Kayrock

Aims to provide an idiomatic Elixir interface to the Kafka protocol.

This work is based on https://github.com/klarna/kafka_protocol for Erlang, which is used in https://github.com/klarna/brod. It is built to work with https://github.com/kafkaex/kafka_ex though there is no reason why it couldn't be used elsewhere.

Basic architecture and usage

Kayrock generates structs for every version of every message in the Kafka protocol. It does this by converting the Erlang-based schema descriptions in :kpro_schema from the kafka_protocol library and is therefore limited to messages and versions included there. Each request message has a serializer (that generates iodata), and each response message has a deserializer. There is also a protocol, Kayrock.Request, that has an implementation created for each request. The Kayrock.Request protocol defines a serializer as well as a response deserializer factory.

This repo includes a mix task, mix gen.kafka_protocol, that produces the code in lib/generated. I chose to check the generated code into the repository to simplify its use. End users should not need to run the mix task unless they are doing development on Kayrock itself.

The generated structs are namespaced as follows: Kayrock.<api>.V<version number>.[Request|Response]. For example, the version 4 fetch api corresponds to Kayrock.Fetch.V4.Request and Kayrock.Fetch.V4.Response.

An example of basic usage, taken (and modified) from Kayrock.broker_sync_call/2:

alias Kayrock.Request

request = %Kayrock.ApiVersions.V1.Request{client_id: "some_client", correlation_id: 0}

# wire_protocol will be iodata representing the serialized request
wire_protocol = Request.serialize(request)

# implementation-specific network request/response handling
:ok = BrokerConnection.send(broker_pid, wire_protocol)
{:ok, resp} = BrokerConnection.recv(broker_pid)

# response_deserializer will be a function that accepts binary data and
# returns {%Kayrock.ApiVersions.V1.Response{}, rest} where rest is any
# extra binary data in the response
response_deserializer = Request.response_deserializer(request)
{deserialized_resp, _} = response_deserializer.(resp)

# deserialized_resp will be a %Kayrock.ApiVersions.V1.Response{} representing
# the api versions supported by the broker

Relationship to other libraries

Kayrock uses only the kpro_schema module from kafka_protocol. kafka_protocol provides quite a lot of functionality (especially when used as part of brod). This repo chooses to limit the integration surface between itself and kafka_protocol because

1. kafka_protocol produces Erlang records whereas Kayrock aims to provide Elixir structs.
2. kafka_protocol provides network-level implementation whereas Kayrock is intended to only provide serialization and deserialization of the protocol.
3. kpro_schema is itself automatically generated from the Java source code of Kafka, meaning that Kayrock could feasibly reproduce this without needing an intermediate dependency.

This is in no way meant to detract from the great work that the kafka_protocol/brod team has done. I have simply chosen a different path here.

You will notice that in this repo there is also a lightweight implementation of a Kafka client. This serves a few purposes:

1. To help with the development of the Kayrock protocol API.
2. To allow for testing against a server without creating a circular dependency on KafkaEx.
3. Because I want to mess around with it.

Regardless, it is not a production-ready implementation. I would refer you to KafkaEx or brod for that.