Why microservices scale independently in NestJS - The Real Reasons

The Big Idea

Discover how breaking big apps into small parts can make your system faster and easier to grow!

The Scenario

Imagine running a big online store where every part--like payments, product listings, and user reviews--is tightly connected in one giant program.

When traffic spikes on just the payment part, you have to make the whole program bigger, even if other parts don't need it.

The Problem

Scaling the entire program wastes resources and slows down updates.

Fixing or improving one part means risking bugs in others because everything is tangled together.

The Solution

Microservices split the big program into small, independent pieces.

Each piece can grow or shrink on its own, so the payment service can handle more users without changing the product listings or reviews.

Before vs After

✗ Before

app.listen(3000); // One big app handles all features

✓ After

paymentService.listen(3001);
productService.listen(3002); // Separate services scale alone

What It Enables

This lets teams work faster and systems handle more users smoothly by scaling only what's needed.

Real Life Example

During a holiday sale, the payment service can add more servers to handle many buyers, while the product catalog stays the same size.

Key Takeaways

Monolithic apps force scaling of everything, wasting resources.

Microservices break apps into parts that scale independently.

This improves performance, reliability, and team productivity.

Practice

1. Why do microservices in NestJS allow independent scaling?

easy

A. Because NestJS combines all microservices into one big app

B. Because all microservices share the same database and scale together

C. Because each microservice runs separately and can be scaled on its own

D. Because microservices cannot be scaled independently

Solution

1. Step 1: Understand microservice independence

   Microservices run as separate processes or servers, each handling a specific task.

2. Step 2: Recognize scaling benefits

   Since they run separately, you can increase resources for one microservice without affecting others.

3. Final Answer:

   Because each microservice runs separately and can be scaled on its own -> Option C

4. Quick Check:

   Microservices run separately = scale independently [OK]

Hint: Microservices run alone, so scale them alone [OK]

Common Mistakes:

• Thinking all microservices share resources and scale together
• Believing NestJS merges microservices into one app
• Assuming microservices cannot scale independently

2. Which NestJS code snippet correctly creates a microservice that can scale independently?

easy

A. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 3001 } });

B. const app = await NestFactory.create(AppModule); app.listen(3001);

C. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.HTTP });

D. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 'not-a-number' } });

Solution

1. Step 1: Identify correct microservice creation syntax

   Using NestFactory.createMicroservice with Transport.TCP and a valid port number is correct.

2. Step 2: Check options validity

   const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 3001 } }); uses port 3001 (a number), which is valid. const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 'not-a-number' } }); uses a string for port, which is invalid.

3. Final Answer:

   const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 3001 } }); -> Option A

4. Quick Check:

   Correct microservice syntax = const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 3001 } }); [OK]

Hint: Use createMicroservice with valid transport and port [OK]

Common Mistakes:

• Using create() instead of createMicroservice() for microservices
• Setting port as a string instead of a number
• Using HTTP transport incorrectly for microservices

3. Given this NestJS microservice code, what will happen when you run it?

const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 4000 } });
await app.listen();
console.log('Microservice running');

medium

A. The microservice fails because listen() needs a port argument

B. The microservice starts on port 4000 and logs 'Microservice running'

C. The microservice starts but on a random port, not 4000

D. The code throws a syntax error

Solution

1. Step 1: Understand createMicroservice with TCP transport

   Setting transport to TCP with port 4000 means the microservice listens on port 4000.

2. Step 2: Check listen() usage

   Calling listen() without arguments works because port is set in options during creation.

3. Final Answer:

   The microservice starts on port 4000 and logs 'Microservice running' -> Option B

4. Quick Check:

   Port set in options + listen() = runs on port 4000 [OK]

Hint: Port in options means listen() needs no argument [OK]

Common Mistakes:

• Thinking listen() always needs a port argument
• Assuming microservice picks random port if none given to listen()
• Confusing syntax errors with runtime behavior

4. What is wrong with this NestJS microservice setup code?

const app = await NestFactory.createMicroservice(AppModule, { transport: Transport.TCP, options: { port: 0 } });
await app.listen();

medium

A. Port 0 lets the OS assign a free port, so it works but port is unknown

B. The code is missing await before app.listen()

C. Port 0 is invalid and causes the microservice to fail

D. Transport.TCP is not supported in NestJS microservices

Solution

1. Step 1: Understand port 0 meaning in networking

   Port 0 tells the OS to assign any free port automatically.

2. Step 2: Check NestJS microservice behavior

   NestJS accepts port 0 and starts microservice on a random free port, so code works but port is not fixed.

3. Final Answer:

   Port 0 lets the OS assign a free port, so it works but port is unknown -> Option A

4. Quick Check:

   Port 0 means OS assigns port [OK]

Hint: Port 0 means OS picks free port automatically [OK]

Common Mistakes:

• Assuming port 0 is invalid and causes failure
• Forgetting await before listen() (though it is present here)
• Believing Transport.TCP is unsupported

5. You have three NestJS microservices: Auth, Payments, and Notifications. You want to scale only Payments during high load without affecting others. Which approach best uses microservices to achieve this?

hard

A. Use a single microservice for all features and scale it vertically

B. Combine all three services into one app and scale the whole app together

C. Run all microservices on the same port to share resources and scale together

D. Run each microservice on its own port and scale Payments service instances independently

Solution

1. Step 1: Understand microservice separation

   Running each microservice on its own port means they operate independently.

2. Step 2: Scale only Payments service

   You can add more instances of Payments microservice without touching Auth or Notifications.

3. Step 3: Avoid combining or sharing ports

   Combining services or sharing ports prevents independent scaling.

4. Final Answer:

   Run each microservice on its own port and scale Payments service instances independently -> Option D

5. Quick Check:

   Separate ports = independent scaling [OK]

Hint: Separate ports let you scale services alone [OK]

Common Mistakes:

• Merging all services into one app losing independent scaling
• Running all microservices on same port causing conflicts
• Using one service for all features limits scaling options