Viscosity

Viscosity is a measure of how much resistance a codebase, system, or team meets when someone tries to make a change. It’s a metaphor borrowed from physics. A low-viscosity fluid such as water flows easily, while a high-viscosity fluid such as honey resists motion. In a high-viscosity codebase, even a small, well-understood change is slow and painful to ship, because the design itself gets in the way.

Several things compound into that drag.

• Excess coupling and outdated patterns that make a small change ripple further than it should.
• Process friction, such as slow reviews, manual release steps, or unclear ownership.
• Accumulated technical debt, taken on to ship quickly, which makes the next change slower still.
• Poor developer experience – weak tooling, missing documentation, inconsistent environments – that adds drag before a line of code is even written.

Robert C Martin named viscosity as one of the symptoms of software rot, alongside rigidity (where a change forces many others), fragility (where a change breaks unrelated parts), and immobility (where a module can’t be reused because it drags in too many dependencies). Where those three describe specific failure modes, viscosity describes the felt experience of the design as a whole. It becomes easier to hack around it than to follow the prevailing design.

References

• Martin, Robert C. (2000). Design Principles and Design Patterns. objectmentor.com.