# System Patterns

*Technical architecture, design patterns, and critical implementation decisions*

## System Architecture Overview

### High-Level Architecture
```
[ASCII or text diagram showing major components and relationships]
```

### Architectural Style
- **Pattern**: [e.g., Microservices, Monolith, Serverless]
- **Rationale**: [Why this architecture was chosen]
- **Trade-offs**: [What we gain vs. what we sacrifice]

## Core Components

### Component 1: [Name]
- **Purpose**: [What it does]
- **Responsibilities**: [Key functions]
- **Technology**: [Implementation details]
- **Interfaces**: [How others interact with it]
- **Location**: [Where to find the code]

### Component 2: [Name]
- **Purpose**: [What it does]
- **Responsibilities**: [Key functions]
- **Technology**: [Implementation details]
- **Interfaces**: [How others interact with it]
- **Location**: [Where to find the code]

## Design Patterns

### Pattern 1: [Pattern Name]
- **Where Used**: [Components/modules using this pattern]
- **Implementation**: [How it's implemented]
- **Benefits**: [Why we use it]
- **Example**: [Code location or snippet]

### Pattern 2: [Pattern Name]
- **Where Used**: [Components/modules using this pattern]
- **Implementation**: [How it's implemented]
- **Benefits**: [Why we use it]

## Data Architecture

### Data Flow
```
[Diagram showing how data moves through the system]
```

### Storage Strategy
| Data Type | Storage | Rationale | Retention |
|-----------|---------|-----------|-----------|
| [Type] | [Where stored] | [Why] | [How long] |
| [Type] | [Where stored] | [Why] | [How long] |

### Data Models
- **Core Entities**: [List main domain objects]
- **Relationships**: [How entities relate]
- **Schemas**: [Location of schema definitions]

## Integration Architecture

### Internal Integrations
```
Service A <--> Service B
   |              |
   v              v
Service C <--> Service D
```

### External Integrations
| System | Integration Type | Purpose | Critical? |
|--------|-----------------|---------|-----------|
| [Name] | [REST/GraphQL/etc] | [Why we integrate] | [Yes/No] |
| [Name] | [Type] | [Purpose] | [Yes/No] |

### API Design Principles
- [Principle 1: e.g., RESTful conventions]
- [Principle 2: e.g., Versioning strategy]
- [Principle 3: e.g., Error handling approach]

## Security Architecture

### Security Layers
1. **Edge Security**: [WAF, DDoS protection]
2. **Application Security**: [Authentication, authorization]
3. **Data Security**: [Encryption at rest/transit]
4. **Infrastructure Security**: [Network isolation, secrets]

### Authentication & Authorization
- **Method**: [JWT, OAuth, SAML, etc.]
- **Implementation**: [How it works]
- **Authorization Model**: [RBAC, ABAC, etc.]

## Critical Implementation Paths

### Path 1: [User Authentication Flow]
1. User submits credentials
2. [Step 2 with component]
3. [Step 3 with component]
4. Token returned

### Path 2: [Core Business Process]
1. [Starting point]
2. [Processing step]
3. [Decision point]
4. [Completion]

## Performance Patterns

### Caching Strategy
- **What's Cached**: [Data types]
- **Where**: [Cache locations]
- **TTL**: [Expiration strategies]
- **Invalidation**: [How cache is cleared]

### Async Processing
- **Queue System**: [Technology used]
- **Use Cases**: [What goes async]
- **Retry Logic**: [Failure handling]

## Scalability Patterns

### Horizontal Scaling
- **Stateless Components**: [Which can scale out]
- **Load Balancing**: [Strategy]
- **Session Management**: [How handled]

### Vertical Scaling
- **Bottlenecks**: [Known limitations]
- **Optimization Points**: [Where to focus]

## Key Technical Decisions

Reference: See `docs/adr/` for detailed Architectural Decision Records

### Decision 1: [Title]
- **Choice**: [What was decided]
- **Rationale**: [Why]
- **Impact**: [Consequences]

### Decision 2: [Title]
- **Choice**: [What was decided]
- **Rationale**: [Why]
- **Impact**: [Consequences]

## Error Handling Patterns

### Global Error Strategy
- **Logging**: [How errors are logged]
- **User Feedback**: [How errors shown to users]
- **Recovery**: [Automatic recovery mechanisms]

### Circuit Breakers
- **Protected Services**: [Which integrations]
- **Thresholds**: [When circuits open]
- **Fallbacks**: [Degraded functionality]

## Monitoring & Observability

### Key Metrics
- **Business Metrics**: [What we track]
- **Technical Metrics**: [Performance indicators]
- **SLIs/SLOs**: [Service level indicators/objectives]

### Logging Strategy
- **Log Levels**: [When to use each]
- **Structured Logging**: [Format/fields]
- **Retention**: [How long kept]

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*This document captures the technical architecture and implementation patterns. It's updated when architectural decisions are made or patterns change.*