SOLID Principles

SOLID is my filter for class- and module-level decisions. Single responsibility keeps each unit with one reason to change; open/closed keeps behaviour extensible without rewriting; and the interface/segmentation principles keep consumers coupled to what they actually use.

The most consequential of the five in real systems is dependency inversion — depending on abstractions decouples modules, which is what lets tests substitute fakes and lets architecture evolve without a rewrite ripple.

Applied as rules they become dogma; applied as diagnosis they are powerful. I use them to name why a design is painful and where to extract, more than as a checklist to satisfy.

Single responsibility is really a statement about change coupling. A class should have one reason to change, not because that sounds tidy, but because a class that changes for many reasons is hard to test, hard to reuse, and breaks in surprising places.

Liskov substitution is the quiet budget-killer when violated. Consumers rely on the contract of a base type; a subclass that bends the contract silently breaks call sites. I treat a subtype that cannot fulfil its base promise as a design problem, not a clever override.

Interface segregation keeps coupling thin. Clients should not depend on methods they never call, so I split fat interfaces into focused ones — which is what lets a consumer use only the slice it needs and stay insulated from the rest.

Dependency inversion is the seam that makes the rest practical. High-level policy does not depend on low-level detail; both depend on an abstraction. That single trick lets tests inject fakes, lets swapping implementations be config, and lets teams build in parallel.

I treat SOLID as a diagnostic vocabulary. When a change ripples, I ask which principle was strained; when a module echoes another, I use them to name the smell and cut. That is more useful than enforcing five laws from on high.

The payoff is a codebase that bends instead of breaks: modules that change in one place, extend without rewrite, and stay genuinely testable. That is the practical worth of SOLID in the long-lived systems I build.

I use single responsibility as a change-coupling test, not a size rule. A class that changes for several reasons is the smell I extract around.

Open/closed is bought with dependency inversion. Policy depends on abstractions and detail supplies them, so behaviour extends without rewriting — and tests can inject fakes.

Liskov is a contract, not a style point. A subtype that bends its base promise breaks callers, so I treat it as a design problem to fix rather than a clever override.

Interface segregation keeps coupling thin. I split fat interfaces so a consumer depends only on the slice it uses, staying insulated from the rest.

The five principles work best as diagnosis. When a change ripples, they tell me which strain caused it and where to cut — more useful than enforcing five laws robotically.

I put the dependency inversion right at the seams. Repositories, gateways, and ports make infrastructure swappable and the domain testable in isolation.

I avoid dogmatism. Where the principles add ceremony without a real problem, I do not force them onto throwaway code.

SOLID composes with DDD. The language and boundaries come from the domain; the module-level discipline comes from these principles, and together they keep long-lived systems honest.

The payoff I measure is structural: a codebase that changes in one place, extends without a rewrite, and stays genuinely testable — verified by how cheap the next feature is.