The Art of the Technical Specification
I've written hundreds of technical specifications over my career. Architecture documents, functional specs, API contracts, migration plans. And I've read thousands more — most of them bad.
A good technical spec is rare because it requires a skill that's surprisingly uncommon among engineers: the ability to communicate complex technical decisions clearly, at the right level of detail, for the right audience.
Why Specs Matter
"Working software over comprehensive documentation" is one of the most misunderstood principles in software development. It doesn't mean "don't write documentation." It means don't write documentation instead of building software.
Specifications serve a different purpose. They're thinking tools. The act of writing a spec forces you to think through decisions you might otherwise defer to implementation time — when they're more expensive to get wrong.
A good spec answers three questions before a single line of code is written:
- What are we building and why?
- How will it work?
- What could go wrong?
The Three-Layer Framework
I structure every spec in three layers, each targeting a different audience:
Layer 1: Conceptual (For Everyone)
One page. What are we doing, why are we doing it, and what does success look like? Written so anyone in the organization — engineering, product, finance, compliance — can understand the intent.
No code. No technical jargon. Just the problem, the approach, and the expected outcome.
Layer 2: Functional (For the Team)
The meat of the spec. How does the system behave? What are the inputs, outputs, and interactions? What are the edge cases?
This layer uses sequence diagrams, data flow diagrams, and API contracts. It describes behavior without prescribing implementation. An engineer should be able to read this layer and understand what to build without being told how to build it.
Layer 3: Technical (For Implementation)
Specific implementation details where they matter. Database schemas, message formats, technology choices, configuration requirements. This layer is deliberately sparse — it only covers decisions that need to be made at the spec level rather than at implementation time.
Common Mistakes
Too much detail. A spec that prescribes every class name and method signature isn't a spec — it's pseudocode. You're robbing the implementing engineer of the opportunity to make good design decisions. Specify the what and the why. Let the team figure out the how.
Too little context. A spec that says "build an API for X" without explaining why, what trade-offs were considered, or what alternatives were rejected is incomplete. Future engineers (including future you) need to understand the reasoning, not just the conclusion.
No failure analysis. What happens when the database is down? When the message queue is full? When the input data is malformed? If your spec doesn't address failure modes, your implementation won't either.
Written and forgotten. A spec that's never updated as the implementation evolves becomes a liability. It's worse than no spec because it's actively misleading. Either commit to maintaining it or mark it clearly as "point-in-time design document."
The Review Process
I treat spec reviews as seriously as code reviews. Before a spec is approved:
- At least two engineers who'll implement it must review and confirm they understand it
- A stakeholder from outside engineering confirms the conceptual layer is accurate
- The spec author walks through the failure analysis and explains how each scenario is handled
This process catches more bugs than code review does, because it catches them before any code is written.
Write to Think
The biggest benefit of writing specs isn't the document itself. It's the thinking the writing forces you to do. When I sit down to write a spec, I discover gaps in my understanding, conflicts in my assumptions, and edge cases I hadn't considered.
If you're an architect or senior engineer and you're not writing specs, you're skipping the most valuable part of the design process. The spec isn't overhead. It's the work.