Client Rewrite

Over several years of usage we have run into a number of complaints about the scala client. This is a proposal for addressing these complaints.

This would likely be done in the timeframe of a 0.9 release.

Current Problems and Fixes

Here is a dump of all the problems we have seen and some solutions:

• Move clients to Java to fix scala problems
  • Javadoc
  • Scala version non-compatability
  • Readability by non-scala users
  • Scary stack traces
  • Leakage of scala classes/interfaces into java api
• Code cleanup and embeddability
  • Both producer and consumer code are extremely hard to understand
  • Redo the request serialization layer to avoid all the custom request definition objects
  • Eliminate the "simple" consumer api and have only a single consumer API with the capabilities of both
  • Remove all threads from the consumer
  • Have a separate client jar with no depedencies
• Generalize APIs
  • Producer
    • Give back a return value containing error code, offset, etc
  • Consumer
    • Enable static partition assignment for stateful data systems
    • Enable consumer-driven offset changes.
• Better support non-java consumers
  • Move to a high-level protocol for consumer group management to centralize complexity on the server for all clients
• Improve performance and operability
  • Make the producer fully async to to allow issuing sends to all brokers simultaneously and having multiple in-flight requests simultaneously. This will dramatically reduce the impact of latency on throughput (which is important with replication).
  • Move to server-side offset management will allow us to scale this facility which is currently a big scalability problem for high-commit rate consumers due to zk non scalability.
  • Server-side group membership will be more scalable with number of partitions then the current consumer co-ordination protocol
  • Improve inefficiencies in compression code

The idea would be to roll out the new api as a separate jar, leaving the existing client intact but deprecated for one or two releases before removing the old client. This should allow a gradual migration.

Proposed Producer API

SendResponse send(KafkaMessage... message);


Usage:
Producer producer = new Producer(new ProducerConfig(props));
SendResponse r = producer.send(new KafkaMessage(topic, key, message));
r.onCompletion(new Runnable() {System.out.println("All done")})
r.getOffset()
r.getError()

Changes from current API

• The key and message in KafkaMessage would have type Object instead of parameterized types. The parameterized types have not played well with the fact that different topics may take different types and since the serializer is instantiated via reflection (at runtime) the parametric types add no actual type safety.
• The producer will always attempt to batch data and will always immediately return a SendResponse which acts as a Future to allow the client to await the completion of the request.

Implementation

As today we will have a single background thread that does message collation, serialization, and compression. However this thread will now run an event loop to simultaneously send requests and receive responses from all brokers.

Consumer API

Here is an example of the proposed consumer API:

Consumer consumer = new Consumer(props);
consumer.addTopic("my-topic"); // consume dynamically assigned partitions
consumer.addTopic("my-other-topic", 3); //
long timeout = 1000;
while(true) {
  List<MessageAndOffset> messages = consumer.poll(timeout);
  process(messages);
  consumer.commit(); // alternately consumer.commit(topic) or consumer.commit(topic, partition)
}

As before the consumer group is set in the properties and for simplicity each consumer can belong to only one group.

addTopic is used to change the set of topics the consumer consumes. If the user gives only the topic name it will automatically be assigned a set of partitions based on the group membership protocol below. If the user specifies a partition they will be statically assigned that partition.

The actual partition assignment can be made pluggable (see the proposal on group membership below) by setting an assignment strategy in the properties.

This api allows alternative methods for managing offsets as today by simply disabling autocommit and not calling commit().

Implementation

The client would be entirely single threaded.

The poll() method executes the network event loop, which includes the following:

• Checks for group membership changes
• Sends a heartbeat to the controller if needed
• Issues a fetch request to all brokers for which the consumer currently is consuming
• Reads any available fetch responses
• Issues metadata requests when requests fail to get the new cluster topology

The timeout the user specifies will be purely to ensure we have a mechanism to give control back to the user even when no messages are delivered. It is up to the user to ensure poll() is called again within the heartbeat frequency set for the consumer group. Internally the timeout on our select() may uses a shorter timeout to ensure the heartbeat frequency is met even when no messages are delivered.

Consumer Group Membership Protocol

We will introduce a set of RPC apis for managing partition assignment on the consumer side. This will be based on the prototype here. This set of APIs is orthogonal to the APIs for producing and consuming data, it is responsible for group membership.

My proposal would be to actually not handle partition assignment at all. The only facility we need to offer is a group membership facility. If all consumers agree on who exists the consumer client implementation can implement any partition assignment it likes.

The use of this protocol would be as follows:

• on startup the consumer issues a list_groups(my_group) to find out the location of the controller for its group (each server is the controller for some groups)
• knowing where the controller is, it connects and issues a join_group command, then it awaits a begin_group_change and group_changed message
• on receiving the join_group command the server issues a begin_group_change message to all alive group members and waits for an acknowledgement from each consumer. The two-phase group change ensures that it is never the case that two consumers have different views of the group at the same time.
• on receiving acknowledgements from all consumers of the group membership change the controller sends the a group_changed message to all the new group members
• on receiving the new group membership the consumer is now able to start consuming its partitions and must now start sending heartbeat messages back to the controller with the current generation id.

Low-level RPC API

TBD we need to design an underlying client network abstraction that can be used to send requests. This should ideally handle metadata and addressing by broker id, multiplex over multiple connections, and support both blocking and non-blocking operations.

Something like

KafkaConnection connection = new KafkaConnection(bootstrapUrl, socket_props);
List<Responses> connection.poll(timeout, Request...requests);

This is an async api so the responses responses returned have no relationship to the request sent. This is meant to be called from within an event loop--i.e. each call to poll corresponds to one iteration of the select() loop. poll() with no request simply checks for any new responses to previous requests or non-client-initiated communication.

Misc Notes and questions

Currently we do a lot of logging and metrics recording in the client. I think instead of handling reporting of metrics and logging we should instead incorporate this feedback in the client apis and allow the user to log or monitor this in the manner they chose. This will avoid dependence on a particular logging or metrics library and seems like a good policy.

It is not clear how to support changing the offset either manually or programmatically while consumption is happening. Simply using the api to set the offset will likely collide with commit calls from the active consumer. Perhaps this is not needed?

How many jars should we have? I think we could do either

• Option A: kafka-clients.jar and kafka-server.jar or
• Option B: kakfa-common.jar, kafka-producer.jar, kafka-consumer.jar, and kafka-server.jar

I prefer Option A as it is simpler if we add an AdminApi--we won't need to introduce a whole new jar for it.