examples: add a WebSocket example

There is nothing really to do in order to support events over websockets.
Since a `CloudEvent` can easily be represented in full with JSON, it can
be sent over a websocket as `event.toString()`.

Fixes: #156

Author: lance

examples/websocket/README.md

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+# WebSocket Example
+
+This example shows how simple it is to use CloudEvents over a websocket
+connection. The code here shows backend communication from two server
+side processes. However, this could be adapted to work within a browser.
+
+## Running the Example
+
+This simple project consists of a server and a client. The server receives
+`CloudEvents` from the client over a local websocket connection.
+
+
+To get started, first install dependencies.
+
+```sh
+npm install
+```
+
+### Server
+The server opens a websocket and waits for incoming connections. It expects that any
+messages it receives will be a CloudEvent. When received, it prints the string
+representation of the event, and then sends an acknowledgement message back over the
+websocket to the connected client. To start the server, run `node server.js`.
+
+### Client
+Upon start, the client prompts a user for a zip code, then fetches the current weather
+for that zip code. It then prints the weather to the console, and constructs a `CloudEvent`
+with the body of the Weather API response as the event data. It sends this event to
+the server via its websocket connection as JSON. When it has received an ACK from the server
+it prints the acknowledge message, then asks again for a zip code.
+
+To terminate the client or server, type CTL-C.

examples/websocket/client.js

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+/* eslint-disable no-console */
+const readline = require("readline");
+const { CloudEvent } = require("cloudevents-sdk");
+
+const WebSocket = require("ws");
+const got = require("got");
+
+const api = "https://api.openweathermap.org/data/2.5/weather";
+const key = "506e9b3eef5b37d0bc745a5c9529788d";
+const ws = new WebSocket("ws://localhost:8080");
+
+const rl = readline.createInterface({
+  input: process.stdin,
+  output: process.stdout
+});
+
+rl.on("close", (_) => console.log("\n\nConnection closed! Press CTL-C to exit."));
+
+function ask() {
+  rl.question("Would you like to see the current weather? Provide a zip code: ", function (zip) {
+    fetch(zip);
+  });
+}
+
+function fetch(zip) {
+  const query = `${api}?zip=${zip}&appid=${key}&units=imperial`;
+  got(query).then((response) => {
+    print(response.body);
+    deliver(response.body);
+  }).catch((err) => {
+    console.error(err.message);
+    ask();
+  });
+}
+
+function print(data) {
+  data = JSON.parse(data);
+  console.log(`\nThe current weather for ${data.name}: ${data.weather[0].main}
+------------------------------------------
+With ${data.weather[0].description}, the temperature is ${Math.round(data.main.temp)}F
+and the wind is blowing at ${Math.round(data.wind.speed)}mph.`);
+}
+
+function deliver(event) {
+  console.log("\nSending weather data to remote server");
+  ws.send(new CloudEvent({
+    dataContentType: "application/json",
+    type: "current.weather",
+    source: api,
+    data: event
+  }).toString());
+}
+
+ws.on("message", function incoming(data) {
+  console.log(`Receieved response from server:\n${data}\n\n`);
+  ask();
+});
+
+ask();
+
+

examples/websocket/package.json

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+{
+  "name": "websocket-cloudevents",
+  "version": "0.0.1",
+  "description": "An example application that sends and receives CloudEvents over a websocket",
+  "main": "server.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1",
+    "start": "node server.js"
+  },
+  "keywords": [
+    "cloudevents",
+    "example",
+    "websocket"
+  ],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "cloudevents-sdk": "^2.0.2",
+    "got": "^11.3.0",
+    "ws": "^7.3.0"
+  }
+}

examples/websocket/server.js

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+/* eslint-disable no-console */
+const { CloudEvent } = require("cloudevents-sdk");
+const WebSocket = require("ws");
+const wss = new WebSocket.Server({ port: 8080 });
+
+console.log("WebSocket server started. Waiting for events.");
+
+wss.on("connection", function connection(ws) {
+  console.log("Connection received");
+  ws.on("message", function incoming(message) {
+    const event = new CloudEvent(JSON.parse(message));
+    console.log(`Message received: ${event.toString()}`);
+
+    ws.send(`Got ${event.type} from ${event.source} id ${event.id}`);
+  });
+});