Why microservices scale independently in NestJS - Why It Works This Way

Overview - Why microservices scale independently

What is it?

Microservices are a way to build software by splitting a big app into smaller, independent parts called services. Each service does one job and can run on its own. Scaling independently means each service can grow or shrink based on its own needs without affecting others. This helps apps handle more users or data smoothly.

Why it matters

Without independent scaling, the whole app must grow together, which wastes resources and slows things down. Imagine a busy store where every section must add staff even if only one area is crowded. Independent scaling lets each part add resources only when needed, saving money and improving speed. This makes apps more reliable and easier to update.

Where it fits

Before learning this, you should understand basic web apps and how servers work. After this, you can learn about containerization, orchestration tools like Kubernetes, and advanced microservice patterns like service meshes.

Mental Model

Core Idea

Each microservice is like its own small business that can hire more workers or close down independently based on its workload.

Think of it like...

Think of a shopping mall with many stores. Each store manages its own staff and customers. If one store gets busy, it can add more cashiers without forcing other stores to do the same.

┌───────────────┐   ┌───────────────┐   ┌───────────────┐
│ Microservice 1│   │ Microservice 2│   │ Microservice 3│
│  (Orders)    │   │ (Payments)    │   │ (Inventory)   │
└──────┬────────┘   └──────┬────────┘   └──────┬────────┘
       │                   │                   │
       │ Scale independently│ Scale independently│ Scale independently
       ▼                   ▼                   ▼
  More instances       More instances       More instances
  when busy            when busy            when busy

Build-Up - 6 Steps

1

Foundation

Understanding Microservices Basics

Concept: Microservices break an app into small, focused services that work independently.

Imagine a big app as a team project. Instead of one big team doing everything, microservices split tasks into small teams. Each team handles one part, like user login or product catalog. They talk to each other but work separately.

Result

You see how apps become easier to manage and update because each part is separate.

Understanding that microservices are separate parts helps you see why they can scale on their own.

2

Foundation

What Scaling Means in Software

Concept: Scaling means adding more resources to handle more users or data.

Think of a restaurant. When many customers come, it needs more tables and staff. In software, scaling means adding more servers or instances to handle more requests.

Result

You grasp why apps need to grow resources to stay fast and reliable.

Knowing what scaling is sets the stage for understanding why independent scaling matters.

3

Intermediate

How Microservices Scale Independently

🤔Before reading on: Do you think all microservices must scale together or can they scale separately? Commit to your answer.

Concept: Each microservice runs on its own and can add or remove instances without affecting others.

Because microservices are separate apps, you can run more copies of one service if it gets busy, while others stay the same. For example, if the payment service is slow, you add more payment servers only.

Result

You understand that scaling is flexible and efficient, targeting only the busy parts.

Knowing that microservices are independent apps explains how scaling can be targeted and cost-effective.

4

Intermediate

Role of Communication in Independent Scaling

🤔Before reading on: Does independent scaling mean microservices never talk to each other? Commit to yes or no.

Concept: Microservices communicate through APIs or messages but remain independent in scaling.

Services send requests or messages to each other but run separately. This means one service can grow without forcing others to change, as long as communication stays clear.

Result

You see how communication and independence work together for smooth scaling.

Understanding communication helps you realize scaling doesn’t break service collaboration.

5

Advanced

Using NestJS to Build Scalable Microservices

🤔Before reading on: Do you think NestJS supports microservices scaling out of the box or requires heavy custom setup? Commit to your answer.

Concept: NestJS provides tools to create microservices that can scale independently using built-in transport layers.

NestJS lets you build microservices with different communication methods like TCP, Redis, or Kafka. Each service can run multiple instances, and NestJS handles message routing. This makes scaling easier and more reliable.

Result

You can build microservices in NestJS that scale independently with minimal extra work.

Knowing NestJS’s built-in support simplifies building scalable microservices and reduces complexity.

6

Expert

Challenges and Trade-offs in Independent Scaling

🤔Before reading on: Does scaling microservices independently always improve performance without downsides? Commit to yes or no.

Concept: Independent scaling improves flexibility but introduces complexity in coordination, data consistency, and deployment.

When services scale separately, you must manage more instances, handle failures gracefully, and keep data synchronized. Tools like service discovery, load balancing, and distributed tracing become essential. Also, over-scaling a service wastes resources.

Result

You appreciate that independent scaling is powerful but requires careful design and monitoring.

Understanding trade-offs prevents naive scaling that can cause bugs or wasted costs in production.

Under the Hood

Each microservice runs as a separate process or container with its own memory and CPU. When demand rises, orchestration tools start more instances of that service. Load balancers distribute requests among instances. Services communicate asynchronously or synchronously via APIs or message brokers, allowing each to scale without blocking others.

Why designed this way?

Microservices were designed to overcome monolithic app limits where scaling meant duplicating the entire app. Independent scaling saves resources and improves fault isolation. Early designs favored modularity and loose coupling to enable this flexibility.

┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│ Service A     │◄──────│ Load Balancer │──────►│ Service B     │
│ (3 instances) │       └───────────────┘       │ (5 instances) │
└───────────────┘                               └───────────────┘
       ▲                                               ▲
       │                                               │
  Scale up/down                                  Scale up/down
  independently                                  independently

Myth Busters - 4 Common Misconceptions

Quick: Do you think scaling one microservice automatically scales others? Commit yes or no.

Common Belief: Scaling one microservice means the whole app scales together.

Reality: Each microservice scales independently; scaling one does not affect others unless designed to do so.

Why it matters: Believing otherwise leads to inefficient resource use and confusion about system behavior.

Quick: Do you think microservices must share the same database to scale independently? Commit yes or no.

Common Belief: Microservices share a single database, so scaling one means scaling the database for all.

Reality: Microservices usually have separate databases or data stores, allowing independent scaling of data layers.

Why it matters: Assuming a shared database limits scaling and can cause bottlenecks.

Quick: Do you think independent scaling removes all complexity from system management? Commit yes or no.

Common Belief: Independent scaling makes system management simple and straightforward.

Reality: It adds complexity in deployment, monitoring, and coordination between services.

Why it matters: Ignoring this leads to unexpected failures and maintenance challenges.

Quick: Do you think microservices always improve performance by scaling independently? Commit yes or no.

Common Belief: Independent scaling always improves app performance.

Reality: Improper scaling or poor design can cause latency, data inconsistency, or resource waste.

Why it matters: Overconfidence can cause costly mistakes in production environments.

Expert Zone

1

Scaling decisions often depend on traffic patterns and service dependencies, not just raw load.

2

Effective independent scaling requires robust service discovery and dynamic load balancing to avoid bottlenecks.

3

Data consistency models (eventual vs strong consistency) influence how services scale and communicate.

When NOT to use

Independent scaling is less useful for tightly coupled services or simple apps where overhead outweighs benefits. In such cases, a monolithic or modular monolith approach may be better.

Production Patterns

Real-world systems use container orchestration (like Kubernetes) to manage scaling, combined with monitoring tools to auto-scale services based on metrics like CPU, memory, or request rate.

Connections

Container Orchestration

Builds-on

Understanding microservices scaling helps grasp how orchestration tools manage service instances dynamically.

Event-Driven Architecture

Complementary pattern

Knowing independent scaling clarifies why event-driven communication suits microservices by decoupling services further.

Supply Chain Management

Analogous system

Just like microservices scale independently, supply chain nodes adjust inventory and capacity separately to meet demand efficiently.

Common Pitfalls

#1 Scaling all microservices together regardless of load.

Wrong approach:

kubectl scale deployment service-a --replicas=5
kubectl scale deployment service-b --replicas=5
kubectl scale deployment service-c --replicas=5

Correct approach:

kubectl scale deployment service-a --replicas=5
# Only scale service-a which is under load
kubectl scale deployment service-b --replicas=1
kubectl scale deployment service-c --replicas=1

Root cause:

Misunderstanding that each service has different load and scaling needs.

#2 Using a single shared database for all microservices causing scaling bottlenecks.

Wrong approach:

All microservices connect to one large monolithic database instance.

Correct approach:

Each microservice uses its own database or schema optimized for its needs.

Root cause:

Not recognizing the importance of data independence for scaling.

#3 Ignoring communication overhead when scaling services independently.

Wrong approach:

Scaling services without updating API gateways or message brokers leading to dropped requests.

Correct approach:

Scale services along with communication infrastructure and monitor message queues.

Root cause:

Overlooking the need for coordinated scaling of communication layers.

Key Takeaways

Microservices split an app into small, independent parts that can grow or shrink on their own.

Independent scaling saves resources by targeting only the busy parts of an app.

Services communicate through APIs or messages but remain separate in how they scale.

NestJS supports building microservices that scale independently with built-in tools.

Independent scaling adds complexity and requires careful design, monitoring, and coordination.

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