Testability

What it means

Testability describes how readily a component can be verified with reliable tests. Clear boundaries, deterministic inputs, observable outputs, controllable time, and limited hidden state generally improve testability. The concept applies to unit, integration, and system levels, with different tradeoffs at each level.

Example

A payment policy reads the system clock and constructs a provider client internally. Injecting a clock and a provider port lets tests exercise expiry and failure behavior without network calls.

Limitations

Adding seams can make production code more abstract or diverge from real integration behavior. High testability at unit level does not prove system behavior. Use the smallest faithful test boundary.

How to use the signal

A test seam earns its keep when it makes an important behavior faster or more diagnosable without severing the integration path.

Evaluate setup cost, determinism, observability, and failure diagnosis together when deciding whether a seam improves verification.

Weave can help compare testability-oriented changes with test feedback time, review time, and rework. Keep test results and coverage in the test system; use Weave for workflow context.