# Architecture Decision Record (ADR) Template ## Overview An Architecture Decision Record (ADR) captures an important architectural decision along with its context and consequences. --- ## ADR Template ```markdown # ADR-[NUMBER]: [TITLE] ## Status [Proposed | Accepted | Deprecated | Superseded by ADR-XXX] ## Date YYYY-MM-DD ## Context [Describe the issue motivating this decision, the context, and any constraints.] What is the issue that we're seeing that is motivating this decision or change? ## Decision [Describe the change that we're proposing or have made.] We will... ## Consequences ### Positive - [Benefit 1] - [Benefit 2] ### Negative - [Tradeoff 1] - [Tradeoff 2] ### Neutral - [Neither positive nor negative consequence] ## Alternatives Considered ### Alternative 1: [Name] - **Pros**: [Benefits] - **Cons**: [Drawbacks] - **Reason for rejection**: [Why not chosen] ### Alternative 2: [Name] - **Pros**: [Benefits] - **Cons**: [Drawbacks] - **Reason for rejection**: [Why not chosen] ## Related - [Link to related ADRs] - [Link to requirements] - [Link to design documents] ``` --- ## ADR Example ```markdown # ADR-001: Use PostgreSQL as Primary Database ## Status Accepted ## Date 2025-01-15 ## Context We need to choose a primary database for our e-commerce platform. The system needs to: - Handle complex relational data (users, orders, products) - Support ACID transactions for payment processing - Scale to 10,000+ concurrent users - Provide full-text search capabilities Our team has experience with both PostgreSQL and MySQL. The project has 12 months to production. ## Decision We will use **PostgreSQL 16** as our primary database. Specific configuration: - Managed PostgreSQL on AWS RDS - Primary-replica setup for read scaling - pg_trgm extension for full-text search - Connection pooling via PgBouncer ## Consequences ### Positive - Rich feature set (JSON, full-text search, extensions) - Strong ACID compliance for financial transactions - Excellent open-source community support - Team has PostgreSQL experience - Cost-effective (open source) ### Negative - More complex configuration than MySQL - Requires dedicated DBA knowledge for optimization - Memory-intensive for complex queries ### Neutral - Similar performance to MySQL for our use case - Learning curve for advanced features (CTEs, window functions) ## Alternatives Considered ### Alternative 1: MySQL 8.0 - **Pros**: Simpler setup, lower memory footprint, wide hosting support - **Cons**: Weaker JSON support, limited extensions - **Reason for rejection**: PostgreSQL's richer feature set better suits complex e-commerce queries ### Alternative 2: MongoDB - **Pros**: Flexible schema, horizontal scaling, JSON-native - **Cons**: Weak transactions, eventual consistency concerns for payments - **Reason for rejection**: ACID transactions required for financial operations ### Alternative 3: CockroachDB - **Pros**: Distributed, PostgreSQL-compatible, auto-scaling - **Cons**: Higher cost, newer technology, fewer DBAs available - **Reason for rejection**: Overkill for initial scale, can migrate later if needed ## Related - REQ-NF-001: Database must support ACID transactions - REQ-NF-002: System must scale to 10,000 concurrent users - ADR-002: Use Redis for Caching (complementary decision) ``` --- ## ADR Categories ### Technology Selection - Database choice - Framework selection - Cloud provider - Third-party services ### Architecture Patterns - Microservices vs. Monolith - Event-driven architecture - API design (REST vs. GraphQL) - Caching strategy ### Development Practices - Testing strategy - CI/CD approach - Code organization - Dependency management ### Security - Authentication method - Authorization model - Data encryption - Secret management ### Performance - Scaling strategy - Caching layers - Database optimization - CDN usage --- ## ADR Lifecycle ``` ┌──────────────┐ │ Proposed │ └──────┬───────┘ │ Review & Discussion ▼ ┌──────────────┐ │ Accepted │ └──────┬───────┘ │ Over time... ▼ ┌──────────────────────────┐ │ Deprecated OR │ │ Superseded by ADR-XXX │ └──────────────────────────┘ ``` --- ## ADR Best Practices ### Do 1. **Write ADRs early**: Capture decisions when context is fresh 2. **Keep them short**: 1-2 pages maximum 3. **Focus on why**: Context and rationale are most important 4. **Link to requirements**: Show traceability 5. **Review with team**: ADRs should be collaborative 6. **Update status**: Mark deprecated/superseded when things change 7. **Use clear titles**: "Use X for Y" format works well ### Don't 1. **Don't over-document**: Not every decision needs an ADR 2. **Don't delete ADRs**: Keep history, mark as deprecated 3. **Don't forget alternatives**: Show what was considered 4. **Don't ignore consequences**: Be honest about tradeoffs 5. **Don't delay updates**: Keep ADRs current --- ## When to Write an ADR Write an ADR when: - [ ] Decision affects multiple components - [ ] Decision is difficult to reverse - [ ] Decision involves significant tradeoffs - [ ] Team needs to align on approach - [ ] Future developers need to understand "why" Skip ADR when: - [ ] Decision is trivial (variable naming, etc.) - [ ] Decision follows established patterns - [ ] Decision is easily reversible - [ ] Decision is project-specific implementation detail --- ## ADR Numbering ``` ADR-001: First major decision ADR-002: Second major decision ... ADR-NNN: Nth decision Superseded: ADR-005 (Superseded by ADR-012) ``` --- ## ADR Directory Structure ``` docs/ └── adr/ ├── README.md # ADR index ├── template.md # ADR template ├── ADR-001-database.md ├── ADR-002-caching.md ├── ADR-003-auth.md └── ... ``` --- ## ADR Index Template ```markdown # Architecture Decision Records ## Active ADRs | ADR | Title | Date | Status | | ------------------------------ | ---------------------------------- | ---------- | -------- | | [ADR-001](ADR-001-database.md) | Use PostgreSQL as Primary Database | 2025-01-15 | Accepted | | [ADR-002](ADR-002-caching.md) | Use Redis for Caching | 2025-01-16 | Accepted | | [ADR-003](ADR-003-auth.md) | Use JWT for Authentication | 2025-01-17 | Accepted | ## Superseded ADRs | ADR | Title | Superseded By | | ----------------------------- | ------------------ | ------------- | | [ADR-004](ADR-004-old-api.md) | REST API v1 Design | ADR-010 | ## How to Propose a New ADR 1. Copy `template.md` to `ADR-NNN-title.md` 2. Fill in the template 3. Submit PR for review 4. After approval, update this index ```