Architectural Testability
Architectural Testability is the degree to which a system’s design enables fast, reliable, and efficient testing of its components.
It includes practices such as separation of concerns, observability, simulation, mocking, and designing for failure — all of which contribute to a system that supports frequent change with confidence.
Level 1 – Initial (Ad Hoc)
The system is not designed with testability in mind.
Testing is slow, brittle, or heavily reliant on end-to-end validation due to tightly coupled code and poor observability.
- Testing often requires standing up full environments or systems
- Components cannot be tested in isolation
- Failures are difficult to diagnose or reproduce
- Environments are inconsistent or unavailable
- Testing bottlenecks delay releases
Level 2 – Managed (Emerging Practice)
Teams begin to recognise testability concerns and apply workarounds, such as test doubles or external mocking tools.
- Some components are tested in isolation using mocks/stubs
- Environments may be shared but increasingly stable
- Teams begin decoupling services for test support
- Visibility into system state is partial or reactive
- Architectural complexity still limits test coverage
Level 3 – Defined (Standardised)
Systems are designed to be testable by default.
Testability is a deliberate architectural concern across components, services, and data layers.
- Teams apply SOLID principles, ports-and-adapters, or DDD for modularity
- Contracts and interfaces are clearly defined and tested
- System boundaries support mocking, stubbing, or virtualisation
- Teams can run integration tests in CI pipelines quickly and reliably
- Architecture reviews consider testability as a core quality attribute
Level 4 – Quantitatively Managed (Measured & Controlled)
Testability is tracked and optimised through metrics, automation, and architectural health indicators.
- Metrics include test coverage by layer, test execution time, test reliability, and flakiness
- Teams measure time-to-diagnose and mean time to repair (MTTR)
- Architecture enables fault injection, chaos testing, and simulation
- Automated tests validate system behaviour under normal and abnormal conditions
- Testability feedback is used to refactor and evolve the system design
Level 5 – Optimising (Continuous Improvement)
Testability is a foundational architectural principle, enabling resilience, speed, and confidence in delivery.
- Systems are architected to allow rapid, context-aware validation
- Teams leverage contract testing, property-based testing, and behavioural monitoring
- Architecture supports continuous testing in production-like conditions
- Testability drives faster learning and safe innovation
- The system enables proactive risk identification and systemic quality improvements