API gateway concept in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary role of an API Gateway in a microservices architecture?

Choose the best description of the main function of an API Gateway.

A It acts as a single entry point that routes client requests to multiple backend services.

B It stores all the data for the microservices in a centralized database.

C It replaces the need for backend services by handling all business logic itself.

D It directly connects clients to each microservice without any routing.

💡 Hint

Think about how clients interact with many services through one interface.

An API Gateway acts as a single entry point that routes requests to the correct backend services, simplifying client communication.

❓ Architecture

intermediate

Which component is NOT typically handled by an API Gateway?

Select the feature that an API Gateway usually does NOT provide.

A Request routing and load balancing

B Authentication and authorization

C Direct database query execution

D Response aggregation from multiple services

💡 Hint

Consider what tasks are usually done by backend services versus the gateway.

API Gateways route requests and handle security but do not execute database queries directly; that is done by backend services.

❓ scaling

advanced

How does an API Gateway help improve scalability in a distributed system?

Choose the option that best explains how an API Gateway supports system scalability.

A By storing all user sessions centrally in the gateway

B By directly managing database connections for all services

C By replacing backend services with a single monolithic service

D By caching responses and reducing load on backend services

💡 Hint

Think about how reducing repeated work can help scale better.

API Gateways can cache frequent responses, reducing backend load and improving scalability.

❓ tradeoff

advanced

What is a common tradeoff when using an API Gateway in system design?

Identify the main downside of introducing an API Gateway.

A It can become a single point of failure if not designed for high availability.

B It eliminates the need for backend services, increasing complexity.

C It requires clients to connect directly to multiple services.

D It reduces security by exposing all backend services publicly.

💡 Hint

Consider what happens if the gateway goes down.

The API Gateway centralizes traffic, so if it fails, the whole system can become unreachable unless it is highly available.

❓ component

expert

In an Express-based API Gateway, which middleware is best suited to handle authentication before routing requests?

Select the middleware type that should be used to authenticate requests in an Express API Gateway.

A A middleware that modifies response headers after sending the response

B A global middleware that checks tokens before any route handlers

C A route-specific middleware that runs after forwarding the request

D A middleware that only logs requests without blocking unauthorized ones

💡 Hint

Authentication should happen early to block unauthorized access.

Global middleware runs before routing and can block unauthorized requests early, improving security.

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