Suave is a simple web development F# library providing a lightweight web server and a set of combinators to manipulate route flow and task composition.

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Realtime Messages With WebSockets

It’s easy to set up WebSockets with Suave.

First, define a function that takes WebSocket and HttpContext typed parameters, and returns a socket computation expression:

open Suave.Sockets
open Suave.Sockets.Control
open Suave.WebSocket

let ws (webSocket : WebSocket) (context: HttpContext) =
    socket {
      ...
    }

Next, use the read and send function to receive and send messages to the clients:

    socket {
        let mutable loop = true

        while loop do
            let! msg = webSocket.read()

            match msg with
            | (Text, data, true) ->
                let str = UTF8.toString data
                let response = sprintf "response to %s" str
                let byteResponse =
                    response
                    |> System.Text.Encoding.ASCII.GetBytes
                    |> ByteSegment
                do! webSocket.send Text byteResponse true

            | (Close, _, _) ->
                let emptyResponse = [||] |> ByteSegment
                do! webSocket.send Close emptyResponse true
                loop <- false

            | _ -> ()
      }

Then, use the handShake function to fit it in your web server:

let app : WebPart =
    choose [
      path "/websocket" >=> handShake ws
      GET >=> choose [ path "/" >=> file "index.html"; browseHome ]
      NOT_FOUND "Found no handlers." ]

The complete example can be found here.

Handling connection errors

By default the socket computation expression handles any errors transparently on both writing and reading from the websocket shutting down the connection.

You may want to add your own additional error handling logic to catch and handle any errors raised when reading and writing from a websocket. Some scenarios where this may be useful:

  • Alerting the rest of the application that a connection is closed.
  • Unsubscribing and/or shutting down processes used by the websocket connection.
  • Custom logging of the error.

Example code can be found here.