# Design Document: [Feature Name]

## Overview
[High-level summary of the feature and approach]

## Architecture
[System architecture and component overview]

## Components and Interfaces
[Detailed component descriptions and interactions]

## Data Models
[Data structures and relationships]

## Error Handling
[Error scenarios and response strategies]

## Testing Strategy
[Testing approach and quality assurance]

## Research Findings

### [Topic]
**Sources:** [Links/references]
**Key Insights:**
- [Finding 1]
- [Finding 2]
**Impact on Design:** [How this affects our approach]

## Architecture

### System Overview
[How the overall system works]

### Component Architecture
[Major components and their responsibilities]

### Data Flow
[How information moves through the system]

### Technology Decisions
[Key technology choices and rationale]

graph TD
    A[Client] --> B[API Gateway]
    B --> C[Auth Service]
    B --> D[Feature Service]
    D --> E[Database]
    D --> F[Cache]

## Components and Interfaces

### [Component Name]
**Purpose:** [What this component does]

**Responsibilities:**
- [Responsibility 1]
- [Responsibility 2]

**Interfaces:**
- Input: [What it receives]
- Output: [What it produces]
- Dependencies: [What it requires]

**API Definition:**
```typescript
interface ComponentName {
  method(input: InputType): Promise<OutputType>;
}

### Step 5: Define Data Models

Document all data structures:

```markdown
## Data Models

### [Entity Name]
**Purpose:** [What this entity represents]

**Properties:**
| Field | Type | Required | Description |
|-------|------|----------|-------------|
| id | string | Yes | Unique identifier |
| name | string | Yes | Display name |
| createdAt | Date | Yes | Creation timestamp |

**Validation Rules:**
- [Rule 1]
- [Rule 2]

**Relationships:**
- [Relationship to other entities]

**Example:**
```json
{
  "id": "abc123",
  "name": "Example",
  "createdAt": "2024-01-15T10:30:00Z"
}

### Step 6: Plan Error Handling

Document error scenarios:

```markdown
## Error Handling

### Error Categories
1. **Validation Errors:** Invalid user input
2. **Authentication Errors:** Unauthorized access
3. **External Service Errors:** Third-party failures
4. **System Errors:** Internal failures

### Error Response Strategy

| Error Type | HTTP Code | User Message | System Action |
|------------|-----------|--------------|---------------|
| Validation | 400 | Specific field error | Log, return details |
| Auth | 401 | "Please log in" | Redirect to login |
| Not Found | 404 | "Resource not found" | Log, return error |
| Server | 500 | "Something went wrong" | Log, alert, retry |

### Recovery Mechanisms
- [Retry strategies]
- [Fallback behaviors]
- [Circuit breaker patterns]

## Testing Strategy

### Unit Testing
- **Coverage Target:** 80%+
- **Focus Areas:** [Critical business logic]
- **Mocking Strategy:** [What to mock]

### Integration Testing
- **Scope:** [Component interactions to test]
- **Environment:** [Test environment setup]
- **Data Strategy:** [Test data approach]

### End-to-End Testing
- **Critical Paths:** [User journeys to test]
- **Tools:** [Testing tools]

### Performance Testing
- **Load Targets:** [Expected load]
- **Benchmarks:** [Performance requirements]

### Decision: [Brief Title]

**Context:** [Situation requiring a decision]

**Options Considered:**

**Option 1: [Name]**
- Pros: [Benefits]
- Cons: [Drawbacks]
- Effort: [Low/Medium/High]

**Option 2: [Name]**
- Pros: [Benefits]
- Cons: [Drawbacks]
- Effort: [Low/Medium/High]

**Decision:** [Chosen option]

**Rationale:** [Why this option was selected]

**Implications:** [What this means for implementation]

### Decision: API Architecture

**Options:**
1. **REST API** - Standard HTTP methods, well-understood
2. **GraphQL** - Flexible queries, single endpoint
3. **RPC-style** - Direct operation mapping

**Decision:** REST API
**Rationale:** Standard CRUD operations, team familiarity, good tooling support

### Decision: Database Choice

**Options:**
1. **PostgreSQL** - ACID compliance, complex queries
2. **MongoDB** - Schema flexibility, horizontal scaling
3. **Redis** - High performance, limited queries

**Decision:** PostgreSQL with JSON columns
**Rationale:** Data consistency needs + flexibility for user preferences

### Decision: Authentication Strategy

**Options:**
1. **Session-based** - Server-controlled, simple
2. **JWT tokens** - Stateless, scalable
3. **OAuth 2.0** - External provider, no password management

**Decision:** JWT with refresh token rotation
**Rationale:** Scalability requirements, API-first architecture

# Design Document: User Profile Management

## Overview
Enable users to view and edit their profile information, including personal details, preferences, and profile picture.

## Architecture

### Component Architecture

### Technology Stack
- API: Node.js with Express
- Database: PostgreSQL
- Image Storage: S3-compatible object storage
- Cache: Redis for session data

## Components

### ProfileService
**Purpose:** Manage user profile CRUD operations

**Interface:**
```typescript
interface ProfileService {
  getProfile(userId: string): Promise<UserProfile>;
  updateProfile(userId: string, data: ProfileUpdate): Promise<UserProfile>;
  uploadAvatar(userId: string, image: File): Promise<string>;
  deleteAvatar(userId: string): Promise<void>;
}

## Next Steps

After completing design:
1. Get design review and approval
2. Move to Task Planning phase
3. Break design into implementation tasks
4. Begin systematic implementation