websocket

WebSocket-over-HTTP Example for Bun

This example illustrates the use of GRIP to handle WebSockets using a Bun application as the backend.

For more information about WebSocket-over-HTTP, refer to this page.

Running the example

For instructions on setting up and running the example, either locally or using Fastly Fanout as the GRIP proxy, refer to the README file in the parent directory.

This example also requires wscat, which can be obtained here.

Testing the example

After you have set up Pushpin and started the application, test the example following these steps.

NOTE: If you are using Fastly Fanout as the GRIP proxy, follow these steps, but replace 127.0.0.1:7999 with the public URL of your Fanout Forwarding service.

1. Open a new terminal window, and type the following:

wscat -c http://127.0.0.1:7999/api/websocket

You should see the following prompt:

Connected (press CTRL+C to quit)
> 

wscat is now connected to the GRIP proxy over a WebSocket. The WebSocket is subscribed to a channel internally called test.

2. Enter a line of text at the prompt. The application will echo it back to you through the WebSocket. For example, typing asdf:

Connected (press CTRL+C to quit)
> asdf
< asdf
>

3. Open a separate terminal window, and type the following:

curl -X POST -d "Hello" "https://127.0.0.1:7999/api/broadcast"

This publishes the given message (to the channel test). You should see the message Hello appear in the WebSocket connection held by the first terminal.

How it works

For an explanation of the common startup and initialization code, as well as validating the GRIP header, refer to the README file in the parent directory.

Before starting the request handling, in addition to checking the GRIP status, this example obtains the WebSocket Context. To do this, it uses the isWsOverHttp and getWebSocketContextFromReq functions.

let wsContext = null;
if (gripStatus.isProxied && isWsOverHttp(req)) {
    wsContext = await getWebSocketContextFromReq(req);
}

The request handling section of this example goes on to handle two routes:

1. A POST request at /api/websocket

WebSocket-over-HTTP always uses the POST method to represent WebSocket messages converted to HTTP requests.

This endpoint is intended to be called through your configured GRIP proxy.

The handler checks gripStatus.isProxied to make sure we are being run behind a valid GRIP proxy. This value will be false if the request did not come through a GRIP proxy, or if the signature validation failed.

if (!gripStatus.isProxied) {
   // emit an error
}

It also checks wsContext to make sure this is indeed a WebSocket-over-HTTP message.

if (wsContext == null) {
    return new Response(
        '[not a websocket request]\n',
        {
            status: 400,
            headers: {
                'Content-Type': 'text/plain',
            },
        },
    );
}

If successful, then the handler goes on to process the incoming WebSocket message(s). The WebSocket context includes a buffer of queued-up incoming WebSocket messages as well as a buffer to queue outgoing WebSocket messages.

First, the context is checked for the presence of an OPEH message (wsContext.isOpening()). If so, then the WebSocket is accepted (wsContext.accept() queues up an outgoing OPEN message). It also subscribes the WebSocket to the test channel.

if (wsContext.isOpening()) {
    wsContext.accept();
    wsContext.subscribe('test');
}

Next, a while loop is used to iterate the buffer of incoming WebSocket messages.

Note that while this loop may deceivingly appear to run for the duration of the WebSocket connection, this is not the case. The loop runs against the current batch of incoming messages and exits, while the WebSocket connection with the client is held by the GRIP proxy.

The wsContext.recv() function returns the next item in the queue.

while (wsContext.canRecv()) {
    const message = wsContext.recv();
    // handle message ..
}

If the incoming message was 'CLOSE', this function would have returned null. The application responds by calling wsContext.close() to cleanly close the connection and leave the loop.

if (message == null) {
    wsContext.close();
    break;
}

In this simple example, the incoming message is simply echoed back to the client.

wsContext.send(message);

Function such as wsContext.accept(), wsContext.send(), and wsContext.close() create outgoing messages and add them to the outgoing messages list.

After the loop is exited, these outgoing messages are serialized into the response body.

const events = wsContext.getOutgoingEvents();
const responseBody = encodeWebSocketEvents(events);

Finally, the response is generated and returned, using wsContext.toHeaders() in the response headers.

return new Response(
    responseBody,
    {
        status: 200,
        headers: wsContext.toHeaders(),
    },
);

The above is the mechanism needed to communicate over a WebSocket using GRIP. At this point, the connection between your backend and the GRIP proxy ends, with the GRIP proxy holding the WebSocket connection with the client. Additional client-side messages over the same WebSocket will arrive at the same endpoint.

2. A POST request at /api/broadcast

It's also possible to send messages to an entire channel.

This handler starts by checking to make sure the content type header specifies that the body is of type text/plain. Afterward, the handler reads the request body into a string.

if (request.headers.get('content-type')?.split(';')[0] !== 'text/plain') {
   // emit an error
}
const body = await request.text();

Next, the handler proceeds to package the message in a WebSocketMessageFormat object and then sending them to a call to publisher.publish to the test channel. Any listener on this channel, such as those that have opened a WebSocket through the endpoint described above, will receive the message.

await publisher.publishFormats('test', new WebSocketMessageFormat(body));

Finally, it returns a simple success response message and ends.

return new Response(
    'Ok\n',
    {
        status: 200,
        headers: {
            'Content-Type': 'text/plain',
        },
    },
);