kafka-websocket

kafka-websocket is a simple websocket server interface to the kafka distributed message broker. It supports clients subscribing to topics, including multiple topics at once, and sending messages to topics. Messages may be either text or binary, the format for each is described below.

A client may produce and consume messages on the same connection.

Consuming from topics

Clients subscribe to topics by specifying them in a query parameter when connecting to kafka-websocket:

/v2/broker/?topics=my_topic,my_other_topic

If no topics are given, the client will not receive messages. The format of messages sent to clients is determined by the subprotocol negotiated: kafka-text or kafka-binary. If no subprotocol is specified, kafka-text is used.

By default, a new, unique group.id is generated per session. The group.id for a consumer can be controlled by passing a group.id as an additional query parameter: ?group.id=my_group_id

Producing to topics

Clients publish to topics by connecting to /v2/broker/ and sending either text or binary messages that include a topic and a message. Text messages may optionally include a key to influence the mapping of messages to partitions. A client need not subscribe to a topic to publish to it.

Message transforms

By default, kafka-websocket will pass messages to and from kafka as is. If your application requires altering messages in transit, for example to add a timestamp field to the body, you can implement a custom transform class. Transforms extend us.b3k.kafka.ws.transforms.Transform and can override the initialize methods, or the transform methods for text and binary messages.

Transforms can be applied to messages received from clients before they are sent to kafka (inputTransform) or to messages received from kafka before they are sent to clients (outputTransform). See conf/server.properties for an example of configuring the transform class.

Binary messages

Binary messages are formatted as:

[topic name length byte][topic name bytes (UTF-8)][message bytes]

Text messages

Text messages are JSON objects with two mandatory attributes: topic and message. They may also include an optional key attribute:

{ "topic" : "my_topic", "message" : "my amazing message" }

{ "topic" : "my_topic", "key" : "my_key123", "message" : "my amazing message" }

Configuration

See property files in conf/

TLS/SSL Transport

kafka-websocket can be configured to support TLS transport between client and server (not from kafka-websocket to kafka). Client certificates can also be used, if desired. Client auth can be set to none, optional, or required, each being, I hope, self-explanatory. See conf/server.properties for various configuration options.

Docker

Build a Docker image using the source code in the working directory:

docker build -t kafka-websocket .

After the Docker image is finished building, run it with:

docker run -it -p 7080:7080 kafka-websocket

License

kafka-websocket is copyright 2014 Benjamin Black, and distributed under the Apache License 2.0.