Stable Dependency Principle (SDP)
Components dependencies should be in the direction of stability
Every system has stable components and volatile components.
Stable components are components that aren't expected to change that often.
- Contain high-level policy, OR
- Have stood the test of time, and the component has more or less found it's role within the architecture. All likely use cases that would influence change within the component have been identified and addressed.
Because of this fact, it's more often that we'll write code that depends on stable components.
Volatile components are ones that are more likely to jitter and require frequent changes.
This is because:
- They are low-level details
- They are fairly new and all of the likely use cases that would influence change within the component have not yet been identified.
There's nothing wrong with volatile components, every system has them and that's perfectly fine.
But it's important to know when a component is volatile and ensure that we don't make stable components depend on them.
- ECMAScript approved language constructs:
- Higher-level policies like domain-layer classes: entities, value objects, domain events.
- GraphQL schemas: the client-side relies on it, and the service-side implements it.
- Front ends. Architecturally, front-end applications are very volatile compared to backend services. The styles, layout, html, css, etc are constantly being changed. This makes testing on the client-side challenging and often times fruitless. When writing Cypress.io or Selenium tests, best practices dictate to refrain from writing tests that rely on the implementation details (styles, css tags, ids, pixels, etc). Instead, work to verify correctness by testing against the behaviour. Even with our best efforts, unless an application has completely converged, it's more likely that behaviour on the client-side will change, requiring tests to constantly need to be rewritten.
- Classes with no clear single responsibility. Change originates from the actors/roles that rely on a component. The more actors/roles that rely on a component, the more reasons it has to change. If more than one actor/role relies on a component (or we're not sure who owns it), the component is said to have no singular responsibility.