How to combine websockets and http to create a REST API that keeps data up to date?

When building a REST API with real-time capabilities, combining WebSockets and HTTP is an effective strategy. The following outlines detailed steps and strategies, illustrated with an example to demonstrate implementation.

Step 1: Designing the Basic REST API

First, we need to design a standard REST API to handle client CRUD operations (Create, Read, Update, Delete). This can be implemented with any backend technology, such as Node.js and Express:

javascript
const express = require('express');
const app = express();
app.use(express.json());

let dataStore = []; // This simply simulates a database

app.get('/data', (req, res) => {
    res.json(dataStore);
});

app.post('/data', (req, res) => {
    dataStore.push(req.body);
    res.status(201).send();
});

app.listen(3000, () => console.log('API is running on http://localhost:3000'));

Step 2: Introducing WebSockets

To maintain real-time data updates, we use WebSockets to push changes to all connected clients. Libraries like Socket.io can simplify WebSocket management:

javascript
const server = require('http').createServer(app);
const io = require('socket.io')(server);

io.on('connection', (socket) => {
    console.log('A user connected');

    socket.on('disconnect', () => {
        console.log('User disconnected');
    });
});

server.listen(3000, () => {
    console.log('Server is running with WebSockets support on http://localhost:3000');
});

Step 3: Synchronizing HTTP and WebSocket Communication

When updating data via the HTTP interface, we broadcast changes to all clients using WebSocket. This ensures each client's data remains current:

javascript
app.post('/data', (req, res) => {
    dataStore.push(req.body);
    io.emit('data_updated', dataStore);  // Broadcast the latest data to all clients
    res.status(201).send();
});

Step 4: Client Processing

Clients must handle data updates received through WebSocket. Using JavaScript, this can be implemented as:

javascript
const socket = io('http://localhost:3000');

socket.on('data_updated', newData => {
    console.log('Received updated data:', newData);
    // Update the client interface or perform other processing here
});

Example: Stock Price Update System

Suppose we are developing a real-time stock price update system. The backend uses a REST API to accept new stock price inputs and broadcasts updates via WebSocket. Whenever a new price is submitted through an HTTP POST, all clients subscribed to the WebSocket service receive the latest stock price array, enabling real-time display updates.

This approach not only ensures real-time data updates but also maintains a clear and efficient system architecture.