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1.2 Core Principles of Microservices

🧱 SOLID Principles

SOLID is a set of 5 design principles that help make your software easy to understand, flexible, and maintainable.

Principle Name Simple Meaning
S Single Responsibility Principle (SRP) A class should do only one job. Don't mix responsibilities.
O Open/Closed Principle (OCP) Software should be open for extension, but closed for modification. Add new code without changing old code.
L Liskov Substitution Principle (LSP) A subclass should behave like its parent. Replacing the parent with a child should not break the app.
I Interface Segregation Principle (ISP) Don't force a class to implement unneeded methods. Use small, specific interfaces.
D Dependency Inversion Principle (DIP) Code should depend on abstractions, not on concrete implementations. Use interfaces instead of tightly coupled classes.

solid

✅ Other Common Design Principles

Principle Meaning
DRY (Don't Repeat Yourself) Don’t copy-paste code. Reuse logic through functions, classes, etc.
KISS (Keep It Simple, Stupid) Keep your code simple and avoid unnecessary complexity.
YAGNI (You Ain’t Gonna Need It) Don’t build something until you actually need it.
Separation of Concerns (SoC) Keep different parts of the system separate. UI, logic, and data should be in different layers.
Composition over Inheritance Prefer combining small objects over using large class hierarchies.

🎯 Why are these important?

These principles help you:

  • Write clean, testable, and maintainable code.
  • Reduce bugs, duplication, and tight coupling.
  • Make your app more flexible to change.

🧭 Decentralization in Microservices

Decentralization means that each microservice manages its own logic, data, and decisions, rather than relying on a central system to control everything.

✅ What it means:

  • No central brain: Each service is independent and doesn’t wait for a "master service" to tell it what to do.
  • Own database: Each service has its own database, instead of sharing one big database.
  • Own responsibilities: Each service handles its own tasks, rules, and errors.

📦 Example:

  • In a monolithic app, a central module might handle all payments.
  • In microservices, the payment service does everything related to payments, including rules, storing data, and processing — without needing other services to do it for it.

🎯 Why it matters:

  • Reduces tight coupling.
  • Increases autonomy and scalability.
  • Teams can work independently.
  • Services can be deployed or updated without affecting others.

📈 Scalability and Autonomy in Microservices

🚀 Scalability in Microservices

Scalability means the ability of your system to handle more load (users, data, traffic) by adding more resources.

✅ In Microservices:

  • You can scale only the service that needs more resources, not the whole application.
  • Example: If the "Order Service" is getting more traffic than others, you can deploy more instances of just that service.

🔧 Types of Scalability:

  • Horizontal scaling – Add more service instances (most common).
  • Vertical scaling – Give more CPU/RAM to existing instances (limited and expensive).

scaling

🧠 Autonomy in Microservices

Autonomy means each microservice is independent and can make decisions, run, and be deployed on its own.

✅ In Microservices:

  • Services have their own code, data, and logic.
  • Teams can build, test, and deploy their services without coordinating with others.
  • Example: The "Payment Service" doesn’t need the "User Service" to function or deploy.

microservice (source:https://learn.microsoft.com/en-us/azure/architecture/guide/architecture-styles/microservices)

🎯 Benefits:

Aspect Benefit
Scalability Efficient use of resources, performance under high load.
Autonomy Faster development, independent deployments, fewer dependencies.

📄 API Contracts in Microservices

🧾 What is an API Contract?

An API Contract is a written agreement (or definition) between microservices that defines:

  • What endpoints are available (/orders, /users)
  • What data to send (request)
  • What data to expect back (response)
  • What status codes might be returned

It’s like a mutual promise between services:

“If you call me like this, I will respond like that.”

🤝 Why API Contracts Are Important

In Microservices:

  • Services are independent.
  • They talk via APIs (e.g., REST, gRPC, messaging).
  • You need a clear contract to ensure they communicate correctly.

📦 Example (Simple REST Contract)

Order Service API Contract:

Method Endpoint Request Body Response Body
POST /orders { "productId": 1, "qty": 2 } { "orderId": 123 }
GET /orders/123 — { "orderId": 123, "status": "Shipped" }

This tells consumers (like UI or another service) what to send and what to expect.

🛠 Tools for API Contracts

  • OpenAPI / Swagger – For REST APIs
  • gRPC Protobufs – For gRPC APIs
  • AsyncAPI – For message-based communication
  • Postman / Stoplight – For testing and documenting contracts
  • Pact – For contract testing between services

✅ Benefits

  • Clear communication between services
  • Early validation of integrations
  • Safer changes (breaking changes are avoided)
  • Contract testing ensures compatibility