Software Design

Software design is the detailed planning stage that transforms the architectural vision of a system into concrete solutions for each component, module, or functionality.

It involves defining:

• Internal structures (classes, objects, functions) and how they relate to one another.
• Algorithms and data structures that fulfill business rules.
• Interfaces and contracts (APIs, services) that enable communication between system parts and external systems.
• Cross-cutting concerns such as security, performance, testability, and maintainability, ensuring the code is modular, cohesive, loosely coupled, and easy to evolve.

It is the phase that details how each part of the system will be built within the larger structure defined by the architecture, in order to satisfy functional and non-functional requirements in a clear, reusable, and sustainable way.

The main focus is to ensure that the final product is robust and efficient.

Relationship with Architecture

While architecture defines the “floor plan” (the macro structure), design focuses on internal details.

The design defines how components interact and how each individual piece will be implemented to function harmoniously within the whole.

Main Objectives

Design seeks to balance technical and business needs through:

• Functional Requirements: Ensuring the software does what the user needs.
• Non-Functional Requirements: Ensuring performance, security, availability, and usability.

Pillars of Good Design

For a system to remain healthy over the long term, the design must follow three principles:

1. Clarity: The code and structure should be easy for other developers to understand.
2. Reusability: Components should be modular to avoid rework and duplication.
3. Sustainability: The ease of maintenance and evolution of the system over time, minimizing “technical debt.”

Key Characteristics

Software design possesses the following key characteristics:

How to apply these characteristics in practice

1. Initial Modeling – Use class diagrams, sequence diagrams, or C4 models to visualize modules, interfaces, and flows.
2. Contract Definition – Create clear interfaces before implementing concrete logic.
3. Choosing Design Patterns – Applying patterns like Factory, Strategy, Decorator, or Repository helps achieve cohesion, coupling, and extensibility.
4. Code Reviews – Evaluate if changes preserve high cohesion and low coupling; verify adherence to SOLID principles.
5. Automated Testing – Write unit tests as soon as each class is created; this forces a more decoupled and testable design.
6. Continuous Refactoring – Whenever a principle violation is identified, refactor before moving forward.
7. Light Documentation – Keep diagrams and READMEs updated, but avoid excessive documentation that becomes outdated.

By considering these characteristics during software design, you create solid foundations that facilitate architecture, implementation, and the system’s evolution over time.

Types of Design

“Software design” can be divided into different levels and areas of focus. Each type addresses a specific aspect of the system and helps ensure it meets both functional and non-functional requirements.

Below are the main types recognized in software engineering practice:

How these types relate

1. Natural Hierarchy – Architectural design defines the system’s “skeleton”; component, data, and API designs are built upon it.
2. Iterativity – Often, when detailing one level (e.g., UI), necessary adjustments to the architecture or performance requirements emerge.
3. Cross-cutting concerns – Security, performance, testability, and observability permeate all types of design and must be considered at every layer.

When to focus on each type

Software design covers various types, from the macro architectural view to specific details like algorithms, UI, and deployment strategies. Each type generates its own artifacts and serves clear objectives, but all must be aligned to produce a coherent, secure, scalable, and maintainable system.

Images generated by AI