Building a Custom API Gateway with Node.js and Express Middleware: A Step-by-Step Guide

API gateway is a vital component of modern microservices architecture, providing a single entry point for all external requests to backend services. It serves as a reverse proxy that abstracts away the complexity of the underlying services and provides a unified interface for the clients. An API gateway offers several benefits, including enhanced security, improved scalability, simplified service discovery, and better user experience.

Node.js, a popular open-source runtime environment, provides a scalable and high-performance platform for building server-side applications. It allows developers to use JavaScript on both the front-end and back-end, streamlining the development process. Express middleware, a fast, minimalist web framework, provides essential middleware support for tasks like routing, authentication, and error handling. Combining Node.js with Express middleware provides a flexible and customizable solution for building a custom API gateway that caters to specific requirements.

In this blog post, we will guide you through the process of building a custom API gateway using Node.js and Express middleware. We’ll cover all the essential steps, from setting up the project to securing the gateway and scaling it using load balancers and auto-scaling groups. So, let’s get started!

Setting up the project

The first step in building a custom API gateway with Node.js and Express middleware is setting up the project. Here’s a detailed guide on how to do it:

1. Installing Node.js

To begin, you’ll need to install Node.js and Express on your development machine. Node.js is a cross-platform runtime environment that executes JavaScript code outside the web browser. You can download and install Node.js from the official website (https://nodejs.org/en/download/).

2. Creating a new project directory

After installing Node.js and Express, create a new project directory for your API gateway. You can do this by running the following command in your terminal:

mkdir api-gateway

3. Initializing the project with npm

Once you’ve created the project directory, navigate to it in your terminal and initialize the project with npm by running the following command:

npm init

This will prompt you to answer a series of questions about your project, such as its name, version, description, author, and license. You can press enter to accept the default values or provide your own values.

4. Installing required packages (express, cors, morgan)

With the project initialized, you can now install the required packages for your API gateway. For this tutorial, we’ll be using three packages: Express, CORS, and Morgan. Express is the web framework we’ll be using to create the API endpoints. CORS is a middleware that allows cross-origin resource sharing, which is essential for accessing resources from other domains. Morgan is a middleware that logs HTTP requests and responses.

You can install these packages by running the following command in your terminal:

npm install express cors morgan

This will install the latest version of each package and add them as dependencies to your project’s package.json file.

Congratulations! You’ve successfully set up the project for building a custom API gateway using Node.js and Express middleware. In the next section, we’ll dive into creating the API endpoints for the gateway.

Creating the API endpoints

The next step in building a custom API gateway with Node.js and Express middleware is creating the API endpoints. Here’s a detailed guide on how to do it:

1. Defining the endpoints for the gateway

Before you can start configuring the endpoints, you’ll need to define the API endpoints that the gateway will expose. You can define these endpoints based on the services you want to expose through the gateway. For example, if you have three microservices for authentication, user management, and payment processing, you may define endpoints like ‘/auth’, ‘/users’, and ‘/payments’.

2. Configuring routing using Express middleware

Once you’ve defined the endpoints, you can configure routing using Express middleware. Express provides a built-in router that makes it easy to define and manage routes for your API endpoints. You can create a new router instance by calling the Express.Router() function, as shown below:

const express = require('express');
const router = express.Router();

You can then define the routes for each endpoint using the router object. For example, to define a route for the ‘/users’ endpoint, you can use the router.get() method, as shown below:

router.get('/users', (req, res) => {
  // handle GET request for users
});

You can also define routes for other HTTP methods like POST, PUT, and DELETE.

3. Implementing CORS and logging with morgan middleware

To enable cross-origin resource sharing (CORS) and logging for your API endpoints, you can use the cors and morgan middleware, respectively. CORS allows the gateway to access resources from other domains, while morgan logs HTTP requests and responses.

To use the cors middleware, you can simply call the cors() function and pass it as a middleware to your Express app or router, as shown below:

const cors = require('cors');
router.use(cors());

To use the morgan middleware, you can call the morgan() function and pass it as a middleware to your Express app or router, as shown below:

const morgan = require('morgan');
router.use(morgan('combined'));

This will log HTTP requests and responses in the combined format.

Congratulations! You’ve successfully created the API endpoints for your custom API gateway using Node.js and Express middleware. In the next section, we’ll explore how to secure the gateway and authenticate requests.

Connecting to backend services

The next step in building a custom API gateway with Node.js and Express middleware is connecting to backend services. Here’s a detailed guide on how to do it:

1. Configuring proxy routes for backend services

To connect to your backend services, you’ll need to configure proxy routes in your API gateway. Proxy routes are routes that forward incoming requests to the appropriate backend service. To configure proxy routes, you can use the http-proxy-middleware package. You can install it by running the following command in your terminal:

npm install http-proxy-middleware

You can then create a new instance of the proxy middleware and configure it to forward requests to your backend services, as shown below:

const { createProxyMiddleware } = require('http-proxy-middleware');

const authProxy = createProxyMiddleware({
  target: 'http://localhost:3001',
  changeOrigin: true
});

const userProxy = createProxyMiddleware({
  target: 'http://localhost:3002',
  changeOrigin: true
});

const paymentProxy = createProxyMiddleware({
  target: 'http://localhost:3003',
  changeOrigin: true
});

router.use('/auth', authProxy);
router.use('/users', userProxy);
router.use('/payments', paymentProxy);

In this example, we’re configuring three proxy routes for the authentication, user management, and payment processing services. Each proxy route is defined using the createProxyMiddleware() function and configured with the target URL of the backend service.

2. Handling errors and retries using middleware

When connecting to backend services, it’s important to handle errors and retries to ensure the reliability and availability of the gateway. To handle errors and retries, you can use middleware like the http-errors and retry packages.

The http-errors package provides a set of HTTP error classes that you can use to handle errors in your API endpoints. For example, if the authentication service returns a 401 Unauthorized error, you can throw an error using the Unauthorized() class, as shown below:

const createError = require('http-errors');

router.get('/auth', async (req, res, next) => {
  try {
    // handle authentication request
  } catch (error) {
    next(createError.Unauthorized('Invalid credentials'));
  }
});

The retry package provides middleware that automatically retries failed requests to backend services. You can install it by running the following command in your terminal:

npm install retry

You can then create a new instance of the retry middleware and configure it to retry failed requests, as shown below:

const retry = require('retry');

const retryOptions = {
  retries: 3,
  factor: 2,
  minTimeout: 1000,
  maxTimeout: 10000,
  randomize: true
};

const retryMiddleware = (req, res, next) => {
  const operation = retry.operation(retryOptions);

  operation.attempt(() => {
    // handle request to backend service
  });
};

In this example, we’re configuring the retry middleware to retry failed requests up to three times, with an exponential backoff factor of 2, a minimum timeout of 1 second, a maximum timeout of 10 seconds, and randomized timeouts.

Congratulations! You’ve successfully connected your API gateway to backend services using Node.js and Express middleware. In the next section, we’ll explore how to secure the gateway and authenticate requests.

Securing the API gateway

After connecting your API gateway to backend services, the next step is to secure the gateway and authenticate requests. Here’s a detailed guide on how to do it:

1. Implementing authentication and authorization using middleware

To secure your API gateway, you need to implement authentication and authorization using middleware. Authentication verifies the identity of the user making the request, while authorization determines whether the user has the necessary permissions to access the requested resource.

You can implement authentication and authorization using middleware like Passport.js and JSON Web Tokens (JWT). Passport.js is an authentication middleware that supports a wide range of authentication strategies, such as OAuth, OpenID, and local authentication. JWT is a standard for securely transmitting information between parties as a JSON object.

First, you need to install the necessary packages by running the following command in your terminal:

npm install passport passport-local passport-jwt jsonwebtoken

You can then create a new instance of Passport.js and configure it to use the JWT authentication strategy, as shown below:

const passport = require('passport');
const passportJWT = require('passport-jwt');
const LocalStrategy = require('passport-local').Strategy;
const JWTStrategy = passportJWT.Strategy;
const ExtractJWT = passportJWT.ExtractJwt;

passport.use(new LocalStrategy({
    usernameField: 'email',
    passwordField: 'password'
  },
  async (email, password, done) => {
    // verify user credentials
  }
));

passport.use(new JWTStrategy({
    jwtFromRequest: ExtractJWT.fromAuthHeaderAsBearerToken(),
    secretOrKey: 'your_jwt_secret'
  },
  async (jwtPayload, done) => {
    // verify JWT token
  }
));

In this example, we’re configuring Passport.js to use the Local authentication strategy for verifying user credentials and the JWT authentication strategy for verifying JWT tokens. The secretOrKey parameter is used to specify the secret key for signing and verifying JWT tokens.

Once you’ve configured Passport.js, you can use it to protect your API endpoints by adding the authenticate middleware to your route handlers, as shown below:

router.get('/users', passport.authenticate('jwt', { session: false }), async (req, res, next) => {
  // handle user management request
});

In this example, we’re adding the authenticate middleware with the JWT strategy to the /users route handler, which ensures that only authenticated users with a valid JWT token can access the user management resource.

2. Setting up SSL/TLS certificate for secure communication

To ensure secure communication between your API gateway and clients, you need to set up an SSL/TLS certificate. SSL/TLS is a protocol for encrypting data transmission over the internet and preventing unauthorized access.

You can set up an SSL/TLS certificate by obtaining a certificate from a trusted certificate authority (CA) or by generating a self-signed certificate. To generate a self-signed certificate, you can use the OpenSSL command-line tool, as shown below:

openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365

In this example, we’re generating a new RSA 4096-bit private key and a self-signed X.509 certificate that is valid for 365 days. The key and certificate are saved to the key.pem and cert.pem files, respectively.

Once you’ve generated the SSL/TLS certificate, you can configure your API gateway to use it by creating a new HTTPS server with the certificate, as shown below:

const https = require('https');
const fs = require('fs');

const options = {
  key: fs.readFileSync('key.pem'),
  cert: fs.readFileSync('cert.pem')
};

const server = https.createServer(options, app);

server.listen(port, () => {
  console.log(`API Gateway listening on port ${port} (HTTPS)`);
});

Monitoring and scaling the API gateway