Chapter 1 — Layering in Enterprise Application Architecture

The Concept of Layering

Layering is a structural principle used to organize complexity in large applications. Just like the OSI model in networking, each layer has a distinct role and depends only on the one directly below it. This approach ensures that changes in one layer have minimal impact on others, making systems easier to maintain and evolve.

Layering promotes separation of concerns, meaning that each layer focuses on a specific responsibility — such as user interaction, business logic, or data management. This separation improves testability, scalability, and team collaboration.

The Client–Server Foundation

Most enterprise applications are built around the Client–Server model.

• The client sends requests (for data or actions).
• The server processes those requests and returns the results.

This structure provides the foundation for layering. Originally, it appeared as a two-tier architecture, but modern systems often extend this into multiple tiers for better flexibility and maintainability. In these layers, domain logic mostly lies in the client side while server work as data source layer.

Two-Tier and Three-Tier Architectures

Two-Tier Architecture

• The client handles both the user interface and part of the business logic.
• The server is responsible for data storage and retrieval.

This setup works for smaller systems but becomes difficult to scale or maintain as complexity grows.

Three-Tier Architecture

• Presentation Layer – handles the user interface.
• Domain (Business Logic) Layer – contains rules, workflows, and decisions.
• Data Source Layer – manages data persistence (databases, APIs, file systems).

By introducing the Domain Layer, we separate business logic from the UI and data management. This makes the application modular, reusable, and easier to evolve.

Note: “Layers” and “tiers” are related but not identical. Layers refer to logical divisions in code, while tiers refer to physical deployments. For example, a three-layer system might still run entirely on one machine - imagine using it locally in your machine. Typically, it can be two tiers also because domain layer and the database can be on the same machine.

The Domain (Business Logic) Layer

The Domain Layer is the heart of an enterprise application. It defines how the system behaves — processing requests, applying business rules, and enforcing policies.

Imagine keeping domain logic in the client side, say Desktop Application. Then, with the rise of the internet, you decided to make the web app. You would need to duplicate all the domain logic. Same with other client application like CLI, mobile apps, or any other forms.

Keeping the domain logic on the server side helps avoid duplication across multiple client platforms (like web, mobile, or desktop). This centralization ensures that every client follows the same business rules, reducing inconsistencies and improving maintainability.

Within the domain layer, logic can be organized into:

• Application Services – coordinate tasks or workflows.
• Domain Models – represent the real-world entities and business rules.

This structure aligns with principles of Domain-Driven Design (DDD).

The Data Source Layer

The Data Source Layer is responsible for data management and persistence. It typically resides on the server and interacts directly with databases or APIs.

In some cases, applications need offline functionality, which requires storing data temporarily on the client side. When the connection is restored, the system synchronizes local data with the server.

Common approaches for offline data:

• Using local databases (like IndexedDB, SQLite, or Realm).
• Implementing synchronization strategies to merge data between client and server.

This ensures smooth user experiences even in unstable network environments.

The Presentation Layer

The Presentation Layer manages how users interact with the system — through web interfaces, mobile screens, or command-line tools. Its primary role is to handle input and display information clearly and efficiently.

For web based rich client like HTMl, server prepares the complete form and then returns to the browser. This is easy to maintain but at the same time increases the responsiveness because of round trip of any action. Modern applications often use Single Page Applications (SPAs) to make interactions faster and smoother. SPAs load once and dynamically update the content instead of reloading entire pages. This approach reduces network load and enhances responsiveness.

On the other hand, desktop application cannot be generated by server. It must separate and the whatever logic we keep here, should carefully modified because of compatible issues.

Although some lightweight logic may exist in client side (like form validation or input handling), the core business rules should remain in the domain layer to ensure consistency across different clients. If domain logic has to be in the client, it should be separated from the UI or presentation.

Layering and Dependency Flow

A well-structured layered architecture follows a clear dependency direction:

• The Presentation Layer calls the Domain Layer.
• The Domain Layer interacts with the Data Source Layer.
• Lower layers never depend on higher ones.

This unidirectional dependency flow prevents circular references and keeps the design modular. It also enables easier testing and the potential for swapping out components without breaking the system.

Note that presentation layer interfaces are for the external user provided by us, while data source layer's interfaces is to interact with database or external entities provided by the other system. Data source is not only database, it can be messaging system, and external APIs too.

Scalability and Modularity

As an application grows, each layer can be further divided into modules, packages, or services. This modular design improves scalability and makes it easier to manage teams working on different areas of the system.

Principles that help maintain clean layering include:

• Dependency Inversion Principle (DIP): Upper layers should depend on abstractions, not concrete implementations.
• Clean Architecture and Hexagonal Architecture: These are modern evolutions of layered design, promoting adaptability and testability.

Summary

• Layering organizes software into logical sections, each with its own responsibility.
• The Client–Server model provides the foundation for distributed applications.
• Three-tier architectures separate presentation, domain, and data for clarity and modularity.
• Keeping business logic centralized ensures consistency across clients.
• Offline support and SPAs improve user experience and performance.
• Proper dependency direction and modularity lead to scalable, maintainable systems.