Technical Plans: Bridging Specification and Implementation

Technical Plans: Bridging Specification and Implementation | CodeSignal Learn

Introduction: The Missing Bridge

You have an approved specification scoring \geq75/100 defining WHAT to build. But there's a gap before creating executable tasks. You need to answer HOW : Which components are required? How do they interact? What database changes are needed? Which existing code do we integrate with? This is the technical plan - the bridge between specification and task decomposition.

The SDD Workflow

text ┌────────────────────────────────────────────────────────────────┐ │ 📋 PRD \to 📝 Specification (WHAT) \to 🏗️ Technical Plan (HOW) │ │ (Product Requirements Doc) ◄── CURRENT LESSON │ │ \to ✅ Task Decomposition \to 💻 Implementation 📝 │ └────────────────────────────────────────────────────────────────┘ ┌────────────────────────────────────────────────────────────────┐ │ 📋 PRD \to 📝 Specification (WHAT) \to 🏗️ Technical Plan (HOW) │ │ (Product Requirements Doc) ◄── CURRENT LESSON │ │ \to ✅ Task Decomposition \to 💻 Implementation 📝 │ └────────────────────────────────────────────────────────────────┘ SDD (Specification-Driven Development) is the workflow this course follows: starting from a PRD (Product Requirements Document), you produce a specification, then a technical plan, then decomposed tasks, and finally implementation. A technical plan translates functional requirements into concrete technical decisions. It has 7 key sections.

Reference: CODEX.md

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1. Architecture (Component Interactions)

Markdown## Architecture: Task Comments Component Flow: Client → API Endpoint → CommentService → CommentRepository → Database Data Flow: 1. Client sends POST /api/tasks/{id}/comments with JWT 2. API validates token, calls `CommentService.create_comment()` 3. Service validates: user owns task, content length valid 4. Service calls CommentRepository.create() 5. Repository persists via SQLAlchemy, returns Comment 6. API transforms to CommentSchema, returns 201## Architecture: Task Comments Component Flow: Client → API Endpoint → CommentService → CommentRepository → Database Data Flow: 1. Client sends POST /api/tasks/{id}/comments with JWT 2. API validates token, calls `CommentService.create_comment()` 3. Service validates: user owns task, content length valid 4. Service calls CommentRepository.create() 5. Repository persists via SQLAlchemy, returns Comment 6. API transforms to CommentSchema, returns 201 Why: Prevents conflicting assumptions about layer responsibilities, duplicate validation, and architectural drift. Common mistakes: Reversed dependencies (Service→API), skipping layers (API→DB), ambiguous validation location. How to decide: Start from entry point → follow data transformations → identify decision points → validate against CODEX.md.

2. Data Model (Database Schema)

Markdown ## Data Model: Comment Table: comments - id: UUID, primary key, default=uuid4 - task_id: UUID, FK to tasks.id, NOT NULL - user_id: UUID, FK to users.id, NOT NULL - content: String(5000), NOT NULL - created_at: DateTime, default=utcnow, NOT NULL Relationships: - task: relationship to Task (backref="comments") - author: relationship to User (backref="comments") Indexes: - (task_id, created_at) for chronological listing ## Data Model: Comment Table: comments - id: UUID, primary key, default=uuid4 - task_id: UUID, FK to tasks.id, NOT NULL - user_id: UUID, FK to users.id, NOT NULL - content: String(5000), NOT NULL - created_at: DateTime, default=utcnow, NOT NULL Relationships: - task: relationship to Task (backref="comments") - author: relationship to User (backref="comments") Indexes: - (task_id, created_at) for chronological listing Why: Prevents N+1 queries, missing constraints, orphaned records, and performance issues. Common mistakes: No indexes, missing NOT NULL, undefined relationships, ambiguous column types. How to decide: Indexes for query patterns, NOT NULL for required data, relationships for navigation, FKs for referential integrity.

3. API Implementation (Endpoints)

Markdown## API Implementation POST /api/tasks/{task_id}/comments - Router: src/api/comments.py, async def create_comment() - Dependencies: current_user, comment_service - Response: 201 with CommentSchema - Errors: 400/401/403/404 - Error handling: Catch service exceptions → HTTPException GET /api/tasks/{task_id}/comments - Query params: skip (0), limit (50) - Response: 200 with List[CommentSchema] DELETE /api/comments/{comment_id} - Authorization: comment owner OR task owner - Response: 204 No Content## API Implementation POST /api/tasks/{task_id}/comments - Router: src/api/comments.py, async def create_comment() - Dependencies: current_user, comment_service - Response: 201 with CommentSchema - Errors: 400/401/403/404 - Error handling: Catch service exceptions → HTTPException GET /api/tasks/{task_id}/comments - Query params: skip (0), limit (50) - Response: 200 with List[CommentSchema] DELETE /api/comments/{comment_id} - Authorization: comment owner OR task owner - Response: 204 No Content Why: Prevents inconsistent error codes, missing authorization, and security vulnerabilities. Common mistakes: No error codes specified, missing authorization, unclear dependencies, ambiguous response formats. How to decide: RESTful paths, map exceptions to HTTP status, specify all dependencies, check authorization before service layer.

4. Module Structure (Files)

Markdown## Module Structure New Files: - src/models/comment.py - src/repositories/comment_repository.py - src/services/comment_service.py - src/schemas/comment.py - src/api/comments.py Test Files: - tests/unit/test_comment_{model,repository,service}.py - tests/integration/test_comment_api.py Modified Files: - src/models/task.py (add comments relationship) - src/api/__init__.py (register router) - alembic/versions/XXXX_add_comments_table.py## Module Structure New Files: - src/models/comment.py - src/repositories/comment_repository.py - src/services/comment_service.py - src/schemas/comment.py - src/api/comments.py Test Files: - tests/unit/test_comment_{model,repository,service}.py - tests/integration/test_comment_api.py Modified Files: - src/models/task.py (add comments relationship) - src/api/__init__.py (register router) - alembic/versions/XXXX_add_comments_table.py Why: Prevents files in wrong directories, forgotten tests/migrations, and merge conflicts. Common mistakes: Only listing new files, no test structure, missing migrations, incorrect paths. How to decide: Follow existing structure, one file per concern, test mirrors production, list all modifications.

5. Integration Points (Existing Code)

Markdown ## Integration Points Use Existing Code: 1. Task Model (src/models/task.py) - validate task_id, check ownership 2. User Model (src/models/user.py) - foreign key for author 3. Authentication (src/api/auth.py) - get_current_user dependency 4. Database Session (src/database.py) - get_db dependency 5. Repository Pattern (CODEX.md) - all DB access through repositories 6. Error Handling (CODEX.md) - HTTPException with status codes ## Integration Points Use Existing Code: 1. Task Model (src/models/task.py) - validate task_id, check ownership 2. User Model (src/models/user.py) - foreign key for author 3. Authentication (src/api/auth.py) - get_current_user dependency 4. Database Session (src/database.py) - get_db dependency 5. Repository Pattern (CODEX.md) - all DB access through repositories 6. Error Handling (CODEX.md) - HTTPException with status codes Why: Prevents reinventing functionality, breaking patterns, and import cycles. Common mistakes: Creating new auth, direct DB access, custom error handling, ignoring CODEX.md patterns. How to decide: Scan CODEX.md first \to review existing models \to check api/ for utilities \to validate compliance.

6. Testing Strategy (Coverage)

Markdown ## Testing Strategy Unit Tests (Mocked): - test_comment_repository.py: create, get, list, delete (95%+) - test_comment_service.py: validation, authorization logic (90%+) Integration Tests (TestClient): - test_comment_api.py: all endpoints, auth, errors (85%+) Overall Target: 90%+ ## Testing Strategy Unit Tests (Mocked): - test_comment_repository.py: create, get, list, delete (95%+) - test_comment_service.py: validation, authorization logic (90%+) Integration Tests (TestClient): - test_comment_api.py: all endpoints, auth, errors (85%+) Overall Target: 90%+ Why: Prevents inadequate coverage, untested authorization, and missing edge cases. Common mistakes: "Write some tests", no coverage targets, only unit OR integration, no error case testing. How to decide: One test file per production file, repositories 95%+, services 90%+, APIs 85%+, test authorization and validation critically. (These targets come directly from CODEX.md's Testing Standards.)

7. Security Considerations

Markdown ## Security Authorization: - Verify user owns task before comment creation - DELETE: Allow comment owner OR task owner Input Validation: - Content: 1-5000 characters - Use SQLAlchemy ORM (no raw SQL) Data Leaks: - Use Pydantic schemas for responses (never raw ORM) - Only return comments for user's tasks ## Security Authorization: - Verify user owns task before comment creation - DELETE: Allow comment owner OR task owner Input Validation: - Content: 1-5000 characters - Use SQLAlchemy ORM (no raw SQL) Data Leaks: - Use Pydantic schemas for responses (never raw ORM) - Only return comments for user's tasks Why: Prevents authorization bypasses, injection attacks, and data leaks. Common mistakes: No authorization, missing validation, raw SQL, returning ORM objects directly. How to decide: Check authorization in service layer, validate all inputs, use ORM, always use Pydantic schemas, exclude sensitive fields.

Technical Plan vs Specification

Aspect Specification Technical Plan Audience Product team + developers Developers only Focus User behavior, validation Components, files, patterns Example "Comments: 1-5000 chars" "CommentService validates, raises ValidationError"

Aspect	Specification	Technical Plan
Audience	Product team + developers	Developers only
Focus	User behavior, validation	Components, files, patterns
Example	"Comments: 1-5000 chars"	"CommentService validates, raises ValidationError"

Validation & Usage

Before task decomposition, validate: ✅ Follows repository pattern (API \to Service \to Repository \to DB) ✅ Uses dependency injection ✅ Uses existing models/patterns from CODEX.md ✅ All components specified ✅ Can decompose into clear tasks When to skip: Trivial changes (one field, simple bugs) Exploratory spikes Always create for: New components/tables Multi-component features Architectural changes Generating with Codex: Markdown Given approved specification (specs/task-comments/specification.md), generate technical plan for TaskMaster. Context: Review CODEX.md, analyze existing code Stack: Python 3.11, FastAPI, SQLAlchemy, PostgreSQL Required: Architecture, Data Model, API, Module Structure, Integration Points, Testing, Security Constraints: Repository pattern, DI, type hints, HTTPException Given approved specification (specs/task-comments/specification.md), generate technical plan for TaskMaster. Context: Review CODEX.md, analyze existing code Stack: Python 3.11, FastAPI, SQLAlchemy, PostgreSQL Required: Architecture, Data Model, API, Module Structure, Integration Points, Testing, Security Constraints: Repository pattern, DI, type hints, HTTPException

Introduction: The Missing Bridge

The SDD Workflow

Reference: CODEX.md

Join the 1M+ learners on CodeSignal

1. Architecture (Component Interactions)

2. Data Model (Database Schema)

3. API Implementation (Endpoints)

4. Module Structure (Files)

5. Integration Points (Existing Code)

6. Testing Strategy (Coverage)

7. Security Considerations

Technical Plan vs Specification

How Detailed Should It Be?

Validation & Usage

Summary