Demystifying the Layers in Software Architecture

Software architecture is like the blueprint for a software system It provides an overview of the various components of the system and how they fit together. One way to organize software architecture is through layers, with each layer fulfilling a specific role. Layers allow for separation of concerns, modular design, and abstraction. In this article, we will examine the common layers seen in layered software architectures

The Presentation Layer

The presentation layer also known as the UI (user interface) layer is the topmost layer that users interact with. It presents data to the user and translates user actions into application commands. The presentation layer contains views, controllers, and presenters.

Some examples of presentation layer technologies are:

• HTML, CSS, JavaScript for web applications
• Windows Forms, WPF for desktop applications
• Xamarin for mobile applications

The presentation layer depends on the layers below it for data and business logic, but those lower layers do not depend on it. This allows the presentation layer to be modified easily without affecting the rest of the system.

Some key responsibilities of the presentation layer include:

• Displaying data to the user
• Handling user input and controlling navigation flow
• Basic input validation
• Maintaining the visual theme and layout
• Interacting with the next layer below (often a controller)

By keeping presentation code separate from business logic, we get some benefits like ability to support multiple client types (web, mobile, desktop) and avoiding cluttering business logic code with presentation code.

The Application Layer

Below the presentation layer is the application layer (also called the business logic layer or domain layer). The application layer contains the core business logic and domain objects like entities, services, view models, controllers, and presenters.

Some examples of application layer technologies and patterns are:

• Controllers, services, validators in MVC
• View models in MVVM
• Presenters and interactors in MVP
• Commands, handlers in CQRS
• Transaction script and domain model patterns

The key responsibilities of the application layer include:

• Coordinating application flow and behavior
• Processing and transforming data
• Handling business rules and calculations
• Integrating with external systems
• Interacting with the persistence layer below it

The application layer depends on the persistence layer for long-term data storage but the persistence layer does not depend on the application layer. The application layer defines interfaces that the persistence layer implements.

Keeping business logic separate from the user interface and data access code promotes separation of concerns and modularity. It makes the business logic easier to test, maintain, and reuse across different implementations.

The Business Layer

In some architectures, there is a distinct business layer below the application layer. This handles domain logic, business rules, data validation, calculations, algorithms, and work flows specific to the business.

The business layer defines interfaces consumed by the application layer. The application layer delegates business processing to the business layer rather than implementing it directly. This provides a clear separation between core business logic and application orchestration logic.

Some examples of code you find in a business layer:

• Domain model objects that represent business entities
• Repository interfaces for data access
• Services that encapsulate business rules
• Specifications that implement queries and criteria
• Work flows and processes

The main benefit of the business layer is the ability to reuse business logic across different applications and presentation layers. It also allows you to modify business rules without changing application or UI code.

The Persistence Layer

The persistence layer (also known as the data access layer or DAL) handles interactions with permanent data storage. It provides create, read, update, and delete (CRUD) operations against entities using technologies like:

• ORM frameworks like Hibernate, Entity Framework
• SQL and noSQL databases
• File systems

Some common responsibilities include:

• Implementing repositories interfaces defined by business layer
• Object-relational mapping
• Database access and CRUD operations
• Query construction
• Caching and transaction management

The persistence layer depends on the database layer below it but the database layer does not depend on the persistence layer. This decoupling allows switching out data access technologies fairly easily.

Separating data access code promotes modularity and abstraction. It reduces duplication of data access logic across the application. It also allows you to tune data access for performance without affecting business logic.

The Database Layer

The database layer refers to the actual database (SQL, noSQL, etc.) used for long-term data storage. Code here focuses on:

• Schema design and management
• Indexing, partitioning
• Replication, failover
• Backup and recovery
• User access control and security

Keeping database access separate from business logic enables switching databases with minimal code changes. It also allows database specialists to focus on data storage, performance and scalability.