Ddd Bdd Tdd Flow

DDD → BDD → TDD Development Flow

Follow these phases strictly in order. Never advance to the next phase without user approval.

Phase 1: Requirements Interview

Conduct a structured interview. Ask all questions before proceeding.

Domain & Context

• What problem does this application/feature solve?
• Who are the primary users (actors)?
• What is the core domain? Are there supporting or generic subdomains?
• What are the key business rules and constraints?
• What are the acceptance criteria for success?

Technical Context

• What technology stack will be used? (language, framework, DB, etc.)
• Are there existing systems to integrate with?
• Non-functional requirements: performance, security, scale, availability

Save the collected answers to doc/requirements.md.

Branch Creation & First Commit

Derive a kebab-case feature name from the Phase 1 answers (e.g., "User Authentication" → user-authentication). Create README.md using the template from the README.md Lifecycle section below — fill in the feature name, one-line description, Overview, and set Status to Phase 1 complete. Then:

git checkout -b feature/<new-feature-name>
git add doc/requirements.md README.md
git commit -m "docs(phase1): add requirements for <new-feature-name>"

All subsequent work happens on this branch.

Phase 2: SUDO Modeling (DDD)

Create four Mermaid diagrams. Save all to doc/sudo-model.md.

SUDO stands for:

• Situation: bounded contexts, subdomains, and their relationships
• Usecase: actors and their use cases
• Domain model: aggregates, entities, value objects, domain events
• Object interaction: key sequence/collaboration for each core use case

S — Situation

Visualize bounded contexts, subdomains, and external system boundaries.

C4Context
  title System Context — <feature name>
  Person(user, "User", "Primary actor")
  System(system, "<System>", "Core bounded context")
  System_Ext(ext, "<External>", "External dependency")
  Rel(user, system, "uses")
  Rel(system, ext, "integrates with")

U — Usecase

List every actor and use case derived from the interview.

graph LR
  Actor -->|"register"| System
  Actor -->|"query"| System

D — Domain Model

Model aggregates, entities, value objects, and domain events.

classDiagram
  class AggregateRoot {
    +Id id
    +operation() DomainEvent
  }
  class Entity {
    +Id id
  }
  class ValueObject {
    +value: string
    +equals(other): bool
  }
  AggregateRoot "1" --> "*" Entity
  AggregateRoot ..> ValueObject

O — Object Interaction

Draw a sequence diagram for each core use case.

sequenceDiagram
  participant User
  participant System
  participant Domain
  User->>System: command(data)
  System->>Domain: operation(data)
  Domain-->>System: DomainEvent
  System-->>User: response

→ Present all four diagrams to the user. Ask: "Does this model accurately represent the domain? What is missing or wrong?"

Iterate until the user explicitly approves. Append a ## Review Notes section with each round of feedback.

Commit after approval

Update README.md: fill in the Domain Model section with a link to doc/sudo-model.md and a one-line summary per SUDO diagram. Set Status to Phase 2 complete.

git add doc/sudo-model.md doc/ADR/ README.md
git commit -m "docs(phase2): add SUDO domain model"

Phase 3: BDD Feature Writing

From the approved SUDO model, derive Gherkin feature files. One .feature file per major use case under doc/features/.

Coverage Checklist (mandatory for every use case)

• Happy path
• Boundary values
• Error / rejection cases
• Business rule violations
• Concurrent or idempotency edge cases

Feature File Template

Feature: <Use case name>
  As a <actor>
  I want to <action>
  So that <business value>

  Background:
    Given <common precondition>

  Scenario: Happy path — <name>
    Given <precondition>
    When <actor performs action>
    Then <expected outcome>
    And <additional assertion>

  Scenario: Error — <name>
    Given <invalid state>
    When <actor performs action>
    Then an error "<message>" is returned

  Scenario Outline: Boundary — <name>
    Given a <entity> with "<param>"
    When <action>
    Then the result is "<expected>"
    Examples:
      | param | expected |
      | ...   | ...      |

Save to doc/features/<use-case-name>.feature.

→ Present all feature files to the user. Ask: "Do these scenarios fully capture the expected behavior? Are there missing cases?"

Iterate until the user explicitly approves.

Commit after approval

Update README.md: fill in the Features / Behavior section with a table of .feature files and their descriptions. Set Status to Phase 3 complete.

git add doc/features/ README.md
git commit -m "docs(phase3): add BDD feature files"

Phase 4: Property-Based Tests

4a: Extract Properties

From each approved feature, derive invariants and properties. Document in doc/properties.md.

For every scenario, ask:

• Post-condition invariant: "After this operation, X must always hold"
• Negative invariant: "This operation must never produce Y"
• Roundtrip: "encode → decode yields the original"
• Monotonic: "Adding an item always increases the count"
• Idempotent: "Applying the operation twice is the same as applying it once"
• Equivalence: "Two different paths produce the same observable result"

Format in doc/properties.md:

## <Use case name>

| Property | Type | Expression |
|---|---|---|
| Balance never goes negative | Invariant | `∀ deposit d: balance(after) ≥ 0` |
| Roundtrip serialization | Roundtrip | `decode(encode(x)) == x` |

4b: Integration Tests (Property-Based)

Create test/integration/<feature-name>.test.<ext> for each feature.

Structure:

1. Define generators / arbitraries for valid domain inputs (use the project's PBT library: fast-check / hypothesis / QuickCheck / jqwik / proptest — match the stack)
2. For each property, write a property test with shrinking enabled
3. Include at least one stateful property (model-based) when the feature has mutable state

// fast-check example
import * as fc from "fast-check";

test("balance invariant: never negative after valid deposit", () => {
  fc.assert(
    fc.property(fc.integer({ min: 1, max: 1_000_000 }), (amount) => {
      const account = Account.empty();
      account.deposit(amount);
      expect(account.balance).toBeGreaterThanOrEqual(0);
    })
  );
});

4c: E2E Tests (Property-Based)

Create test/e2e/<feature-name>.e2e.<ext> for each major user journey.

Structure:

1. Drive the full stack (API/UI → DB) — no mocking at the system boundary
2. Use property-based generation for varied but valid inputs
3. Assert system-level invariants: data consistency, API contract, idempotency

Commit after Phase 4

Update README.md: fill in the Testing section with the PBT library used and links to test/integration/ and test/e2e/. Set Status to Phase 4 complete.

git add doc/properties.md test/integration/ test/e2e/ README.md
git commit -m "test(phase4): add property-based integration and e2e tests"

Phase 5: TDD Implementation (t_wada Style)

Follow Red → Green → Refactor strictly. One cycle at a time.

5a: Build the Test List

Before writing any code, enumerate all unit tests needed. Save to doc/test-list.md:

## Test List

- [ ] <scenario: happy path> — unit
- [ ] <scenario: error case> — unit
- [ ] <edge: boundary value> — unit
- [ ] <edge: invalid input> — unit
...

Write the full list in one pass. Do not start coding yet.

5b: TDD Cycle (one item at a time)

Work through the test list one item at a time:

RED

• Pick the next unchecked item from the test list
• Write ONE failing test in test/unit/<module>.test.<ext>
• The test must fail for the right reason: assertion failure, not compile error (fix compile errors first without making the test pass)
• Name the test as a specification: "deposit: increases balance by the deposited amount"
• Use Arrange-Act-Assert

GREEN

• Write the simplest code in src/ that makes the test pass
• Hardcode if that is the simplest path ("fake it till you make it")
• Run all tests — the new test must pass, no existing test must regress

REFACTOR

• Remove duplication between test and implementation
• Improve names, simplify structure, extract abstractions only if warranted
• Run all tests — all must still pass
• Mark the item as [x] in the test list

Repeat until the test list is exhausted.

5c: Triangulation Rule

When the implementation is hardcoded (faked), add a second test case with a different concrete example before refactoring. The second case forces generalization without guessing.

First test:  deposit(100) → balance == 100   ← implementation returns 100 (faked)
Second test: deposit(200) → balance == 200   ← forces the real addition logic
Refactor:    implement `balance += amount`

5d: Integration Gate

After all unit tests pass:

1. Run the property-based integration tests from Phase 4b
2. Run the e2e tests from Phase 4c
3. Fix any failures with additional TDD cycles (add test to the list, loop)

Commit after integration gate passes

Update README.md: fill in the Usage and Development sections (how to run the app and tests). Set Status to Phase 5 complete. Remove any placeholder comments left in earlier phases.

git add src/ test/unit/ doc/test-list.md README.md
git commit -m "feat(<scope>): implement <new-feature-name>"

Architecture Decision Records (ADR)

Whenever a significant architectural or design decision is made during any phase, record it as an ADR.

When to create an ADR:

• A technology or framework is chosen (e.g., "use PostgreSQL over SQLite")
• A design pattern or architectural style is adopted (e.g., "CQRS for the Order aggregate")
• A trade-off is accepted with known consequences
• A rejected alternative is worth preserving for future readers

File: doc/ADR/NNNN-<kebab-case-title>.md (zero-padded four-digit number, e.g. doc/ADR/0001-use-event-sourcing.md)

Format (Nygard):

# ADR-NNNN: <title>

## Status

Proposed | Accepted | Deprecated | Superseded by [ADR-NNNN](NNNN-<title>.md)

## Context

<Describe the situation, forces, and constraints that led to this decision.>

## Decision

<State the decision in active voice: "We will…">

## Consequences

<List positive and negative consequences of this decision.>

Create the ADR immediately when the decision is made — do not batch them at the end of a phase. If the user later changes a decision, update the old ADR's Status to Superseded by ADR-NNNN and create a new one.

Git Workflow

All development happens on a feature/<new-feature-name> branch created at the end of Phase 1. Commit at every phase boundary so that history mirrors the flow.

Rules:

• README.md is always staged and committed at every phase boundary — never skip it.
• Include ADR files (doc/ADR/) in the Phase 2 commit if any were created during modeling.
• Never commit in the middle of a Red-Green-Refactor cycle; always commit only after the Refactor step of the last item in the test list.
• Do not push until the user asks; the branch is local until then.

README.md Lifecycle

README.md lives in the repo root. It is created in Phase 1 and gains a new section at each subsequent phase. Use this progressive template — fill in only what the current phase covers; leave the rest as comments until that phase is reached:

# <Feature Name>

> <one-line description from Phase 1>

## Status

Phase N complete — <phase name>

## Overview

<Problem statement and acceptance criteria — filled in Phase 1>

## Domain Model

<!-- Added in Phase 2 -->
[SUDO model](doc/sudo-model.md) — <one-line summary of S/U/D/O diagrams>

## Features / Behavior

<!-- Added in Phase 3 -->
| Feature file | Description |
|---|---|
| [name.feature](doc/features/name.feature) | ... |

## Testing

<!-- Added in Phase 4 -->
- **Property tests (integration):** `test/integration/` — uses <PBT library>
- **Property tests (e2e):** `test/e2e/` — full-stack, no mocks

## Usage

<!-- Added in Phase 5 -->
<How to run the application or feature>

## Development

<!-- Added in Phase 5 -->
<How to build and run all tests>

Remove placeholder comments as each section is filled in. By Phase 5 the README must have no remaining comment placeholders.

File Structure

doc/
  requirements.md           # Phase 1
  sudo-model.md             # Phase 2 (diagrams + review notes)
  features/
    <use-case>.feature      # Phase 3
  properties.md             # Phase 4a
  test-list.md              # Phase 5a
  ADR/
    0001-<decision>.md      # Any phase — created when a decision is made

test/
  unit/
    <module>.test.<ext>     # Phase 5b
  integration/
    <feature>.test.<ext>    # Phase 4b
  e2e/
    <feature>.e2e.<ext>     # Phase 4c

src/
  <module>.<ext>            # Phase 5

Key Principles

• Phases are sequential. Never skip or reverse.
• User approval is a hard gate before advancing past Phase 2 and Phase 3.
• Tests document behavior, not implementation. Test names read as sentences.
• Small steps in TDD. Each Red-Green-Refactor cycle should take minutes.
• Property tests catch what example tests miss. They are not optional.
• All diagrams use Mermaid — keeps documentation as plain text alongside code.
• Decisions get an ADR immediately. Every significant architectural choice is recorded in doc/ADR/ in Nygard format at the moment it is made, not after the phase ends.

Common Failures to Avoid

Related Skills

• playwright-test — for e2e tests in browser-driven projects
• retrospective-codify — codify insights from the TDD cycle as permanent rules