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name: architecture-documentation description: Master architecture documentation with C4 model, ADRs, diagrams, and technical documentation for clear architecture communication.

Architecture Documentation

Create clear, comprehensive architecture documentation that communicates design decisions and system structure to stakeholders.

When to Use This Skill

• New system design
• Architecture changes
• Team onboarding
• Stakeholder communication
• Technical reviews
• Knowledge transfer
• Compliance requirements
• Decision tracking

Core Concepts

1. Architecture Decision Record (ADR)

Copy# ADR 001: Use PostgreSQL for Primary Database

**Date:** 2024-01-15
**Status:** Accepted
**Deciders:** Tech Lead, Solutions Architect, Backend Team

## Context
Need to select a database for new e-commerce platform. Requirements:
- ACID transactions for order processing
- Complex queries (joins, aggregations)
- Strong consistency
- Support for 100K users initially, 1M within 2 years

## Decision
Use PostgreSQL as primary database.

## Alternatives Considered

**MySQL:**
- ✅ Widely used, good documentation
- ✅ Strong community support
- ❌ Historically weaker for complex queries
- ❌ JSON support not as robust

**MongoDB:**
- ✅ Flexible schema
- ✅ Horizontal scaling
- ❌ No multi-document ACID transactions (at time of decision)
- ❌ Eventual consistency by default

**DynamoDB:**
- ✅ Fully managed, auto-scaling
- ❌ Complex query limitations
- ❌ Vendor lock-in
- ❌ Cost unpredictable at scale

## Rationale
- PostgreSQL offers ACID guarantees needed for financial transactions
- Advanced SQL features (window functions, CTEs) for analytics
- Excellent JSON support for flexible data
- Proven at scale (Instagram, Uber)
- Can add read replicas for scaling
- Strong typing prevents data errors

## Consequences

**Positive:**
- Strong data consistency
- Rich query capabilities
- Battle-tested reliability
- Excellent tooling ecosystem

**Negative:**
- Vertical scaling limits (mitigated with read replicas)
- More complex horizontal sharding (if needed in future)
- Requires DBA expertise for optimization

## Compliance
No specific compliance requirements impacted.

## Notes
Will reassess if we need to shard beyond 10M users.

2. System Architecture Document

Copy# System Architecture: E-Commerce Platform

## Overview
Microservices-based e-commerce platform supporting 1M users with high availability and scalability.

## Architecture Principles
1. Microservices for domain separation
2. API-first design
3. Event-driven communication
4. Cloud-native (AWS)
5. Infrastructure as Code

## System Context (C4 Level 1)
[Diagram showing users, system, external systems]

## Container Diagram (C4 Level 2)

**Frontend:**
- React SPA (S3 + CloudFront)
- Mobile Apps (iOS/Android)

**Backend Services:**
- API Gateway (AWS API Gateway)
- User Service (Node.js, PostgreSQL)
- Product Service (Node.js, PostgreSQL)
- Order Service (Node.js, PostgreSQL)
- Payment Service (Node.js)
- Notification Service (Node.js, SES)

**Data Stores:**
- PostgreSQL (RDS Multi-AZ)
- Redis (ElastiCache)
- S3 (product images)

**Messaging:**
- EventBridge (event bus)
- SQS (queues)

**External:**
- Stripe (payment processing)
- SendGrid (email)

## Technology Stack

**Frontend:**
- React 18
- TypeScript
- Tailwind CSS

**Backend:**
- Node.js 20
- Express.js
- TypeScript
- PostgreSQL 15
- Redis 7

**Infrastructure:**
- AWS (primary cloud)
- Terraform (IaC)
- GitHub Actions (CI/CD)
- DataDog (monitoring)

## Security
- HTTPS only (TLS 1.3)
- JWT authentication
- Role-based access control
- Encryption at rest (AES-256)
- Regular security scans (Snyk, OWASP ZAP)

## Scalability
- Horizontal scaling with Auto Scaling Groups
- Database read replicas
- CDN for static assets
- Redis caching
- Async processing with SQS

## Disaster Recovery
- RPO: 1 hour
- RTO: 4 hours
- Multi-AZ deployment
- Daily database backups
- Cross-region replication for critical data

## Monitoring & Observability
- DataDog APM
- CloudWatch logs
- Custom metrics dashboard
- PagerDuty alerts

Best Practices

1. Keep it current - Update with changes
2. Multiple views - Different stakeholders need different levels
3. Visual diagrams - Architecture diagrams, sequence diagrams
4. Decision rationale - Document "why", not just "what"
5. ADRs for major decisions - Track decision history
6. Living documentation - Evolve with system
7. Version control - Track changes over time
8. Review regularly - Quarterly architecture reviews

Resources

• C4 Model: https://c4model.com
• ADR Templates: https://adr.github.io
• Structurizr: Architecture diagram tool