Why TDDs matter — the question is why and what’s at stake
The question is why clients should insist on Technical Design Documents (TDDs) before work begins. What’s at stake here is time, cost, system reliability and the ability to evolve a solution without rebuilding from scratch. First principles tell us that clarity up front reduces rework downstream; a TDD is the practical artifact that enforces that clarity.
So what does this mean for clients? A TDD converts assumptions into decisions, unknowns into scoped risks, and vague requirements into a repeatable implementation plan. If a project starts without that conversion, the result is often scope creep, finger-pointing, or expensive late design changes.
Why Technical Design Documents Matter
A TDD serves three practical functions for client engagements: alignment, risk control, and operational continuity.
Alignment
A TDD is the contract of understanding between business stakeholders and delivery teams. It records accepted trade-offs, data models, integration points, and authorization flows. When stakeholders later question a design choice, the TDD is the single source that explains the why and the expected outcomes.
Risk control
Designs carry technical and delivery risk. A good TDD lists those risks, assigns mitigations, and sets acceptance criteria. That turns unknowns into project tasks rather than surprises discovered during user acceptance testing (UAT) or, worse, in production.
Operational continuity
Systems outlive individuals. When a client inherits a solution, the TDD is the operational memory: how interfaces behave, where configuration lives, and how to restore or change the system safely. Without it, maintenance requires reverse engineering.
What a good TDD contains
A concise, actionable TDD is not a speculative essay. It focuses on decisions and evidence. Use these sections as a practical template.
- Scope and objectives: Business goals, in-scope/out-of-scope items, and acceptance criteria linked to measurable KPIs.
- Context diagram: High-level systems map showing data flow and authoritative sources.
- Interfaces and contracts: API endpoints, message formats, error handling, authentication, throughput expectations.
- Data model and migrations: Key entities, authoritative fields, transformation rules, and an outline of migration strategy with rollback steps.
- Process flows: Step-by-step sequence diagrams for primary use cases and failure modes.
- Non-functional requirements: SLAs, performance targets, security controls, and compliance considerations.
- Risk register and mitigations: Known unknowns, spike tasks, and decision triggers tied to timeline impacts.
- Deployment and operational runbook: Release steps, monitoring signals, alert thresholds, and recovery procedures.
- Acceptance tests: Concrete scenarios that verify behavior end-to-end.
How to implement TDDs in client projects
Implementing TDDs is a process change as much as a documentation practice. The goal is to make the TDD a working tool rather than a checkbox.
Phase 1 — Plan and constrain
Start with a short discovery (1–2 weeks) to capture constraints and showstoppers. Deliver a one-page intent document that becomes the backbone of the TDD. Constrain scope to what you can validate in the first increment.
Phase 2 — Draft and validate
Produce a concise draft TDD focused on decisions, not prose. Use diagrams and tables. Validate the draft in a workshop with stakeholders and engineers; record objections as decision backlog items. Each unresolved item should map to a spike with a hypothesis and timebox.
Phase 3 — Lock and use
Lock the document for the implementation phase with a clear change control process. Require that any deviation be recorded and reviewed. Make the TDD the entry point for developer onboarding, test-case authoring, and the runbook creation.
Common implementation patterns and pitfalls
Pattern: living artifact
TDDs work best when treated as living artifacts — lightweight, versioned, and tied to the codebase or project tracker. Link the TDD to PRs and acceptance tests so changes are visible and traceable.
Pitfall: over-documenting
Too much detail wastes time and obscures decisions. Aim for the minimal content that allows a new engineer to implement and operate the solution. If a section isn’t needed for decision-making or operations, keep it out.
Pitfall: late delivery
Delivering the TDD after development starts defeats its purpose. Timebox the TDD work into the project cadence and make completion a milestone before broad implementation begins.
Example: NetSuite integration TDD (brief)
For a NetSuite integration, the TDD should identify canonical records, mapping rules for custom fields, error reconciliation strategy for asynchronous jobs, and the schedule for batch vs. real-time processing. It should include the exact SuiteScript or connector entry points, expected API quotas, and how to handle duplicate detection. Those specifics prevent common failures like mis-synced financials or broken automated postings.
How to measure TDD effectiveness
Track a few practical metrics: number of design-related change requests after TDD sign-off, defects traced to missing design decisions, and onboarding time for new engineers. Use these to refine the level of detail and the processes that produce the TDD.
Ultimately, the TDD is not an academic exercise — it’s the instrument that converts planning into predictable delivery. It lowers cost by reducing rework, improves reliability by capturing failure modes, and preserves institutional knowledge.
The takeaway for clients is simple: require a concise, decision-centered TDD as part of project governance. When you make the TDD a gate, projects start with shared assumptions, and delivery teams have a practical roadmap to implement, test, and operate the system.