Build a real-time code-streaming app by using Socket.IO and host it on Azure

Article
08/25/2023

Building a real-time experience like the co-creation feature in Microsoft Word can be challenging.

Through its easy-to-use APIs, Socket.IO has proven itself as a library for real-time communication between clients and a server. However, Socket.IO users often report difficulty around scaling Socket.IO's connections. With Web PubSub for Socket.IO, developers no longer need to worry about managing persistent connections.

Overview

This article shows how to build an app that allows a coder to stream coding activities to an audience. You build this application by using:

Monaco Editor, the code editor that powers Visual Studio Code.
Express, a Node.js web framework.
APIs that the Socket.IO library provides for real-time communication.
Host Socket.IO connections that use Web PubSub for Socket.IO.

The finished app

The finished app allows the user of a code editor to share a web link through which people can watch the typing.

Screenshot of the finished code-streaming app.

To keep the procedures focused and digestible in around 15 minutes, this article defines two user roles and what they can do in the editor:

A writer, who can type in the online editor and the content is streamed
Viewers, who receive real-time content typed by the writer and can't edit the content

Architecture

Item	Purpose	Benefits
Socket.IO library	Provides a low-latency, bidirectional data exchange mechanism between the back-end application and clients	Easy-to-use APIs that cover most real-time communication scenarios
Web PubSub for Socket.IO	Hosts WebSocket or poll-based persistent connections with Socket.IO clients	Support for 100,000 concurrent connections; simplified application architecture

Diagram that shows how the Web PubSub for Socket.IO service connects clients with a server.

Prerequisites

To follow all the steps in this article, you need:

An Azure account. If you don't have an Azure subscription, create an Azure free account before you begin.
The Azure CLI (version 2.29.0 or later) or Azure Cloud Shell to manage Azure resources.
Basic familiarity with Socket.IO's APIs.

Create a Web PubSub for Socket.IO resource

Use the Azure CLI to create the resource:

az webpubsub create -n <resource-name> \
                    -l <resource-location> \
                    -g <resource-group> \
                    --kind SocketIO \
                    --sku Free_F1

Get a connection string

A connection string allows you to connect with Web PubSub for Socket.IO.

Run the following commands. Keep the returned connection string somewhere, because you'll need it when you run the application later in this article.

az webpubsub key show -n <resource-name> \ 
                      -g <resource-group> \ 
                      --query primaryKey \
                      -o tsv

Write the application's server-side code

Start writing your application's code by working on the server side.

Build an HTTP server

Create a Node.js project:

mkdir codestream
cd codestream
npm init

Install the server SDK and Express:

npm install @azure/web-pubsub-socket.io
npm install express

Import required packages and create an HTTP server to serve static files:

/*server.js*/

// Import required packages
const express = require('express');
const path = require('path');

// Create an HTTP server based on Express
const app = express();
const server = require('http').createServer(app);

app.use(express.static(path.join(__dirname, 'public')));

Define an endpoint called /negotiate. A writer client hits this endpoint first. This endpoint returns an HTTP response. The response contains an endpoint that the client should use to establish a persistent connection. It also returns a room value that the client is assigned to.

/*server.js*/
app.get('/negotiate', async (req, res) => {
    res.json({
        url: endpoint
        room_id: Math.random().toString(36).slice(2, 7),
    });
});

// Make the Socket.IO server listen on port 3000
io.httpServer.listen(3000, () => {
    console.log('Visit http://localhost:%d', 3000);
});

Create the Web PubSub for Socket.IO server

Import the Web PubSub for Socket.IO SDK and define options:

/*server.js*/
const { useAzureSocketIO } = require("@azure/web-pubsub-socket.io");

const wpsOptions = {
    hub: "codestream",
    connectionString: process.argv[2]
}

Create a Web PubSub for Socket.IO server:

/*server.js*/

const io = require("socket.io")();
useAzureSocketIO(io, wpsOptions);

The two steps are slightly different from how you would normally create a Socket.IO server, as described in this Socket.IO documentation. With these two steps, your server-side code can offload managing persistent connections to an Azure service. With the help of an Azure service, your application server acts only as a lightweight HTTP server.

Implement business logic

Now that you've created a Socket.IO server hosted by Web PubSub, you can define how the clients and server communicate by using Socket.IO's APIs. This process is called implementing business logic.

After a client is connected, the application server tells the client that it's logged in by sending a custom event named login.
```
/*server.js*/
io.on('connection', socket => {
    socket.emit("login");
});
```
Each client emits two events that the server can respond to: joinRoom and sendToRoom. After the server gets the room_id value that a client wants to join, you use socket.join from Socket.IO's API to join the target client to the specified room.
```
/*server.js*/
socket.on('joinRoom', async (message) => {
    const room_id = message["room_id"];
    await socket.join(room_id);
});
```

After a client is joined, the server informs the client of the successful result by sending a message event. When the client receives a message event with a type of ackJoinRoom, the client can ask the server to send the latest editor state.

/*server.js*/
socket.on('joinRoom', async (message) => {
    // ...
    socket.emit("message", {
        type: "ackJoinRoom", 
        success: true 
    })
});

/*client.js*/
socket.on("message", (message) => {
    let data = message;
    if (data.type === 'ackJoinRoom' && data.success) {
        sendToRoom(socket, `${room_id}-control`, { data: 'sync'});
    }
    // ... 
});

When a client sends a sendToRoom event to the server, the server broadcasts the changes to the code editor state to the specified room. All clients in the room can now receive the latest update.

socket.on('sendToRoom', (message) => {
    const room_id = message["room_id"]
    const data = message["data"]

    socket.broadcast.to(room_id).emit("message", {
        type: "editorMessage",
        data: data
    });
});

Write the application's client-side code

Now that the server-side procedures are finished, you can work on the client side.

Initial setup

You need to create a Socket.IO client to communicate with the server. The question is which server the client should establish a persistent connection with. Because you're using Web PubSub for Socket.IO, the server is an Azure service. Recall that you defined a /negotiate route to serve clients an endpoint to Web PubSub for Socket.IO.

/*client.js*/

async function initialize(url) {
    let data = await fetch(url).json()

    updateStreamId(data.room_id);

    let editor = createEditor(...); // Create an editor component

    var socket = io(data.url, {
        path: "/clients/socketio/hubs/codestream",
    });

    return [socket, editor, data.room_id];
}

The initialize(url) function organizes a few setup operations together:

Fetches the endpoint to an Azure service from your HTTP server
Creates a Monaco Editor instance
Establishes a persistent connection with Web PubSub for Socket.IO

Writer client

As mentioned earlier, you have two user roles on the client side: writer and viewer. Anything that the writer types is streamed to the viewer's screen.

Get the endpoint to Web PubSub for Socket.IO and the room_id value:

/*client.js*/

let [socket, editor, room_id] = await initialize('/negotiate');

When the writer client is connected with the server, the server sends a login event to the writer. The writer can respond by asking the server to join itself to a specified room. Every 200 milliseconds, the writer client sends the latest editor state to the room. A function named flush organizes the sending logic.
```
/*client.js*/

socket.on("login", () => {
    updateStatus('Connected');
    joinRoom(socket, `${room_id}`);
    setInterval(() => flush(), 200);
    // Update editor content
    // ...
});
```

If a writer doesn't make any edits, flush() does nothing and simply returns. Otherwise, the changes to the editor state are sent to the room.

/*client.js*/

function flush() {
    // No changes from editor need to be flushed
    if (changes.length === 0) return;

    // Broadcast the changes made to editor content
    sendToRoom(socket, room_id, {
        type: 'delta',
        changes: changes
        version: version++,
    });

    changes = [];
    content = editor.getValue();
}

When a new viewer client is connected, the viewer needs to get the latest complete state of the editor. To achieve this, a message that contains sync data is sent to the writer client. The message asks the writer client to send the complete editor state.

/*client.js*/

socket.on("message", (message) => {
    let data = message.data;
    if (data.data === 'sync') {
        // Broadcast the full content of the editor to the room
        sendToRoom(socket, room_id, {
            type: 'full',
            content: content
            version: version,
        });
    }
});

Viewer client

Like the writer client, the viewer client creates its Socket.IO client through initialize(). When the viewer client is connected and receives a login event from the server, it asks the server to join itself to the specified room. The query room_id specifies the room.
```
/*client.js*/

let [socket, editor] = await initialize(`/register?room_id=${room_id}`)
socket.on("login", () => {
    updateStatus('Connected');
    joinRoom(socket, `${room_id}`);
});
```

When a viewer client receives a message event from the server and the data type is ackJoinRoom, the viewer client asks the writer client in the room to send the complete editor state.

/*client.js*/

socket.on("message", (message) => {
    let data = message;
    // Ensures the viewer client is connected
    if (data.type === 'ackJoinRoom' && data.success) { 
        sendToRoom(socket, `${id}`, { data: 'sync'});
    } 
    else //...
});

If the data type is editorMessage, the viewer client updates the editor according to its actual content.

/*client.js*/

socket.on("message", (message) => {
    ...
    else 
        if (data.type === 'editorMessage') {
        switch (data.data.type) {
            case 'delta':
                // ... Let editor component update its status
                break;
            case 'full':
                // ... Let editor component update its status
                break;
        }
    }
});

Implement joinRoom() and sendToRoom() by using Socket.IO's APIs:

/*client.js*/

function joinRoom(socket, room_id) {
    socket.emit("joinRoom", {
        room_id: room_id,
    });
}

function sendToRoom(socket, room_id, data) {
    socket.emit("sendToRoom", {
        room_id: room_id,
        data: data
    });
}

Run the application

Locate the repo

The preceding sections covered the core logic related to synchronizing the editor state between viewers and the writer. You can find the complete code in the examples repository.

Clone the repo

You can clone the repo and run npm install to install project dependencies.

Start the server

node server.js <web-pubsub-connection-string>

This is the connection string that you received in an earlier step.

Play with the real-time code editor

Open http://localhost:3000 on a browser tab. Open another tab with the URL displayed on the first webpage.

If you write code on the first tab, you should see your typing reflected in real time on the other tab. Web PubSub for Socket.IO facilitates message passing in the cloud. Your express server only serves the static index.html file and the /negotiate endpoint.

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