Case Study: Refactoring with Interface Segregation Principle

Three months into a CMS refactor, our team kept hitting the same wall: changing one thing broke three others. The culprit wasn't missing tests or poor naming. It was bloated interfaces. The Interface Segregation Principle (ISP) says clients shouldn't depend on methods they never use. Simple rule. We'd been ignoring it. Here's what we found, what we changed, and what got measurably better.

Introduction to the Codebase

The CMS had been in production for two years. New features landed, requirements shifted, and nobody paused to ask whether the interfaces still made sense. They didn't. By the time we inherited it, components were tightly coupled across the board and SOLID principles, ISP especially, had been largely ignored.

Identifying the Problem Areas

Two problems showed up immediately.

Bloated Interfaces

IContentService and IAuthorizationService had each accumulated dozens of methods spanning unrelated responsibilities. That's a direct ISP violation. Clients were forced to implement methods they'd never call, and every new method added risk for everyone implementing the interface.

Tight Coupling

Every class that touched those interfaces was coupled to their full surface area. Testability suffered. A change to one method signature could ripple through a dozen unrelated files.

Applying ISP: Refactoring Strategies

Three moves drove the refactor.

Interface Segregation

We split monolithic interfaces into smaller, focused ones. IContentService became IContentReader and IContentWriter. Each interface now owns only the methods relevant to its single responsibility, nothing borrowed from a neighbor.

Dependency Injection

We introduced dependency injection so that concrete implementations depend on abstractions, not on each other. Swapping a component in tests went from a multi-file surgery to changing a single binding. Flexibility improved. So did confidence.

Composition over Inheritance

We favored composition wherever inheritance had been used just to share code. Objects now get composed with only the interfaces they actually need, which keeps the ISP intact and prevents the "inherit everything" trap from recurring.

Results and Benefits

The gains showed up quickly.

Improved Readability

Interfaces scoped to a single responsibility are just easier to read. A developer picking up the codebase for the first time can look at IContentReader, understand what it does, and move on. No scanning through forty unrelated methods to find the three that matter.

Enhanced Modifiability

Adding a feature used to mean tracing ripple effects through bloated interfaces. Now it means touching the one interface that owns that responsibility. Scope is obvious. Side effects are rare.

Testability and Maintainability

Decoupled components with injected dependencies are straightforward to test in isolation. Unit tests got shorter. Feedback loops tightened. The maintenance burden dropped in proportion to how much coupling we removed.

Conclusion

The real cost of ignoring ISP isn't the refactor itself. It's the months of slow, risky changes before you finally decide to do it. If your interfaces are already feeling heavy, that's the signal. Split them now, while the blast radius is small. The next developer who touches that code (maybe you, six months from now) will thank you for it.