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From Neha Narkhede <neha.narkh...@gmail.com>
Subject New Consumer API discussion
Date Mon, 10 Feb 2014 18:54:42 GMT
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

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