API gateway concept in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an API Gateway in system design?

An API Gateway is a server that acts as a single entry point for all client requests to multiple backend services. It handles routing, authentication, rate limiting, and sometimes response aggregation.

beginner

Name two main responsibilities of an API Gateway.

1. Routing client requests to the correct backend service.
2. Handling security like authentication and authorization.

intermediate

Why use an API Gateway instead of direct client-to-service communication?

It simplifies client interactions by providing one endpoint, improves security by centralizing access control, and can improve performance by caching and aggregating responses.

intermediate

How does an API Gateway help with scalability?

It can balance load across multiple backend services and apply rate limiting to protect services from overload, helping the system scale smoothly.

advanced

What is a common challenge when using an API Gateway?

It can become a single point of failure or a bottleneck if not designed or scaled properly.

What role does an API Gateway play in a microservices architecture?

A Acts as a single entry point for client requests

B Stores all user data

C Replaces backend services

D Manages database transactions

An API Gateway acts as a single entry point that routes client requests to the appropriate backend services.

Which of the following is NOT a typical function of an API Gateway?

A Direct database management

B Authentication and authorization

C Request routing

D Rate limiting

API Gateways do not manage databases directly; they route requests and handle security and traffic control.

How does an API Gateway improve security?

A By encrypting backend databases

B By replacing firewalls

C By storing user passwords

D By centralizing authentication and authorization

API Gateways centralize security by managing authentication and authorization for all incoming requests.

What is a potential risk of using an API Gateway?

A It increases client complexity

B It can slow down the database

C It can become a single point of failure

D It eliminates the need for backend services

If not properly designed or scaled, the API Gateway can become a single point of failure.

Which feature helps an API Gateway protect backend services from too many requests?

A Caching

B Rate limiting

C Load balancing

D Data encryption

Rate limiting controls how many requests a client can make, protecting backend services from overload.

Explain the role and benefits of an API Gateway in a microservices system.

Think about how clients communicate with many services and how the gateway helps.

Describe common challenges when implementing an API Gateway and how to address them.

Consider what happens if the gateway goes down or gets overloaded.

Practice

1. What is the primary role of an API gateway in a system using Express?

easy

A. It directly serves static files to users.

B. It stores all the data for the application.

C. It acts as a single entry point to route requests to multiple backend services.

D. It replaces the database in the backend.

Solution

1. Step 1: Understand the role of API gateway

   An API gateway acts as a single entry point that routes client requests to appropriate backend services.

2. Step 2: Eliminate incorrect roles

   Storing data, replacing databases, or serving static files are not responsibilities of an API gateway.

3. Final Answer:

   It acts as a single entry point to route requests to multiple backend services. -> Option C

4. Quick Check:

   API gateway = single entry point [OK]

Hint: API gateway routes requests centrally [OK]

Common Mistakes:

• Thinking API gateway stores data
• Confusing API gateway with database
• Assuming it serves static files directly

2. Which Express middleware is commonly used to forward requests in an API gateway setup?

easy

A. http-proxy-middleware

B. express.static()

C. body-parser

D. cors

Solution

1. Step 1: Identify middleware for proxying requests

   The http-proxy-middleware package is used to forward requests from the gateway to backend services.

2. Step 2: Differentiate from other middleware

   express.static() serves static files, body-parser parses request bodies, and cors manages cross-origin requests, none forward requests.

3. Final Answer:

   http-proxy-middleware -> Option A

4. Quick Check:

   Proxy middleware = http-proxy-middleware [OK]

Hint: Proxy requests with http-proxy-middleware [OK]

Common Mistakes:

• Using express.static() to proxy requests
• Confusing body-parser with proxying
• Assuming cors forwards requests

3. Given this Express API gateway code snippet, what will be the output when a client requests /api/users?

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

app.use('/api/users', createProxyMiddleware({ target: 'http://localhost:5000', changeOrigin: true }));

app.listen(3000);

medium

A. The request results in a 404 error

B. The request is served by Express app on port 3000

C. The request is blocked by CORS policy

D. The request is forwarded to http://localhost:5000/api/users

Solution

1. Step 1: Analyze the proxy middleware setup

   The middleware forwards requests matching /api/users to http://localhost:5000, preserving the path.

2. Step 2: Understand request forwarding behavior

   The request /api/users on port 3000 is proxied to http://localhost:5000/api/users.

3. Final Answer:

   The request is forwarded to http://localhost:5000/api/users -> Option D

4. Quick Check:

   Proxy forwards /api/users to target URL [OK]

Hint: Proxy forwards matching paths to target server [OK]

Common Mistakes:

• Assuming Express serves the request itself
• Expecting 404 without backend running
• Confusing CORS blocking with proxy behavior

4. Identify the error in this Express API gateway code snippet:

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

app.use('/api', createProxyMiddleware({ target: 'http://localhost:4000' }));

app.listen(3000);

// Client requests /api/users but gets no response

medium

A. Missing changeOrigin: true causes backend to reject requests

B. The target URL is incorrect and unreachable

C. The middleware is not mounted on the correct path

D. Express app is not listening on port 3000

Solution

1. Step 1: Check proxy middleware options

   Without changeOrigin: true, the Host header sent to backend remains the gateway's, which may cause backend to reject requests.

2. Step 2: Verify other parts

   The target URL and path are correct, and app listens on port 3000, so those are not errors.

3. Final Answer:

   Missing changeOrigin: true causes backend to reject requests -> Option A

4. Quick Check:

   changeOrigin fixes Host header issues [OK]

Hint: Add changeOrigin: true to fix backend rejection [OK]

Common Mistakes:

• Ignoring changeOrigin option
• Assuming wrong target URL without checking
• Thinking app.listen is missing

5. You want to design an Express API gateway that routes requests to multiple backend services and also logs each request's method and URL. Which approach best fits this requirement?

hard

A. Use only express.static() to serve all backend services and log requests.

B. Use http-proxy-middleware for routing and add a custom middleware before it to log requests.

C. Implement routing logic inside each backend service and skip the gateway.

D. Use body-parser middleware to route and log requests.

Solution

1. Step 1: Identify routing and logging needs

   The API gateway must route requests to multiple backends and log method and URL of each request.

2. Step 2: Choose appropriate middleware

   http-proxy-middleware handles routing, and a custom Express middleware can log requests before proxying.

3. Step 3: Eliminate incorrect options

   express.static() serves static files only, backend routing skips gateway, and body-parser parses bodies but does not route or log.

4. Final Answer:

   Use http-proxy-middleware for routing and add a custom middleware before it to log requests. -> Option B

5. Quick Check:

   Proxy + custom middleware = routing + logging [OK]

Hint: Log requests before proxying with custom middleware [OK]

Common Mistakes:

• Using express.static() for routing
• Skipping gateway for routing
• Confusing body-parser with routing