Requirements Engineering
Structured approaches for capturing what to build before how to build it.
Core Principle
Specify WHAT, not HOW. Requirements describe outcomes and behaviors, not implementation details.
EARS Format
Easy Approach to Requirements Syntax - structured acceptance criteria.
| Pattern | Template | Use Case |
|---|---|---|
| Event-driven | WHEN [event] THEN [system] SHALL [response] | User actions, triggers |
| Conditional | IF [condition] THEN [system] SHALL [response] | Business rules |
| State-driven | WHILE [state] [system] SHALL [response] | Ongoing behaviors |
| Ubiquitous | WHERE [feature] [system] SHALL [response] | Always-true rules |
| Unconditional | [system] SHALL [response] | Basic capabilities |
Examples
| Type | Example |
|---|---|
| Event | WHEN user clicks "Submit" THEN system SHALL validate all required fields |
| Conditional | IF user is not authenticated THEN system SHALL redirect to login |
| State | WHILE file is uploading system SHALL display progress indicator |
| Ubiquitous | WHERE data is displayed system SHALL use consistent date format |
User Story Format
As a [role]
I want [capability]
So that [benefit]
Good vs Bad
| Bad | Good |
|---|---|
| As a user I want to login | As a returning customer I want to login with my email so that I can access my order history |
| As a user I want a button | As a content creator I want a publish button so that I can share my work publicly |
Key concept: The "so that" clause reveals the actual need - often the stated "want" isn't what's really needed.
Requirements Document Structure
| Section | Content |
|---|---|
| Problem Statement | What problem are we solving? |
| User Stories | Who wants what and why? |
| Acceptance Criteria | How do we know it's done? (EARS) |
| Non-Functional | Performance, security, accessibility |
| Out of Scope | What we're NOT building |
| Success Metrics | How do we measure success? |
Constitution (Governing Principles)
For complex projects, establish guiding principles before features:
| Element | Purpose |
|---|---|
| Core Values | What matters most (simplicity > cleverness) |
| Technical Principles | Architecture standards |
| Decision Framework | How to make trade-offs |
| Quality Standards | Testing, review requirements |
Key concept: Constitution prevents scope creep and ensures consistency across features.
Clarification Techniques
Before implementation, resolve ambiguities:
| Technique | When to Use |
|---|---|
| Ask "what if" | Edge cases, error scenarios |
| Present options | Multiple valid approaches |
| Challenge assumptions | "Is this actually required?" |
| Define boundaries | What's in/out of scope |
Questions to Ask
- What happens when [edge case]?
- What does success look like?
- What's the minimum viable version?
- What can we defer to later?
Non-Functional Requirements
| Category | Questions to Answer |
|---|---|
| Performance | Response time? Throughput? Concurrent users? |
| Security | Authentication? Authorization? Data protection? |
| Accessibility | WCAG level? Screen reader support? |
| Scalability | Expected growth? Peak load? |
| Reliability | Uptime requirements? Recovery time? |
Anti-Patterns
| Don't | Why |
|---|---|
| Specify implementation | "Use React" locks in decisions |
| Assume requirements | Validate with stakeholders |
| Skip edge cases | They become bugs later |
| Mix requirements with design | Conflates what with how |
| Write vague criteria | "System should be fast" is untestable |
Testable vs Untestable
| Untestable | Testable |
|---|---|
| "Fast response" | "Response under 200ms" |
| "User-friendly" | "Task completable in <3 clicks" |
| "Secure" | "Passwords hashed with bcrypt" |
| "Scalable" | "Handles 10K concurrent users" |
Key concept: If you can't write a test for it, it's not a requirement - it's a wish.