Horizontal vs Vertical Code Design

In the realm of software architecture, the strategic separation of concerns plays a pivotal role in creating systems that are both scalable and maintainable. This separation can be approached through two primary dimensions: horizontal and vertical. Understanding the distinction between these two, along with their application, is fundamental for architects aiming to design robust systems.

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Horizontal Separation: Layers of Responsibility

Horizontal separation, or layering, involves dividing a system into distinct layers or tiers, each responsible for a specific aspect of the system's functionality. This approach typically includes:

●??Presentation Layer: The user interface and user experience components.

●??Business Logic Layer: The core computational logic, rules, and processes.

●??Data Persistence Layer: The storage mechanisms, databases, and data access layers.

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The key advantage of horizontal separation is the isolation of concerns(Figure 4-1). For instance, separating the main business logic from the persistence logic ensures that changes in the database do not necessitate alterations in the business rules or processes. This isolation grants flexibility, allowing for database changes without impacting the core business logic, and vice versa.

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Figure 4-1. Layers of responsabilities diagram

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Vertical Separation: Functional Segmentation

Vertical separation slices the system into functional segments, each dedicated to a distinct set of features or business capabilities, such as user management, product catalog, and checkout processes. This form of separation enables each segment to operate somewhat independently, focusing on its specific responsibilities.

An effective example of vertical separation is the delineation of different functional areas from each other(Figure 4-2). By maintaining low coupling (minimal dependencies between components) and high cohesion (related functionalities grouped together), the system benefits in several ways:

● Scalability: Each functional segment can be scaled independently, responding to specific demands without affecting the entirety of the system.

● Maintainability: Changes or updates within one segment have limited or no impact on others, simplifying maintenance efforts.

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Figure 4-2. Domain components

Advocating for Vertical Separation

In the pursuit of enhanced cohesion and well-defined domain boundaries, I strongly advocate for teams to embrace vertical separation in their architectural approach. Vertical separation involves organizing the codebase into distinct segments or modules, each dedicated to a specific domain or functionality, such as User Management, Product Catalog, or Checkout Process(Figure 4-2). This method stands in contrast to horizontal separation, which layers the codebase according to technical roles like presentation, business logic, and data access(Figure 4-1).

Why Vertical Separation?

Vertical separation brings numerous benefits that align with our goals for a clean, maintainable, and scalable architecture:

●?Enhanced Cohesion: By keeping all aspects of a domain within a single vertical slice, we ensure that related functionalities stay together. This not only simplifies understanding and development within that domain but also naturally promotes higher cohesion.

●?Well-defined Domain Boundaries: Vertical separation makes the boundaries between different domains and functionalities explicit. Each vertical slice acts as a self-contained unit with its own responsibilities, reducing dependencies on other parts of the system. This clarity in domain boundaries is crucial for maintaining modular and flexible codebases.

●?Facilitates Scalability: With domains encapsulated within distinct verticals, scaling becomes more manageable. Teams can focus on scaling or optimizing individual domains based on their specific requirements without impacting the broader system.

●?Encourages Domain-driven Design: This approach aligns well with domain-driven design principles, encouraging teams to think in terms of business capabilities and domain logic, rather than technical layers. It fosters a deeper understanding of the business problems being solved and how the software architecture supports these goals.

Implementing Vertical Separation

To effectively implement vertical separation, teams should start by identifying the core domains or functionalities their system needs to address. From there, organizing the codebase into vertical modules that encapsulate all aspects of each domain — from the user facing API down to data persistence — sets a solid foundation for a cohesive and well-structured system.

Adopting vertical separation doesn't preclude the use of horizontal layers within each vertical module; rather, it reframes how we think about structuring our code. It encourages us to prioritize business logic and domain needs over technical categorization, leading to software that is more aligned with business objectives and easier to adapt and extend.

By following vertical separation, teams can achieve a balance of low coupling and high cohesion, facilitating a development environment where changes are localized, impact is clear, and the overall health of the codebase is preserved.