of > > > > > > > > > > > > > > send > > > > > > > > > > > > > > > > calls that get blocked because the broker > that > > > is the > > > > > > > leader > > > > > > > > > > for > > > > > > > > > > > a > > > > > > > > > > > > > > certain > > > > > > > > > > > > > > > > partition is not responding. I'm not sure > that > > > we > > > > > need > > > > > > > to do > > > > > > > > > > > > > anything > > > > > > > > > > > > > > > > special here. On the other hand, we could > make > > > the > > > > > case > > > > > > > for > > > > > > > > > a > > > > > > > > > > > > > generic > > > > > > > > > > > > > > "max > > > > > > > > > > > > > > > > number of outstanding sends" configuration to > > > prevent > > > > > > > > > surprise > > > > > > > > > > > OOMs > > > > > > > > > > > > > in > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > existing cases, plus the new one we're > adding. > > > But > > > > > this > > > > > > > > > feels > > > > > > > > > > > > like a > > > > > > > > > > > > > > bit > > > > > > > > > > > > > > > > of a scope expansion. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Right, this is an existing problem, however the > > > > > > > asynchronous > > > > > > > > > send > > > > > > > > > > > > could > > > > > > > > > > > > > > > cause unexpected behavior. For example, if a > > client > > > > > pinned > > > > > > > > > > > > > > > topics/partitions to individual send threads, > > then > > > > > memory > > > > > > > > > > couldn't > > > > > > > > > > > be > > > > > > > > > > > > > > > exhausted by a single topic since a blocking > send > > > would > > > > > > > prevent > > > > > > > > > > > > further > > > > > > > > > > > > > > > records from being buffered on that topic. The > > > > > compromise > > > > > > > could > > > > > > > > > > be > > > > > > > > > > > > that > > > > > > > > > > > > > > we > > > > > > > > > > > > > > > only ever permit one outstanding record batch > > for a > > > > > topic, > > > > > > > > > which > > > > > > > > > > > > would > > > > > > > > > > > > > > keep > > > > > > > > > > > > > > > the code simple and wouldn't permit a single > > topic > > > to > > > > > > > consume > > > > > > > > > all > > > > > > > > > > > > > > available > > > > > > > > > > > > > > > memory. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > They may be connected, but I'm not sure they > > > should > > > > > be > > > > > > > the > > > > > > > > > > same. > > > > > > > > > > > > > > Perhaps > > > > > > > > > > > > > > > > expiry should be 4x the typical fetch rate, > for > > > > > example. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > That's true. You could also make the case for a > > > faster > > > > > > > expiry > > > > > > > > > > than > > > > > > > > > > > > > > refresh > > > > > > > > > > > > > > > as well. Makes sense to separate this out. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hmm.... are you sure this is an N^2 problem? > > If > > > you > > > > > > > have T1 > > > > > > > > > > and > > > > > > > > > > > > T2, > > > > > > > > > > > > > > you > > > > > > > > > > > > > > > > fetch metadata for T1 and T2, right? I guess > > you > > > > > could > > > > > > > argue > > > > > > > > > > > that > > > > > > > > > > > > we > > > > > > > > > > > > > > often > > > > > > > > > > > > > > > > fetch metadata for partitions we don't care > > > about, > > > > > but > > > > > > > that > > > > > > > > > > > doesn't > > > > > > > > > > > > > > make it > > > > > > > > > > > > > > > > O(N^2). Maybe there's something about the > > > > > implementation > > > > > > > > > that > > > > > > > > > > > I'm > > > > > > > > > > > > > > missing. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > My apologies, I left out the context. One issue > > the > > > > > KIP is > > > > > > > > > trying > > > > > > > > > > > to > > > > > > > > > > > > > > > resolve is the metadata storm that's caused by > > > > > producers > > > > > > > > > starting > > > > > > > > > > > up. > > > > > > > > > > > > > In > > > > > > > > > > > > > > > the worst case, where the send call is only > > > performed > > > > > from > > > > > > > a > > > > > > > > > > single > > > > > > > > > > > > > > thread > > > > > > > > > > > > > > > (i.e. no possible batching), fetching metadata > > for > > > 1K > > > > > > > topics > > > > > > > > > will > > > > > > > > > > > > > > generate > > > > > > > > > > > > > > > 1K RPCs, with payload 1+2+...+1K topics' > > metadata. > > > > > Being > > > > > > > smart > > > > > > > > > > > about > > > > > > > > > > > > > the > > > > > > > > > > > > > > > topics being refreshed would still generate 1K > > RPCs > > > > > for 1 > > > > > > > topic > > > > > > > > > > > each, > > > > > > > > > > > > > and > > > > > > > > > > > > > > > asynchronous behavior would permit batching > (note > > > > > > > steady-state > > > > > > > > > > > > > refreshing > > > > > > > > > > > > > > > doesn't require the asynchronous behavior to > > > batch). > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > In general, we need to take advantage of > > > batching to > > > > > do > > > > > > > this > > > > > > > > > > well > > > > > > > > > > > > > (one > > > > > > > > > > > > > > > > reason why I think we should steer clear of > > > > > > > ultra-granular > > > > > > > > > > > timeout > > > > > > > > > > > > > > > > tracking). It's best to do 1 RPC asking for > 10 > > > > > topics, > > > > > > > not > > > > > > > > > 10 > > > > > > > > > > > RPCs > > > > > > > > > > > > > > asking > > > > > > > > > > > > > > > > for a single topic each, even if that means > > some > > > of > > > > > the > > > > > > > topic > > > > > > > > > > > > > timeouts > > > > > > > > > > > > > > are > > > > > > > > > > > > > > > > not *exactly* aligned with the configured > > value. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Absolutely, agreed. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > best, > > > > > > > > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Mon, Nov 11, 2019 at 11:47 AM Colin > > McCabe < > > > > > > > > > > > > cmcc...@apache.org> > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi Brian, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks for the KIP. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Starting the metadata fetch before we > need > > > the > > > > > > > result is > > > > > > > > > > > > > > definitely a > > > > > > > > > > > > > > > > > > great idea. This way, the metadata fetch > > > can be > > > > > > > done in > > > > > > > > > > > > parallel > > > > > > > > > > > > > > with > > > > > > > > > > > > > > > > all > > > > > > > > > > > > > > > > > > the other stuff e producer is doing, > rather > > > than > > > > > > > forcing > > > > > > > > > > the > > > > > > > > > > > > > > producer > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > > > periodically come to a halt periodically > > > while > > > > > > > metadata > > > > > > > > > is > > > > > > > > > > > > > fetched. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Maybe I missed it, but there seemed to be > > > some > > > > > > > details > > > > > > > > > > > missing > > > > > > > > > > > > > > here. > > > > > > > > > > > > > > > > When > > > > > > > > > > > > > > > > > > do we start the metadata fetch? For > > > example, if > > > > > > > topic > > > > > > > > > > > metadata > > > > > > > > > > > > > > expires > > > > > > > > > > > > > > > > > > every 5 minutes, perhaps we should wait 4 > > > > > minutes, > > > > > > > then > > > > > > > > > > > > starting > > > > > > > > > > > > > > > > fetching > > > > > > > > > > > > > > > > > > the new metadata, which we would expect > to > > > > > arrive by > > > > > > > the > > > > > > > > > 5 > > > > > > > > > > > > minute > > > > > > > > > > > > > > > > > > deadline. Or perhaps we should start the > > > fetch > > > > > even > > > > > > > > > > earlier, > > > > > > > > > > > > > > around > > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > > > 2.5 minute mark. In any case, there > should > > > be > > > > > some > > > > > > > > > > > discussion > > > > > > > > > > > > > > about > > > > > > > > > > > > > > > > what > > > > > > > > > > > > > > > > > > the actual policy is. Given that > > > > > > > metadata.max.age.ms is > > > > > > > > > > > > > > configurable, > > > > > > > > > > > > > > > > > > maybe that policy needs to be expressed > in > > > > > terms of > > > > > > > a > > > > > > > > > > > > percentage > > > > > > > > > > > > > > of > > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > > > refresh period rather than in terms of an > > > > > absolute > > > > > > > delay. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The KIP correctly points out that the > > current > > > > > > > metadata > > > > > > > > > > > fetching > > > > > > > > > > > > > > policy > > > > > > > > > > > > > > > > > > causes us to "[block] in a function > that's > > > > > > > advertised as > > > > > > > > > > > > > > asynchronous." > > > > > > > > > > > > > > > > > > However, the KIP doesn't seem to spell > out > > > > > whether we > > > > > > > > > will > > > > > > > > > > > > > > continue to > > > > > > > > > > > > > > > > > > block if metadata can't be found, or if > > this > > > > > will be > > > > > > > > > > > abolished. > > > > > > > > > > > > > > > > Clearly, > > > > > > > > > > > > > > > > > > starting the metadata fetch early will > > reduce > > > > > > > blocking in > > > > > > > > > > the > > > > > > > > > > > > > > common > > > > > > > > > > > > > > > > case, > > > > > > > > > > > > > > > > > > but will there still be blocking in the > > > uncommon > > > > > case > > > > > > > > > where > > > > > > > > > > > the > > > > > > > > > > > > > > early > > > > > > > > > > > > > > > > fetch > > > > > > > > > > > > > > > > > > doesn't succeed in time? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > To address (2), the producer currently > > > > > maintains > > > > > > > an > > > > > > > > > > expiry > > > > > > > > > > > > > > threshold > > > > > > > > > > > > > > > > > > for > > > > > > > > > > > > > > > > > > > every topic, which is used to remove a > > > topic > > > > > from > > > > > > > the > > > > > > > > > > > > working > > > > > > > > > > > > > > set > > > > > > > > > > > > > > > > at a > > > > > > > > > > > > > > > > > > > future time (currently hard-coded to 5 > > > > > minutes, > > > > > > > this > > > > > > > > > > > should > > > > > > > > > > > > be > > > > > > > > > > > > > > > > modified > > > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > > > > use metadata.max.age.ms). While this > > does > > > > > work to > > > > > > > > > > reduce > > > > > > > > > > > > the > > > > > > > > > > > > > > size > > > > > > > > > > > > > > > > of > > > > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > > > > topic working set, the producer will > > > continue > > > > > > > fetching > > > > > > > > > > > > > metadata > > > > > > > > > > > > > > for > > > > > > > > > > > > > > > > > > these > > > > > > > > > > > > > > > > > > > topics in every metadata request for > the > > > full > > > > > > > expiry > > > > > > > > > > > > duration. > > > > > > > > > > > > > > This > > > > > > > > > > > > > > > > > > logic > > > > > > > > > > > > > > > > > > > can be made more intelligent by > managing > > > the > > > > > > > expiry > > > > > > > > > from > > > > > > > > > > > > when > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > topic > > > > > > > > > > > > > > > > > > > was last used, enabling the expiry > > > duration > > > > > to be > > > > > > > > > > reduced > > > > > > > > > > > to > > > > > > > > > > > > > > improve > > > > > > > > > > > > > > > > > > cases > > > > > > > > > > > > > > > > > > > where a large number of topics are > > touched > > > > > > > > > > intermittently. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Can you clarify this part a bit? It > seems > > > like > > > > > we > > > > > > > have a > > > > > > > > > > > > > metadata > > > > > > > > > > > > > > > > > > expiration policy now for topics, and we > > will > > > > > have > > > > > > > one > > > > > > > > > > after > > > > > > > > > > > > this > > > > > > > > > > > > > > KIP, > > > > > > > > > > > > > > > > but > > > > > > > > > > > > > > > > > > they will be somewhat different. But > it's > > > not > > > > > clear > > > > > > > to > > > > > > > > > me > > > > > > > > > > > what > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > > > differences are. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > In general, if load is a problem, we > should > > > > > probably > > > > > > > > > > consider > > > > > > > > > > > > > > adding > > > > > > > > > > > > > > > > some > > > > > > > > > > > > > > > > > > kind of jitter on the client side. There > > are > > > > > > > definitely > > > > > > > > > > > cases > > > > > > > > > > > > > > where > > > > > > > > > > > > > > > > people > > > > > > > > > > > > > > > > > > start up a lot of clients at the same > time > > in > > > > > > > parallel > > > > > > > > > and > > > > > > > > > > > > there > > > > > > > > > > > > > > is a > > > > > > > > > > > > > > > > > > thundering herd problem with metadata > > > updates. > > > > > > > Adding > > > > > > > > > > jitter > > > > > > > > > > > > > would > > > > > > > > > > > > > > > > spread > > > > > > > > > > > > > > > > > > the load across time rather than > creating a > > > spike > > > > > > > every 5 > > > > > > > > > > > > minutes > > > > > > > > > > > > > > in > > > > > > > > > > > > > > > > this > > > > > > > > > > > > > > > > > > case. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > best, > > > > > > > > > > > > > > > > > > Colin > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Nov 8, 2019, at 08:59, Ismael > Juma > > > wrote: > > > > > > > > > > > > > > > > > > > I think this KIP affects when we block > > > which is > > > > > > > > > actually > > > > > > > > > > > user > > > > > > > > > > > > > > visible > > > > > > > > > > > > > > > > > > > behavior. Right? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Ismael > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Fri, Nov 8, 2019, 8:50 AM Brian > Byrne > > < > > > > > > > > > > > > bby...@confluent.io> > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hi Guozhang, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Regarding metadata expiry, no access > > > times > > > > > other > > > > > > > than > > > > > > > > > > the > > > > > > > > > > > > > > initial > > > > > > > > > > > > > > > > > > lookup[1] > > > > > > > > > > > > > > > > > > > > are used for determining when to > expire > > > > > producer > > > > > > > > > > > metadata. > > > > > > > > > > > > > > > > Therefore, > > > > > > > > > > > > > > > > > > > > frequently used topics' metadata will > > be > > > aged > > > > > > > out and > > > > > > > > > > > > > > subsequently > > > > > > > > > > > > > > > > > > > > refreshed (in a blocking manner) > every > > > five > > > > > > > minutes, > > > > > > > > > > and > > > > > > > > > > > > > > > > infrequently > > > > > > > > > > > > > > > > > > used > > > > > > > > > > > > > > > > > > > > topics will be retained for a minimum > > of > > > five > > > > > > > minutes > > > > > > > > > > and > > > > > > > > > > > > > > currently > > > > > > > > > > > > > > > > > > > > refetched on every metadata update > > during > > > > > that > > > > > > > time > > > > > > > > > > > period. > > > > > > > > > > > > > The > > > > > > > > > > > > > > > > > > sentence is > > > > > > > > > > > > > > > > > > > > suggesting that we could reduce the > > > expiry > > > > > time > > > > > > > to > > > > > > > > > > > improve > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > latter > > > > > > > > > > > > > > > > > > > > without affecting (rather slightly > > > > > improving) the > > > > > > > > > > former. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Keep in mind that in most all cases, > I > > > > > wouldn't > > > > > > > > > > > anticipate > > > > > > > > > > > > > > much of > > > > > > > > > > > > > > > > a > > > > > > > > > > > > > > > > > > > > difference with producer behavior, > and > > > the > > > > > extra > > > > > > > > > logic > > > > > > > > > > > can > > > > > > > > > > > > be > > > > > > > > > > > > > > > > > > implemented > > > > > > > > > > > > > > > > > > > > to have insignificant cost. It's the > > > > > > > large/dynamic > > > > > > > > > > topic > > > > > > > > > > > > > corner > > > > > > > > > > > > > > > > cases > > > > > > > > > > > > > > > > > > that > > > > > > > > > > > > > > > > > > > > we're trying to improve. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > It'd be convenient if the KIP is no > > > longer > > > > > > > necessary. > > > > > > > > > > > > You're > > > > > > > > > > > > > > right > > > > > > > > > > > > > > > > in > > > > > > > > > > > > > > > > > > that > > > > > > > > > > > > > > > > > > > > there's no public API changes and the > > > > > behavioral > > > > > > > > > > changes > > > > > > > > > > > > are > > > > > > > > > > > > > > > > entirely > > > > > > > > > > > > > > > > > > > > internal. I'd be happy to continue > the > > > > > discussion > > > > > > > > > > around > > > > > > > > > > > > the > > > > > > > > > > > > > > KIP, > > > > > > > > > > > > > > > > but > > > > > > > > > > > > > > > > > > > > unless otherwise objected, it can be > > > retired. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > [1] Not entirely accurate, it's > > actually > > > the > > > > > > > first > > > > > > > > > time > > > > > > > > > > > > when > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > client > > > > > > > > > > > > > > > > > > > > calculates whether to retain the > topic > > > in its > > > > > > > > > metadata. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Thu, Nov 7, 2019 at 4:48 PM > Guozhang > > > Wang > > > > > < > > > > > > > > > > > > > > wangg...@gmail.com> > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hello Brian, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Could you elaborate a bit more on > > this > > > > > > > sentence: > > > > > > > > > > "This > > > > > > > > > > > > > logic > > > > > > > > > > > > > > can > > > > > > > > > > > > > > > > be > > > > > > > > > > > > > > > > > > made > > > > > > > > > > > > > > > > > > > > > more intelligent by managing the > > expiry > > > > > from > > > > > > > when > > > > > > > > > the > > > > > > > > > > > > topic > > > > > > > > > > > > > > was > > > > > > > > > > > > > > > > last > > > > > > > > > > > > > > > > > > > > used, > > > > > > > > > > > > > > > > > > > > > enabling the expiry duration to be > > > reduced > > > > > to > > > > > > > > > improve > > > > > > > > > > > > cases > > > > > > > > > > > > > > > > where a > > > > > > > > > > > > > > > > > > large > > > > > > > > > > > > > > > > > > > > > number of topics are touched > > > > > intermittently." > > > > > > > Not > > > > > > > > > > sure > > > > > > > > > > > I > > > > > > > > > > > > > > fully > > > > > > > > > > > > > > > > > > understand > > > > > > > > > > > > > > > > > > > > > the proposal. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Also since now this KIP did not > make > > > any > > > > > > > public API > > > > > > > > > > > > changes > > > > > > > > > > > > > > and > > > > > > > > > > > > > > > > the > > > > > > > > > > > > > > > > > > > > > behavioral changes are not > > considered a > > > > > public > > > > > > > API > > > > > > > > > > > > contract > > > > > > > > > > > > > > (i.e. > > > > > > > > > > > > > > > > > > how we > > > > > > > > > > > > > > > > > > > > > maintain the topic metadata in > > producer > > > > > cache > > > > > > > is > > > > > > > > > > never > > > > > > > > > > > > > > committed > > > > > > > > > > > > > > > > to > > > > > > > > > > > > > > > > > > > > users), > > > > > > > > > > > > > > > > > > > > > I wonder if we still need a KIP for > > the > > > > > > > proposed > > > > > > > > > > change > > > > > > > > > > > > any > > > > > > > > > > > > > > more? > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Guozhang > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Thu, Nov 7, 2019 at 12:43 PM > Brian > > > > > Byrne < > > > > > > > > > > > > > > bby...@confluent.io > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Hello all, > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I'd like to propose a vote for a > > > producer > > > > > > > change > > > > > > > > > to > > > > > > > > > > > > > improve > > > > > > > > > > > > > > > > > > producer > > > > > > > > > > > > > > > > > > > > > > behavior when dealing with a > large > > > > > number of > > > > > > > > > > topics, > > > > > > > > > > > in > > > > > > > > > > > > > > part by > > > > > > > > > > > > > > > > > > > > reducing > > > > > > > > > > > > > > > > > > > > > > the amount of metadata fetching > > > > > performed. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The full KIP is provided here: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-526%3A+Reduce+Producer+Metadata+Lookups+for+Large+Number+of+Topics > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > And the discussion thread: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > https://lists.apache.org/thread.html/b2f8f830ef04587144cf0840c7d4811bbf0a14f3c459723dbc5acf9e@%3Cdev.kafka.apache.org%3E > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > > > > > > > > > > > > > > Brian > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > > > > > > > > > > > > -- Guozhang > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > > -- Guozhang > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > -- Guozhang >
