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.
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 you're arguing that the batch sizes are too small or time interval too short, that's fine, and I'll be happy to increase them. However, there is a breaking point to justify refreshing a subset of a producer's topics. Even if producers out number brokers by 10x, the fixed overhead cost of an RPC that occurs every 1-second is probably not worth scraping for performance, but worth sanitizing. 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 > > > > > > > > > > > > > > > > > > > > >
