Define WebSocket endpoint and Lambda handler functions.


This app.arc file defines both HTTP and WebSocket endpoints:


  "architect": {
    "app": "myapp"
    "ws": {}
  "start": "npx sandbox",
  "dependencies": {
    "@architect/architect": "latest"
app: testapp

ws: ~

Running arc create generates the following WebSocket handlers, each mapping to a required WebSocket event (referred to as an action):

├── src/ws/
│   ├── connect
│   ├── default
│   └── disconnect
├── app.arc
└── package.json

Each handler responds to WebSocket actions from clients. In the payload delivered to the function there is a connectionId that uniquely identifies a client. Use this connectionId to send messages to the correct client (if needed).

Default actions

Each action handler created by the @ws pragma receives events from WebSocket clients.

  • connect - Invoked when a WebSocket client connects to the application
  • default - Invoked when a WebSocket client sends any (un-routed) message to the application
  • disconnect - Invoked when a WebSocket client disconnects from your application

Custom actions

In addition to the three default WebSocket actions (connect, default, disconnect), you can create custom actions to be routed via message payloads like so:


These will generate additional handlers in your src/ws dir (e.g. src/ws/some-custom-action/). Custom action invocation routing is performed by sending a JSON payload with the corresponding action property; for example:

// Assuming the client is already connected
  whatever: 'some data'
})) // Invokes `default`

  action: 'some-custom-action',
  whatever: 'related data'
})) // Invokes `some-custom-action`


WebSocket event payloads may contain a fair bit of data, but here are a few key bits:

Argument Description
req The WebSocket request payload
req.requestContext.connectionId An ID that uniquely identifies the client
req.body Body payload sent by the client (if present)

Send messages

To publish a message to a WebSocket client you can use Arc’s runtime library @architect/functionsws.send method. You can call this method from any of your application’s functions so long as you have a valid connectionId.

Docs: Node.js | Ruby | Python