An engineering leader’s guide to SOLID principles

Estimated reading time: 12 minutes

As a technical leader, one of your most important responsibilities is to create an environment where systems – and the teams building them – can thrive in the long run.

Good software architecture means ensuring that systems remain scalable, maintainable, and adaptable to change. This is where SOLID principles come in.

First introduced by Robert C. “Uncle Bob” Martin nearly 30 years ago, SOLID is a set of five foundational principles in object-oriented software design. These principles still provide a framework for writing modular, resilient code. 

SOLID stands for: 

Single Responsibility Principle – Ensure that each class has a single, well-defined purpose. 

Open/Closed Principle – Promote extensibility without modifying existing code. 

Liskov Substitution Principle – Ensures that derived classes behave consistently with their base classes.

Interface Segregation Principle – Prevents unnecessary dependencies by encouraging small, focused interfaces.

Dependency Inversion Principle – Reduces tight coupling by ensuring high-level modules depend on abstractions rather than concrete implementations.

Implementing these principles isn’t just about improving the code, it’s about fostering a culture where design quality is a shared priority. This means encouraging your team to think critically about their designs, invest time in improving existing systems, and embrace processes that make clean architecture a natural outcome of their work.

More than just coding best practices, SOLID principles serve as a foundation for building resilient software systems that empower both engineers and the business.

5 principles for effective code architecture

1. Single Responsibility Principle (SRP)

One of the most common mistakes I’ve encountered is the “God class,” where a single class (or template for creating objects) is burdened with too many responsibilities. A class handling everything from validation to database access is impossible to maintain and even harder to test.

For example, in a user management system, a class that handles both authentication and database persistence will quickly become a bottleneck. If a security update is needed, you risk unintended side effects in unrelated parts of the system. By separating authentication into an `Authenticator` class and data persistence into a `UserRepository` class, you create clear boundaries that make testing, debugging, and scaling easier.

Beyond code, SRP applies to DevOps workflows as well. A single CI/CD pipeline handling build, deployment, and monitoring may seem convenient, but it introduces unnecessary complexity. Breaking it into separate pipeline stages allows teams to improve and troubleshoot independently – making failures easier to isolate and resolve.

2. Open/Closed Principle (OCP)

Good architecture anticipates change. One of the biggest challenges in growing a system is introducing new functionality without breaking what’s already there. OCP addresses this by ensuring that software components are open for extension but closed for modification.

In practice, this often involves relying on abstractions. Take a payment processing system as an example: instead of modifying existing code to add a new payment method like cryptocurrency, you can create a new class implementing the existing payment interface. Design patterns like `Strategy` or `Factory` make this principle easier to apply, allowing your system to evolve without rewriting core logic.

By following OCP, you ensure that introducing new payment methods requires no changes to the core system. This significantly reduces testing overhead and accelerates our ability to support new revenue streams.

3. Liskov Substitution Principle (LSP)

On the surface, LSP seems simple: derived classes should behave like their base classes. But in practice, it’s a principle that prevents some of the trickiest runtime errors. A violation happens when a subclass overrides or extends behavior in a way that breaks the assumptions of the base class.

For example, imagine a Bird class with a `fly()` method. If you introduce a Penguin subclass that throws an error when `fly()` is called, you’re violating LSP. Ensuring derived classes remain true to the base class’s contract creates predictable, reliable behavior across your system. It’s a safeguard against surprises down the line.

4. Interface Segregation Principle (ISP)

Large, monolithic interfaces can make systems harder to use and maintain. ISP advocates for splitting large interfaces into smaller, focused ones, making them easier to implement and reducing unnecessary dependencies.

For instance, in a vehicle management system, having a single Vehicle interface with methods like `drive()`, `fly()`, and `sail()` forces unrelated classes to implement methods they don’t need. By breaking it into smaller interfaces like `Drivable` or `Flyable`, you ensure each class only handles the behavior it actually supports. The leadership takeaway here is that simplicity drives scalability. Guide teams toward designs that avoid overcomplication, reducing both technical debt and cognitive load.

5. Dependency Inversion Principle (DIP)

DIP is the cornerstone of scalable and testable architecture. It flips the traditional dependency structure on its head, ensuring that high-level modules don’t depend on low-level ones. Instead, both depend on abstractions.

Consider a logging system. If your application directly calls a `FileLogger` class, changing the logging mechanism requires rewriting code throughout the application. By introducing an abstraction like `ILogger` and injecting the implementation at runtime, you decouple your code from specific implementations. Dependency injection frameworks like Spring or Guice make applying this principle seamless. Decoupling in this way not only improves flexibility but also makes it far easier to test your code in isolation.

Challenges and lessons learned implementing SOLID

While SOLID principles are powerful, implementing them effectively isn’t always straightforward. 

Overengineering
One of the most common issues is applying SOLID principles where they don’t belong. I’ve seen junior developers split a simple data transfer object into half a dozen classes just to follow the Single Responsibility Principle. While the intention is good, the result is overly complex code that becomes harder to maintain.

The lesson here is to apply SOLID thoughtfully, not mechanically. Ask yourself: Is this design solving a real problem, or am I creating unnecessary abstractions? Encourage your team to focus on areas where the complexity is justified – like modules that frequently change or interact with multiple systems. Simplicity is still the ultimate goal.

Misinterpreting principles
Principles like SRP and ISP are especially prone to misinterpretation. For example, SRP doesn’t mean every class should only do one small thing; it means a class should only have one reason to change. Similarly, ISP doesn’t mean breaking every interface into tiny fragments – it means designing interfaces that are meaningful and relevant to their consumers.

To address this, I’ve found it helpful to host team workshops where we analyze existing code and discuss how SOLID principles could (or should) be applied. Real-world examples from our codebase ground the discussion and clarify misconceptions.

Team alignment
Implementing SOLID isn’t a solo effort. A common challenge is ensuring that everyone, from senior engineers to junior developers, understands the principles and applies them consistently. Without alignment, you risk inconsistent designs that undermine the benefits of SOLID.

One approach I’ve used is creating a shared architecture guide. This guide includes examples, do’s and don’ts, and clear guidelines for applying SOLID principles. Combine this with regular code reviews that focus on architecture, not just functionality. These reviews are an opportunity to mentor team members and reinforce good design practices.

Legacy Code
When dealing with legacy systems, introducing SOLID principles can feel like an impossible task. Refactoring large classes, separating concerns, and introducing abstractions take time – and they often compete with the pressure to ship features.

The solution? Tackle refactoring incrementally. Focus on high-impact areas of the codebase first – modules that are frequently modified or prone to bugs. Tools like SonarQube or CodeClimate can help identify code smells, making it easier to prioritize where to apply SOLID principles.

A great starting point is the Boy Scout Rule: leave the code better than you found it. Even small improvements, like extracting a single responsibility into its own class, can snowball into meaningful architectural change over time.

Driving Cultural Change
The shift toward SOLID principles isn’t just technical – it’s cultural. As a leader, it’s your responsibility to foster an environment where good design is valued. That means advocating for design reviews, setting aside time for refactoring, and celebrating examples of clean, maintainable code.

One strategy I’ve found effective is pairing new developers with experienced engineers for architecture-focused sessions. During these sessions, they can explore how to design around SOLID principles, focusing on real-world scenarios. Not only does this help the junior developer learn, but it also reinforces best practices across the team.

Applying SOLID

The SOLID principles provide a framework for sustainable growth, but their true value comes from how they’re applied in real-world scenarios.

• Fostering modular design with SRP

Start by encouraging your team to apply the Single Responsibility Principle. This is about more than refactoring classes – it’s about promoting a mindset of modularity.

By splitting responsibilities into smaller, focused components, teams can make changes faster, reduce bugs, and collaborate more effectively. For example, when we broke down a massive `ReportGenerator` class into separate classes for data fetching, formatting, and exporting, it didn’t just simplify the code – it improved communication across teams by clarifying ownership.

As a leader, ask: Are we building systems that are easy to extend? Are responsibilities clearly defined within the code and across the team?

• Designing for change with OCP 

One of the biggest risks in growing systems is the fragility that comes from constantly editing existing code. Encourage discussions around extensibility during design reviews and help engineers focus on solutions that minimize disruption. A system that’s easy to extend not only reduces risk but also accelerates delivery in the long run.

In an e-commerce project, for instance, our discount engine initially relied on a single, sprawling method full of `if-else` conditions for various promotions. When new requirements came in, the team struggled to add them without introducing bugs. Refactoring this system to use the `Strategy pattern` allowed us to add new promotions by creating new classes, leaving the core logic untouched.

• Ensuring predictable behavior with LSP 

Predictability isn’t just a technical goal – it’s about building trust within the team and reducing uncertainty in delivery timelines. The Liskov Substitution Principle ensures that derived components behave predictably, which is critical when working in large teams or across multiple modules. 

Help your team avoid these issues by reinforcing the importance of predictable behavior in design reviews. In one project, a violation of LSP caused unexpected errors when a subclass didn’t align with its base class. Fixing this required restructuring the hierarchy to ensure consistency, but the larger lesson was about setting expectations. When teams understand that every component must adhere to a shared contract, they can work with greater confidence.

• Simplifying systems with ISP

Large, unfocused interfaces can make systems – and teams – harder to manage. Encourage your engineers to apply the Interface Segregation Principle by designing interfaces that are focused and intuitive. For example, in a logging system, breaking a monolithic `Logger` interface into smaller ones—such as `FileLogger` and `ConsoleLogger`—didn’t just improve the codebase. It made it easier for teams to integrate logging into their systems without unnecessary complexity.

• Decoupling teams with DIP

Dependency Inversion is a principle that mirrors how effective teams work. Decoupled systems create decoupled teams. When you remove unnecessary dependencies you give teams the freedom to innovate and deliver faster.

For example, a payment service initially relied directly on a `StripePaymentProcessor`. When the business added PayPal support, this tight coupling led to significant rework. By refactoring the system to depend on an abstract `PaymentProcessor`, we reduced dependencies between teams, enabling different groups to work independently on payment integrations without stepping on each other’s toes. This improved deployment speed and reduced cross-team bottlenecks. 

Leadership in Action: Applying SOLID as a Strategy

The real power of SOLID principles lies in the conversations they spark. Encourage your team to think critically about design and prioritize maintainability over short-term fixes. Ask questions like:

• How does this design anticipate future changes?
• Are we creating unnecessary dependencies between components or teams?
• How can we make this system easier to understand for the next developer who touches it?

These discussions are about creating a culture where quality and sustainability are valued.

Where to start?

Successfully applying SOLID principles requires the right tools, well-defined processes, and a team culture that prioritizes clean, maintainable design. 

Static code analysis platforms like SonarQube and CodeClimate help identify areas where code violates SOLID principles, flagging large, complex classes or unnecessary dependencies before they become a problem. 

Modern IDEs such as IntelliJ IDEA and Visual Studio offer powerful refactoring features that make it easy to extract classes, restructure methods, and reinforce modularity. Dependency Injection frameworks like Spring, Guice, and Autofac simplify applying the Dependency Inversion Principle by automating dependency management, reducing tight coupling in the codebase.

Beyond tooling, solid engineering practices help ensure these principles become part of your team’s daily workflow. Design reviews should focus on maintainability and scalability by encouraging discussions about modularity, extensibility, and proper separation of concerns. Code reviews provide another opportunity to reinforce these concepts. 

Teams should ask: 

• Does this design minimize unnecessary dependencies?
• Is it structured to accommodate future changes without major rewrites?

Integrating these questions into regular reviews shifts the focus from short-term fixes to long-term sustainability.

Refactoring should be seen as an ongoing effort. Scheduled “Refactoring Fridays” or as part of regular sprint work ensures that improving code quality isn’t sacrificed in favor of feature development. 

Investing in team education through workshops, internal tech talks, or discussions in retrospectives gives engineers the knowledge and confidence to apply SOLID effectively. Encouraging experimentation with design patterns and recognizing well-structured code in team meetings reinforces the behaviors that lead to better systems.

Finally, the true measure of success is cleaner code, improved team efficiency, and system stability. A well-structured codebase should lead to faster onboarding for new engineers, fewer regressions, and more predictable delivery timelines. When engineers can add features without breaking existing functionality or introducing unnecessary complexity, the benefits of SOLID become clear—not just in the code but in the way the team operates.

Final thoughts

Whether it’s refactoring a monolithic class to align with the Single Responsibility Principle, designing for extensibility with the Open/Closed Principle, or reducing unnecessary dependencies through the Dependency Inversion Principle, the impact of SOLID principles is far-reaching. These practices empower teams to collaborate more effectively, reduce technical debt, and respond quickly to new challenges and opportunities.

More than a set of rules, these principles act as a guiding philosophy for how software should evolve. As a leader, embedding SOLID into your engineering culture helps ensure that the systems you build today can support the needs of tomorrow.

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