What is Spec Driven Development?
It seems like everyone is talking about Spec Driven Development - but maybe you are, like me, wondering what it is all about. SDD is an approach where specifications (or "specs") - often written as structured documents, test cases, or code-based contracts - drive the implementation of software features. You begin by creating clear, detailed specs for what a feature, function, or API should do, and then build your code to pass these specs.
Think of it as building your house from a blueprint, not just going with what feels right each step of the way.
The What
Specs can be written in plain language, but often take the form of:
- Unit tests (assert my_code(…) == 42)
- API contracts (e.g., OpenAPI/Swagger docs)
- Formal requirement documents (e.g., Gherkin for BDD)
- JSON/XML schemas
The development team implements features to fulfill these specs. Every change is validated against the spec, ensuring reliability and consistency
The Why
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- Reduces ambiguity: Everyone knows exactly what’s being built
- Facilitates collaboration: QA, frontend, and backend teams share the same understanding
- Prevents scope creep: The spec is the agreed-upon contract - no surprises
- Automates verification: Specs double as test cases, making regression testing much easier
- Improves agility: You can safely refactor code knowing your specs will catch deviations
The How
Typical SDD process:
- Write a Spec: Define what the functionality should be, often as formal tests or contracts
- Develop to the Spec: Implement code that satisfies the spec
- Validate: Run tests or check the contract to ensure code matches the specification
- Iterate: Refine the spec as needed based on feedback, new requirements, or edge cases
When to Use SDD
- Building public APIs
- Working across distributed teams
- Developing reusable libraries
- When requirements are likely to change and you want a single “source of truth”
Fun fact
The NASA space shuttle software was famously spec-driven - every function was verified against huge libraries of specs! Spec driven development helps achieve reliability that’s aerospace-grade.
Resources to learn more: