WebSocket: Publish message to clients

Continuation of the discussion that we have at #41

As using combining DynamoDB and WebSocket is a common use case, laconia can make users life easier by publishing the below public contracts.

Discussions

• Is broadcasting a common scenario?
• If WebSocket is used to reply to individual clients, how do we know which client do we want to retrieve? It looks like there’s an additional info from the user land that we need to store if we want to support this

On connect / disconnect

const { websocketHandler } = require('@laconia/websocket')

// Understands event type of $connect and $disconnect, automatically store and delete connection ID in DynamoDB
exports.handler = websocketHandler('WEBSOCKET_CONNECTION_TABLE_NAME')

On sending message API is inspired from: https://github.com/websockets/ws

const { createWss } = require("@laconia/websocket");

// wss object is injected
const app = async (event, { wss }) => {
  // Automatically get all client objects from DynamoDB
  const clients = await wss.getClients()

  // Sends data to a client, wraps apigwManagementApi
  // What's the best way to find the client here?
  await clients[0].send("something")

  // Broadcast, also wraps apigwManagementApi
  await Promise.all(clients.map(c => c.send('something')))
  await wss.broadcast('something')
};

exports.handler = laconia(app).register(createWss('WEBSOCKET_CONNECTION_TABLE_NAME'));

[ceilfors]

This diagram from AWS should give a high level overview:

[hugosenari]

We are not dependant of serverless.js but is currently most easily way to deploy lambda and serverless/components have two interesting components PubSub (push message with websocket isn’t too different) and chat-app (that is what we tried to achieve) we could mashup both.

off-topic: maybe we could abstract some other components. 🤔