Core Microservices Design Principles

Back to: Microservices using ASP.NET Core Tutorials

Core Microservices Design Principles

In this article, I will explain the Core Microservices Design Principles with Examples. Microservices are a software architecture pattern that has become popular for building large-scale, complex applications. It is all about breaking down a monolithic application into smaller, independent services that work seamlessly together. 

What Are Microservices?

Before we understand the Core Design Principles of Microservices, it is essential to understand what microservices are. Microservices architecture involves breaking down a large monolithic application into smaller, independently deployable services. Each service runs in its own process and communicates with others over a network (typically via HTTP/REST). These services are loosely coupled, meaning a change in one service doesn’t require a change in others, providing greater scalability, flexibility, and maintainability.

Example:

Imagine an e-commerce platform. Instead of building a single, monolithic application that handles everything (from product catalogue, payment processing, order management, to shipping), you break it down into smaller services:

• Product Service: Handles everything related to products (e.g., product info, categories).
• Order Service: Manages customer orders and their statuses.
• Payment Service: Handles payments and payment transactions.
• User Service: Handles user registration, login, profile management, and user-related functionality.

Each service has its own database and can scale independently. For a better understanding, please have a look at the following image. The following diagram shows how a Monolithic Application can be broken down into independent microservices.

Characteristics of Microservices

Microservices have specific characteristics that make them unique and desirable for modern software development. Let’s understand these characteristics.

Small and Independent Services

Each microservice is designed to handle a specific business functionality. These services are small but can work together to form the entire application.

• Small: A service has a narrow, well-defined function, such as “Processing Payments” or “Managing Inventory.”
• Independent: Each microservice can be deployed and maintained independently of others.

Autonomous

Microservices are autonomous and do not depend on the internal workings of other services. Each service can be updated or replaced without affecting other services.

• Example: If the Product Service is down, the Payment Service and Order Service can continue to function as usual, processing orders and payments.

Scalable

Each microservice can be scaled independently depending on the demand. If one service, like the Payment Service, needs more capacity due to high traffic (e.g., during a sales event), you can scale only that service without affecting others.

Core Microservices Design Principles

Let’s explore the core design principles that help shape a robust microservices architecture.

Principle-1: Loose Coupling

Loose coupling ensures that services do not depend on each other’s internal workings. If one service changes or fails, it doesn’t affect others. For a better understanding, please have a look at the following diagram.

For example, imagine you have a Product Service and a Payment Service. The Payment Service can access pricing data from the Product Service through a simple API call. However, it doesn’t need to know the product service’s internal workings.

Principle-2: High Cohesion

High cohesion means that each service should focus on doing a single task very well. This helps keep the services well-organized and easier to manage. For a better understanding, please have a look at the following diagram.

For example, the Order Service should only handle the order lifecycle, from order creation to status updates. It should not handle payments, which is a concern for the Payment Service. By doing so, each service is highly focused on one specific task, making it easier to maintain.

Principle-3: Single Responsibility Principle (SRP)

The Single Responsibility Principle (SRP) is one of the key design principles that also applies to microservices. It states that a class, function, or, in our case, a service, should have only one reason to change. In the microservices context, each microservice should focus on one business domain.

• Example: The Product Service should only manage product data (e.g., adding, removing, updating products). It should not handle order processing, which is the responsibility of the Order Service. Similarly, payment processing should only be the job of the Payment Service.

When services adhere to SRP, they are easier to maintain, extend, and scale, as each service only needs to change based on changes to its business domain.

Principle-4: Bounded Context

A Bounded Context refers to the boundary within which a specific model or domain logic applies consistently. Each service should have its own bounded context, meaning it operates under its own set of business rules and terminologies. For a better understanding, please have a look at the following diagram:

The above diagram shows how different microservices (such as Product, Order, Payment, and User) have their own distinct contexts with specific entities and responsibilities.

Product Context:

• Entities: Product, Category, Inventory.
• Purpose: This context is responsible for managing product data, including adding, removing, and updating products, and managing the inventory.

Order Context:

• Entities: Order, OrderItem, Customer.
• Purpose: This context is responsible for managing the lifecycle of orders, including order creation, order status, and customer details.

Payment Context:

• Entities: Payment, Transaction, PaymentMethod.
• Purpose: This context manages payment processing, transactions, and payment methods, ensuring that payments are securely processed.

User Context:

• Entities: User, Role, Profile.
• Purpose: This context handles user registration, authentication, role management (e.g., Admin, Customer), and user profiles, keeping track of user-specific data and preferences.

Principle-5: Domain-Driven Design (DDD)

Domain-Driven Design (DDD) is a methodology that focuses on modelling software based on the business domain. DDD helps organize services around bounded contexts and ensures that the business logic aligns with real-world business processes.

DDD helps create clear, focused services representing distinct business functions, making the application easier to maintain, scale, and extend. In an E-commerce Platform, DDD helps identify important domains such as Products, Orders, and Payments, allowing us to develop separate services for each domain. For a better understanding, please have a look at the following diagram:

Core Concepts of Domain-Driven Design (DDD):

Domain Model:

• This is the central part of DDD, representing the application’s core business logic. The Domain Model describes the rules and behaviors of the business domain.
• Think of it as a blueprint that shows how different business elements work together. For example, in an e-commerce system, the domain model would define how orders, products, and customers interact.

Bounded Context:

• A Bounded Context refers to the boundary around a specific part of your application where a particular domain model is defined and consistently used. This boundary ensures that a model is used within that context.
• For example, in the e-commerce system, the Product context (how products are managed) could be separate from the Order context (how orders are handled). Each of these contexts has its own models, terminologies, and rules.

Why is Bounded Context Important?

Within a bounded context, all the terms, models, and behaviors are consistent. Outside this context, the same terms could mean different things, which is why defining clear boundaries between them avoids confusion and errors.

Subdomains:

A Subdomain is a smaller, more specific part of the business domain for which each microservice is responsible. In our example, the subdomains are:

• Product: Deals with everything related to products (e.g., adding, updating, removing products).
• Order: Manages orders and their lifecycle, including order creation, tracking, and status updates.
• Payment: Handles payment transactions, methods, and processing.
• User: Manages user-related functionalities, like authentication, profile management, etc.

These are the components that each microservice handles, and each one is focused on a specific set of business operations.

How They Work Together:

• The Domain Model in the middle represents the central logic or business rules of the system. It outlines how each service will communicate with each other and what each service’s responsibility is.
• The Bounded Context defines the scope in which that logic operates. Anything within the bounded context has consistent rules and relationships.
• The Subdomains (Product, Order, Payment, User) are parts of the business that interact with the Domain Model. Each subdomain is responsible for a specific business area, and each service inside a microservice architecture usually corresponds to one subdomain.

Example:

Let’s imagine an E-commerce platform:

• Product Service manages everything related to the product (product details, categories, etc.). The logic around product management is defined inside the “Product” subdomain.
• Order Service deals with customer orders, order statuses, and other rules about orders (e.g., which products can be ordered, and the customer’s shipping address). The “Order” subdomain contains all these rules.
• Payment Service processes payments. It doesn’t care about products or orders; its sole focus is handling transactions and payment methods. This is the “Payment” subdomain.
• User Service handles user management, such as registration, login, and authentication. This operates within the “User” subdomain.

Why is this approach beneficial?

• Separation of Concerns: By separating each business area (product, order, payment, user), we ensure that each service is focused on a single task. This makes the system easier to manage and scale.
• Clearer Communication: Teams working on these services can use consistent models and terminology specific to their domain, which reduces ambiguity and confusion.
• Independent Development: Since each service operates within its bounded context, developers can work on them independently without interfering with each other.

So, Domain-Driven Design (DDD) helps us organize the application by defining clear business boundaries (contexts) and ensuring that each service focuses on one specific business area (subdomain). This leads to more maintainable, scalable, and understandable software architecture.

Example to Develop: Online Shopping Platform

To put everything together, let’s imagine we are building an Online Shopping Platform with the following Microservices:

• Product Service: Manages product information.
• Order Service: Handles customer orders.
• Payment Service: Processes payments.
• User Service: Handles user registrations, authentication, and profile management.

Each service can be developed, deployed, and scaled independently. They interact with each other through well-defined APIs, with each service focusing on its domain:

• Product Service has its own database for storing product information and is focused purely on product management.
• Order Service manages customer orders and their states independently of product management or payment processing.
• Payment Service processes payments and deals only with payment-related logic, without involving product or order management.
• User Service is responsible for handling all user-related operations. This service deals with everything related to user accounts, registration, authentication, and profile management.

Conclusion: Why Microservices Work

By adhering to the Core Design Principles of Microservices, Loose Coupling, High Cohesion, Single Responsibility, Bounded Context, and Domain-Driven Design, we can build flexible, maintainable, and scalable systems. Microservices allow each part of an application to develop independently, ensuring that a change in one service does not affect the whole system.

As we move forward with our learning journey into Microservices Using ASP.NET Core Web API, we will implement these principles through hands-on projects, providing a deeper understanding of how they work in practice.

In the next article, I will discuss Clean Architecture in ASP.NET Core Web API with Examples. In this article, I explain Core Microservices Design Principles. I hope you enjoy this article, Core Microservices Design Principles.