| name | Architecture Decision Records (ADR) |
| description | Documenting significant architectural decisions with context, consequences, and rationale for future reference. |
Architecture Decision Records (ADR)
Overview
Architecture Decision Records (ADRs) are lightweight documents that capture important architectural decisions, their context, and consequences. They create a historical record of why systems are built the way they are.
Core Principle: "Document the 'why' behind architectural decisions, not just the 'what'."
1. What is an ADR?
An ADR documents:
- What decision was made
- Why it was made (context)
- What alternatives were considered
- What the consequences are
When to Write an ADR
- Choosing a database (PostgreSQL vs MongoDB)
- Selecting an architecture pattern (microservices vs monolith)
- Picking a framework (React vs Vue)
- Defining API standards (REST vs GraphQL)
- Security decisions (OAuth2 vs JWT)
2. ADR Template (MADR Format)
# ADR-001: Use PostgreSQL for Primary Database
## Status
Accepted
## Context
We need a database for our e-commerce platform that handles:
- Complex transactions (orders, payments, inventory)
- ACID compliance requirements
- Relational data (users, products, orders)
- Expected scale: 100K users, 10K orders/day
## Decision
We will use PostgreSQL as our primary database.
## Alternatives Considered
### MongoDB
- **Pros**: Flexible schema, horizontal scaling
- **Cons**: Weaker transaction support, eventual consistency
- **Why rejected**: We need strong ACID guarantees for financial transactions
### MySQL
- **Pros**: Mature, widely used, good performance
- **Cons**: Less advanced features than PostgreSQL
- **Why rejected**: PostgreSQL's JSON support and advanced indexing better fit our needs
## Consequences
### Positive
- Strong ACID compliance for financial transactions
- Rich query capabilities (CTEs, window functions)
- JSON support for flexible product attributes
- Mature ecosystem and tooling
### Negative
- Vertical scaling limitations (can be mitigated with read replicas)
- More complex to operate than managed NoSQL solutions
- Learning curve for team members unfamiliar with SQL
## Implementation Notes
- Use Prisma ORM for type-safe database access
- Set up read replicas for scaling reads
- Use connection pooling (PgBouncer)
## Related Decisions
- ADR-002: Use Prisma ORM
- ADR-005: Database migration strategy
## Date
2024-01-15
## Authors
@alice, @bob
3. ADR Statuses
| Status | Meaning |
|---|
| Proposed | Under discussion |
| Accepted | Decision made and approved |
| Deprecated | No longer recommended |
| Superseded | Replaced by another ADR |
| Rejected | Considered but not chosen |
4. Directory Structure
docs/adr/
├── README.md
├── 0001-use-postgresql.md
├── 0002-use-prisma-orm.md
├── 0003-adopt-microservices.md
├── 0004-use-react-frontend.md
└── template.md
5. ADR Tools
adr-tools (CLI)
npm install -g adr-log
adr new "Use PostgreSQL for primary database"
adr list
adr generate toc
Log4brains (Web UI)
npm install -g log4brains
log4brains init
log4brains preview
log4brains build
6. Lightweight ADR Template
For smaller decisions:
# Use Tailwind CSS for styling
**Status**: Accepted
**Date**: 2024-01-15
## Decision
Use Tailwind CSS for all component styling.
## Rationale
- Utility-first approach speeds up development
- Consistent design system
- Smaller bundle size with PurgeCSS
- Team already familiar with it
## Consequences
- Need to learn Tailwind conventions
- HTML can become verbose
- Harder to share styles across projects
7. Superseding an ADR
# ADR-003: Adopt Microservices Architecture
## Status
~~Accepted~~ **Superseded by ADR-010**
## Superseded By
[ADR-010: Return to Modular Monolith](0010-modular-monolith.md)
## Original Decision
[Keep original content for historical reference]
## Why Superseded
After 2 years of microservices:
- Operational complexity exceeded benefits
- Team size (5 engineers) too small to manage 12 services
- Network latency impacting user experience
- Distributed tracing and debugging challenges
See ADR-010 for new approach.
8. ADR Review Process
## ADR Review Checklist
- [ ] **Context Clear**: Is the problem well-defined?
- [ ] **Alternatives Listed**: Were other options considered?
- [ ] **Trade-offs Analyzed**: Pros and cons documented?
- [ ] **Consequences Identified**: Both positive and negative?
- [ ] **Stakeholders Consulted**: Relevant teams reviewed?
- [ ] **Implementation Plan**: Next steps clear?
- [ ] **Date and Authors**: Documented?
9. Common ADR Topics
Technology Choices
- Database selection
- Programming language
- Framework selection
- Cloud provider
Architecture Patterns
- Microservices vs monolith
- Event-driven architecture
- CQRS and Event Sourcing
- API design (REST vs GraphQL)
Security
- Authentication method
- Encryption standards
- API security
Operations
- Deployment strategy
- Monitoring approach
- Logging standards
10. ADR Best Practices
Do
✅ Write ADRs for significant decisions
✅ Include alternatives considered
✅ Document trade-offs honestly
✅ Keep ADRs immutable (don't edit, supersede instead)
✅ Make ADRs easy to find (in repo, searchable)
Don't
❌ Write ADRs for trivial decisions
❌ Make ADRs too long (aim for 1-2 pages)
❌ Skip the "why" (context is crucial)
❌ Delete old ADRs (mark as superseded)
❌ Write ADRs after the fact (document during decision)
11. ADR Integration with Development
In Pull Requests
## PR Description
This PR implements the decision from [ADR-005: Use Redis for caching](../docs/adr/0005-redis-caching.md)
Changes:
- Add Redis client
- Implement cache layer
- Update documentation
In Code Comments
export function verifyToken(token: string) {
}
12. ADR Checklist
Related Skills
59-architecture-decision/tradeoff-analysis59-architecture-decision/tech-stack-selection59-architecture-decision/architecture-review00-meta-skills/decision-making
