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

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)

# Install
npm install -g adr-log

# Create new ADR
adr new "Use PostgreSQL for primary database"

# List all ADRs
adr list

# Generate table of contents
adr generate toc

Log4brains (Web UI)

# Install
npm install -g log4brains

# Initialize
log4brains init

# Preview
log4brains preview

# Build static site
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

// Per ADR-003, we use JWT for authentication
// See: docs/adr/0003-jwt-authentication.md
export function verifyToken(token: string) {
  // ...
}

12. ADR Checklist

• Template Available: ADR template in repository?
• Directory Structure: Organized and numbered?
• Review Process: Who approves ADRs?
• Discoverability: ADRs linked in README?
• Tooling: CLI or web UI for managing ADRs?
• Integration: ADRs referenced in code/PRs?
• Maintenance: Old ADRs marked as superseded?
• Culture: Team writes ADRs consistently?

Related Skills

• 59-architecture-decision/tradeoff-analysis

• 59-architecture-decision/tech-stack-selection

• 59-architecture-decision/architecture-review

• 00-meta-skills/decision-making