BDD Methodology for Acceptance Test Design

Core Philosophy

Test units of behavior, not units of code. Acceptance tests validate business outcomes through public interfaces, decoupled from implementation.

Outside-In Double-Loop TDD

The acceptance-designer creates the outer loop of Outside-In TDD. Development starts from user perspective, drives inward.

Outer loop (acceptance/BDD): Hours to days | User perspective, business language | Defines "done" | Scenarios describe user goals and observable outcomes, not internals | Failing outer-loop test is the starting signal for implementation

Inner loop (unit/TDD): Minutes | Developer perspective, technical terms | Software-crafter owns this loop

Workflow:

1. Write failing acceptance test from user perspective (outer loop -- outside)
2. Software-crafter drops to inner loop: unit tests to implement components (inside)
3. Iterate inner loop until acceptance test passes
4. Passing acceptance test proves user value delivered
5. Repeat for next behavior

Outer loop defines WHAT users need (outside). Inner loop drives HOW to build it (inside).

Given-When-Then Structure

Scenario: [Business-focused title describing one behavior]
  Given [preconditions - system state in business terms]
  When [single user action or business event]
  Then [observable business outcome]

Scenario Writing Rules

Rule 1: One scenario, one behavior -- Split multi-behavior scenarios.

Rule 2: Declarative, not imperative -- Business outcomes, not UI interactions. "When I log in with valid credentials" not "When I click Login button and enter email."

Rule 3: Concrete examples, not abstractions -- "Given my account balance is $100.00" not "Given the user has sufficient funds."

Rule 4: Keep scenarios short (3-5 steps) -- Longer means testing multiple behaviors or irrelevant details.

Rule 5: Background for shared Given steps only -- Only Given steps. Actions/validations in scenarios.