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Gradle setup

    compile group: "org.http4k", name: "http4k-core", version: "3.241.0"
    compile group: "org.http4k", name: "http4k-server-jetty", version: "3.241.0"
    compile group: "org.http4k", name: "http4k-client-websocket", version: "3.241.0"
    compile group: "org.http4k", name: "http4k-format-jackson", version: "3.241.0"

http4k provides Websocket support using a simple, consistent, typesafe, and testable API on supported server backends (see above). Websocket communication consists of 3 main concepts:

  1. WsHandler - represented as a typealias: WsHandler = (Request) -> WsConsumer?. This is responsible for matching an HTTP request to a websocket.
  2. WsConsumer - represented as a typealias: WsConsumer = (WebSocket) -> Unit. This function is called on connection of a websocket and allow the API user to react to events coming from the connected websocket.
  3. WsMessage - a message which is sent or received on a websocket. This message can take advantage of the typesafety accorded to other entities in http4k by using the Lens API. Just like the http4k HTTP message model, WsMessages are immutable data classes.

Websocket as a Function

The simplest possible Websocket can be mounted as a WsConsumer function onto a server with:

{ ws: Websocket -> ws.send(WsMessage("hello")) }.asServer(Jetty(9000)).start()

Mixing HTTP and Websocket services

Both Websockets and Http handlers in http4k are routed using a similar path-based API. We combine them into a single PolyHandler which can handle both http:// and ws://, and then convert to a Server as usual:

Automarshalling Websockets messages

Using the standard Lens API, we can auto-convert Websocket messages on and off the wire. This example uses the Jackson for the marshalling:

Testing Websockets

http4k provides Websockets that are both typesafe (via the Lens API), and testable. Both WsHandlers and PolyHandlers are convertible to a WsClient which provides a synchronous API for testing reactions to Websocket events in an offline environment.

In the below example, we have gone one step further - defining a contract test case and then providing 2 implementations of it - one for unit-testing (in memory), one using a server. http4k provides clients with an identical interface for both cases, meaning it's possible reuse the same test logic: