Hi, I am following this link:
https://cwiki.apache.org/confluence/display/KAFKA/0.8.0+SimpleConsumer+Example for a consumer design using kafka_2.9.2 version 0.8.1.1 (what I can find in maven central). I have a couple of questions about the consumer. I checked the archives and didn't see these exact questions asked already, but I may have missed them -- I apologize if that is the case. When I create a consumer I give it a consumer ID. I assumed that it would store my consumer's name as well as the last readOffset in zookeeper, but looking in zookeeper that doesn't seem to be the case. So it seems to me that when my consumers come up they need to either get the entire history from the start of time (which could take a long time, as I have 14 day durability); or else they need to somehow keep track of the read offset themselves. I have redis in my system already, so I have the choice of keeping track of this in either redis or zookeeper. It seems like zookeeper would be a better idea. Am I right, though, that the SimpleConsumer and the example I linked above don't keep track of this, so if I want to do that I would have to do it myself? Second question: in the example consumer, there is an error handler that checks if you received an OffsetOutOfRange response from kafka. If so, it gets a new read offset .LatestTime(). My interpretation of this is that you have asked it for an offset which doesn't make sense, so it just scans you to the end of the stream. That's a guaranteed data loss. A simple alternative would be to take the beginning of the stream, which if you have idempotent processing would be fine - it would be a replay - but it could take a long time. I don't know for sure what would cause you to get an OffsetOutOfRange - the only thing I can really think of is that someone has changed the underlying stream on you (like they deleted and recreated it and didn't tell all the consumers). I guess it's possible that if I have a 1 day stream durability and I stop my consumer for 3 days that it could ask for a readOffset that no longer exists; it's not clear to me whether or not that would result in an OffsetOutOfRange error or not. Does that all make sense? Third question: I set a .maxWait(1000) interpreting that to mean that when I make my fetch request the consumer will time out if there are no new messages in 1 second. It doesn't seem tow ork - my call to consumer.fetch() seems to return immediately. Is that expected? Final question: just to confirm: new FetchRequestBuilder().addFetch( topic, shardNum, readOffset, FETCH_SIZE ) FETCH_SIZE is in bytes, not number of messages, so presumably it fetches as many messages as will fit into that many byte buffer? Is that right? Thanks. Christopher Piggott Sr. Staff Engineer Golisano Institute for Sustainability Rochester Institute of Technology
