Web Socket API

Lenses provides a rich set of WebSocket APIs as well to interact with Apache Kafka data in real-time. It can be used to build powerful Front-End Web applications with full Lenses SQL support.

To speed up integration we provide a Javascript lib for Kafka developers . if you are using React/Redux you can get going fast and focus on your business requirements.

npm install rxjs redux-lenses-streaming --save

Introduction 

The API is straightforward. Once a WebSocket connection has been opened, the client needs to make sure it follows the protocol. Here is the template for each message the client can send to the back end:

    {
        "type":" SUBSCRIBE/UNSUBSCRIBE/PUBLISH/COMMIT/LOGIN",
        "content":"The json text for the specific request",
        "correlationId":1000,
        "authToken" : "Authorization token or empty"
    }

Below you can find the list of supported functionality:

• A client can and should AUTHENTICATE first to obtain the token allowing all other requests to be accepted.
• A client can SUBSCRIBE to a topic via SQL. Please check the Lenses Kafka SQL section for the full details on how to set filters and use functions.
• A client can UNSUBSCRIBE from a topic.
• A client can PUBLISH messages to a topic. The current version supports only string/json. In the future, we will add support for Avro.
• A client can COMMIT the (topic, partition) offsets.
• A client subscription must specify the decoder type. This allows reading correctly the content of the Kafka Message Key/Value parts.

The following decoders are supported

• STRING - the byte[] payload received from Kafka is read as a String
• INT - the byte[] payload received from Kafka is read as an Int
• LONG - the byte[] payload received from Kafka is read as a Long
• AVRO - the byte[] payload received from Kafka is read as an Avro
• JSON - the byte[] payload received from Kafka is read as an Avro
• BINARY - the byte[] payload received from Kafka is kept as it is in Binary format

The response received from the back end follows this template:

    {
        "correlationId": Long,
        "type"  : "ERROR/INVALIDREQUEST/KAFKAMSG/HEARTBEAT/SUCCESS",
        "content": String
    }

Protocol Definition 

All requests made are constrained by user permissions on the back end. If the user has only Read access then publishing a record to a topic will not be allowed. If the user has only No-Data user role, then retrieving messages from Kafka will not be allowed either. See the security section for role definitions.

Login 

The first thing to do when a WebSocket connection has been opened is to obtain an authorization token. To do so the client will have to send the following LOGIN request format:

    {
       "type" : "LOGIN",
       "content" : "{
                     "user" : String,
                     "password" : String,
                    }",
       "correlationId": Long,
       "authToken": String
    }

Note: The content field value is a string containing a JSON!

A successful login response will look like this:

    {
      "correlationId" : Long,
      "type" : "SUCCESS",
      "content" : String
    }

If the user or password provided is not correct, the client will receive an error response. In this case, the response format looks like this:

    {
      "correlationId": Long,
      "type" : "ERROR",
      "content": String
    }

Publishing 

In order to publish a message to a topic the client has to send the following request:

    {
       "type" : "PUBLISH",
       "content" : "{
                     "topic" : String,
                     "key" : String,
                     "value" : String
                    }",
       "correlationId": Long,
       "authToken": String
    }

Note: Remember, the content for key/value are sent to the target Kafka topic are sent as String! The content field value is a string containing a json!

Subscription 

To receive messages from a Kafka topic the client has to send a SUBSCRIBE request.

    {
       "type" : "SUBSCRIBE",
       "content" : "{
           "sqls" : [
               String,
               String
            ]
       }",
       "correlationId" : Long,
       "authToken" : String
    }

SELECT *
FROM $TOPIC
WHERE  _ktype='INT/LONG/JSON/STRING/AVRO'
       AND _vtype='INT/LONG/JSON/STRING/AVRO'
       [AND ...]

You can provide more than one LSQL statement if you want to subscribe to more than 1 topic. Please visit the Lenses Kafka SQL section for full details on what it supports. The response from the back end can be a SUCCESS or an ERROR.

Once the subscription has been successful, messages arriving in the Kafka topic(-s) and matching the filter will be delivered. A message received by the client will have this structure:

    {
      "content": [
        {
           "key" : "...",
           "value" : "{...}",
           "topic" : "topicA",
           "partition" : Int,
           "offset" : Long,
           "timestamp" : Long
        },
        ..
      ],
      "correlationId": Long,
      "type" : "KAFKAMSG"
    }

Note: The timestamp field requires Kafka 0.10.2+ and correct broker settings/or client publishing the timestamp.

Un-subscribe 

A client can choose at any point to stop receiving messages from a given topic(-s). In order to do so it has to send the following message:

    {
       "type" :"UNSUBSCRIBE",
       "content": "{
          "topics": [
             "topic":String,
             ..
          ]
       }",
       "correlationId": Long,
       "authToken" : String,
    }

Although the subscription allows you to specify via LSQL the partitions to subscribe to, the unsubscribe does not support selective partition dropping from the subscription.

Note: Executing a subscribe call with a new LSQL for a topic already in the subscription, will unsubscribe first and subscribe again.

Offsets Commit 

The JavaScript client can decide when to commit the offset in Kafka. This way, when the client reopens a connection and resubscribes to the same Kafka topic it will receive the Kafka messages from where it left it.

To commit offsets the client has to send the following message structure:

    {
       "type" :"COMMIT",
       "content": "{
          "commits": [
             {
                "topic": String,
                "partition": Int,
                "offset" : Long
             },
             ...
          ]
       }",
       "correlationId": Long,
       "authToken" : String
    }

Since the commits field is an array, more than one (topic, partition, offset) tuple can be provided at once.

Note: The content field value is a string containing a json!

Heartbeat 

The REST API makes sure it keeps the connection open in case there is no data going back and forth between the client and the back end. As a result the client should be able to handle messages with the following structure:

    {
      "type" : "HEARTBEAT"
    }

When such messages are received the client can discard them.