Hi Jay, I apologize for derailing the conversation about the consumer API. We should start a new discussion about hierarchical topics, if we want to keep talking about it. My final thought on the matter is that, hierarchical topics is still an important feature to have in Kafka, because it gives us flexibility to do namespace level access controls.
Getting back to the topic of the Consumer API: 1. Any thoughts on consistency for method arguments and return types? 2. lastCommittedOffsets() method returns a List<TopicPartitionOffset>where as the confluence page suggested a Map<TopicPartition, Long>. I would think that a Map is the more appropriate return type. On Tue, Feb 11, 2014 at 8:04 AM, Jay Kreps <jay.kr...@gmail.com> wrote: > Hey Pradeep, > > That wiki is fairly old and it predated more flexible subscription > mechanisms. In the high-level consumer you currently have wildcard > subscription and in the new proposed interface you can actually subscribe > based on any logic you want to create a "union" of streams. Personally I > think this gives you everything you would want with a hierarchy and more > actual flexibility (since you can define groupings however you want). What > do you think? > > -Jay > > > On Mon, Feb 10, 2014 at 3:37 PM, Pradeep Gollakota <pradeep...@gmail.com > >wrote: > > > WRT to hierarchical topics, I'm referring to > > KAFKA-1175<https://issues.apache.org/jira/browse/KAFKA-1175>. > > I would just like to think through the implications for the Consumer API > if > > and when we do implement hierarchical topics. For example, in the > > proposal< > > https://cwiki.apache.org/confluence/display/KAFKA/Hierarchical+Topics# > > >written > > by Jay, he says that initially wildcard subscriptions are not going > > to be supported. But does that mean that they will be supported in v2? If > > that's the case, that would change the semantics of the Consumer API. > > > > As to having classes for Topic, PartitionId, etc. it looks like I was > > referring to the TopicPartition and TopicPartitionOffset classes (I > didn't > > realize these were already there). I was only looking at the confluence > > page which shows List[(String, Int, Long)] instead of > > List[TopicParitionOffset] (as is shown in the javadoc). However, I did > > notice that we're not being consistent in the Java version. E.g. we have > > commit(TopicPartitionOffset... offsets) and > > lastCommittedOffsets(TopicPartition... partitions) on the one hand. On > the > > other hand we have subscribe(String topic, int... partitions). I agree > that > > creating a class for TopicId today would probably not make too much sense > > today. But with hierarchical topics, I may change my mind. This is > exactly > > what was done in the HBase API in 0.96 when namespaces were added. 0.96 > > HBase API introduced a class called 'TableName' to represent the > namespace > > and table name. > > > > > > On Mon, Feb 10, 2014 at 3:08 PM, Neha Narkhede <neha.narkh...@gmail.com > > >wrote: > > > > > Thanks for the feedback. > > > > > > Mattijs - > > > > > > - Constructors link to > > > http://kafka.apache.org/documentation.html#consumerconfigs for valid > > > configurations, which lists zookeeper.connect rather than > > > metadata.broker.list, the value for BROKER_LIST_CONFIG in > ConsumerConfig. > > > Fixed it to just point to ConsumerConfig for now until we finalize the > > new > > > configs > > > - Docs for poll(long) mention consumer.commit(true), which I can't find > > in > > > the Consumer docs. For a simple consumer setup, that call is something > > that > > > would make a lot of sense. > > > Missed changing the examples to use consumer.commit(true, offsets). The > > > suggestions by Jay would change it to commit(offsets) and > > > commitAsync(offsets), which will hopefully make it easier to understand > > > those commit APIs. > > > - Love the addition of MockConsumer, awesome for unittesting :) > > > I'm not quite satisfied with what it does as of right now, but we will > > > surely improve it as we start writing the consumer. > > > > > > Jay - > > > > > > 1. ConsumerRebalanceCallback > > > a. Makes sense. Renamed to onPartitionsRevoked > > > b. Ya, it will be good to make it forward compatible with Java 8 > > > capabilities. We can change it to PartitionsAssignedCallback and > > > PartitionsRevokedCallback or RebalanceBeginCallback and > > > RebalanceEndCallback? > > > c. Ya, I thought about that but then didn't name it just > > > RebalanceCallback since there could be a conflict with a controller > side > > > rebalance callback if/when we have one. However, you can argue that at > > that > > > time we can name it ControllerRebalanceCallback instead of polluting a > > user > > > facing API. So agree with you here. > > > 2. Ya, that is a good idea. Changed to subscribe(String topic, > > > int...partitions). > > > 3. lastCommittedOffset() is not necessarily a local access since the > > > consumer can potentially ask for the last committed offsets of > partitions > > > that the consumer does not consume and maintain the offsets for. That's > > the > > > reason it is batched right now. > > > 4. Yes, look at > > > > > > > > > http://people.apache.org/~nehanarkhede/kafka-0.9-consumer-javadoc/doc/kafka/clients/consumer/ConsumerConfig.html#AUTO_OFFSET_RESET_CONFIG > > > 5. Sure, but that is not part of the consumer API right? I think you're > > > suggesting looking at OffsetRequest to see if it would do that > properly? > > > 6. Good point. Changed to poll(long timeout, TimeUnit) and poll with a > > > negative timeout will poll indefinitely? > > > 7. Good point. Changed to commit(...) and commitAsync(...) > > > 8. To commit the current position for all partitions owned by the > > consumer, > > > you can use commit(). If you don't use group management, then > > > commit(customListOfPartitions) > > > 9. Forgot to include unsubscribe. Done now. > > > 10. positions() can be called at any time and affects the next fetch on > > the > > > next poll(). Fixed the places that said "starting fetch offsets" > > > 11. Can we not look that up by going through the messages returned and > > > getting the offset from the ConsumerRecord? > > > > > > One thing that I really found helpful for the API design was writing > out > > > actual code for different scenarios against the API. I think it might > be > > > good to do that for this too--i.e. enumerate the various use cases and > > code > > > that use case up to see how it looks > > > The javadocs include examples for almost all possible scenarios of > > consumer > > > usage, that I could come up with. Will add more to the javadocs as I > get > > > more feedback from our users. The advantage of having the examples in > the > > > javadoc itself is to that the usage is self explanatory to new users. > > > > > > Pradeep - > > > > > > 2. Changed to poll(long, TimeUnit) and a negative value for the timeout > > > would block in the poll forever until there is new data > > > 3. We don't have hierarchical topics support. Would you mind explaining > > > what you meant? > > > 4. I'm not so sure that we need a class to express a topic which is a > > > string and a separate class for just partition id. We do have a class > for > > > TopicPartition which uniquely identifies a partition of a topic > > > > > > Thanks, > > > Neha > > > > > > > > > On Mon, Feb 10, 2014 at 12:36 PM, Pradeep Gollakota < > > pradeep...@gmail.com > > > >wrote: > > > > > > > Couple of very quick thoughts. > > > > > > > > 1. +1 about renaming commit(...) and commitAsync(...) > > > > 2. I'd also like to extend the above for the poll() method as well. > > > poll() > > > > and pollWithTimeout(long, TimeUnit)? > > > > 3. Have you guys given any thought around how this API would be used > > with > > > > hierarchical topics? > > > > 4. Would it make sense to add classes such as TopicId, PartitionId, > > etc? > > > > Seems like it would be easier to read code with these classes as > > opposed > > > to > > > > string and longs. > > > > > > > > - Pradeep > > > > > > > > > > > > On Mon, Feb 10, 2014 at 12:20 PM, Jay Kreps <jay.kr...@gmail.com> > > wrote: > > > > > > > > > A few items: > > > > > 1. ConsumerRebalanceCallback > > > > > a. onPartitionsRevoked would be a better name. > > > > > b. We should discuss the possibility of splitting this into two > > > > > interfaces. The motivation would be that in Java 8 single method > > > > interfaces > > > > > can directly take methods which might be more intuitive. > > > > > c. If we stick with a single interface I would prefer the name > > > > > RebalanceCallback as its more concise > > > > > 2. Should subscribe(String topic, int partition) should be > > > > subscribe(String > > > > > topic, int...partition)? > > > > > 3. Is lastCommittedOffset call just a local access? If so it would > be > > > > more > > > > > convenient not to batch it. > > > > > 4. How are we going to handle the earliest/latest starting position > > > > > functionality we currently have. Does that remain a config? > > > > > 5. Do we need to expose the general ability to get known positions > > from > > > > the > > > > > log? E.g. the functionality in the OffsetRequest...? That would > make > > > the > > > > > ability to change position a little easier. > > > > > 6. Should poll(java.lang.Long timeout) be poll(long timeout, > TimeUnit > > > > > unit)? Is it Long because it allows null? If so should we just add > a > > > > poll() > > > > > that polls indefinitely? > > > > > 7. I recommend we remove the boolean parameter from commit as it is > > > > really > > > > > hard to read code that has boolean parameters without named > > arguments. > > > > Can > > > > > we make it something like commit(...) and commitAsync(...)? > > > > > 8. What about the common case where you just want to commit the > > current > > > > > position for all partitions? > > > > > 9. How do you unsubscribe? > > > > > 10. You say in a few places that positions() only impacts the > > starting > > > > > position, but surely that isn't the case, right? Surely it controls > > the > > > > > fetch position for that partition and can be called at any time? > > > > Otherwise > > > > > it is a pretty weird api, right? > > > > > 11. How do I get my current position? Not the committed position > but > > > the > > > > > offset of the next message that will be given to me? > > > > > > > > > > One thing that I really found helpful for the API design was > writing > > > out > > > > > actual code for different scenarios against the API. I think it > might > > > be > > > > > good to do that for this too--i.e. enumerate the various use cases > > and > > > > code > > > > > that use case up to see how it looks. I'm not sure if it would be > > > useful > > > > to > > > > > collect these kinds of scenarios from people. I know they have > > > > sporadically > > > > > popped up on the mailing list. > > > > > > > > > > -Jay > > > > > > > > > > > > > > > On Mon, Feb 10, 2014 at 10:54 AM, Neha Narkhede < > > > neha.narkh...@gmail.com > > > > > >wrote: > > > > > > > > > > > As mentioned in previous emails, we are also working on a > > > > > re-implementation > > > > > > of the consumer. I would like to use this email thread to discuss > > the > > > > > > details of the public API. I would also like us to be picky about > > > this > > > > > > public api now so it is as good as possible and we don't need to > > > break > > > > it > > > > > > in the future. > > > > > > > > > > > > The best way to get a feel for the API is actually to take a look > > at > > > > the > > > > > > javadoc< > > > > > > > > > > > > > > > > > > > > > http://people.apache.org/~nehanarkhede/kafka-0.9-consumer-javadoc/doc/kafka/clients/consumer/KafkaConsumer.html > > > > > > >, > > > > > > the hope is to get the api docs good enough so that it is > > > > > self-explanatory. > > > > > > You can also take a look at the configs > > > > > > here< > > > > > > > > > > > > > > > > > > > > > http://people.apache.org/~nehanarkhede/kafka-0.9-consumer-javadoc/doc/kafka/clients/consumer/ConsumerConfig.html > > > > > > > > > > > > > > > > > > > Some background info on implementation: > > > > > > > > > > > > At a high level the primary difference in this consumer is that > it > > > > > removes > > > > > > the distinction between the "high-level" and "low-level" > consumer. > > > The > > > > > new > > > > > > consumer API is non blocking and instead of returning a blocking > > > > > iterator, > > > > > > the consumer provides a poll() API that returns a list of > records. > > We > > > > > think > > > > > > this is better compared to the blocking iterators since it > > > effectively > > > > > > decouples the threading strategy used for processing messages > from > > > the > > > > > > consumer. It is worth noting that the consumer is entirely single > > > > > threaded > > > > > > and runs in the user thread. The advantage is that it can be > easily > > > > > > rewritten in less multi-threading-friendly languages. The > consumer > > > > > batches > > > > > > data and multiplexes I/O over TCP connections to each of the > > brokers > > > it > > > > > > communicates with, for high throughput. The consumer also allows > > long > > > > > poll > > > > > > to reduce the end-to-end message latency for low throughput data. > > > > > > > > > > > > The consumer provides a group management facility that supports > the > > > > > concept > > > > > > of a group with multiple consumer instances (just like the > current > > > > > > consumer). This is done through a custom heartbeat and group > > > management > > > > > > protocol transparent to the user. At the same time, it allows > users > > > the > > > > > > option to subscribe to a fixed set of partitions and not use > group > > > > > > management at all. The offset management strategy defaults to > Kafka > > > > based > > > > > > offset management and the API provides a way for the user to use > a > > > > > > customized offset store to manage the consumer's offsets. > > > > > > > > > > > > A key difference in this consumer also is the fact that it does > not > > > > > depend > > > > > > on zookeeper at all. > > > > > > > > > > > > More details about the new consumer design are > > > > > > here< > > > > > > > > > > > > > > > > > > > > > https://cwiki.apache.org/confluence/display/KAFKA/Kafka+0.9+Consumer+Rewrite+Design > > > > > > > > > > > > > > > > > > > Please take a look at the new > > > > > > API< > > > > > > > > > > > > > > > > > > > > > http://people.apache.org/~nehanarkhede/kafka-0.9-consumer-javadoc/doc/kafka/clients/consumer/KafkaConsumer.html > > > > > > >and > > > > > > give us any thoughts you may have. > > > > > > > > > > > > Thanks, > > > > > > Neha > > > > > > > > > > > > > > > > > > > > >
