adr-documentation

# ADR Documentation ## Overview ### What are ADRs? Architecture Decision Records (ADRs) are lightweight documents that capture important architectural decisions along with their context and consequences. They serve as a historical record of why certain choices were made, helping teams avoid revisiting settled debates and providing context for future developers. **Key characteristics:** - **Immutable**: Once accepted, ADRs are rarely modified (supersede instead) - **Context-rich**: Capture the environment and constraints at decision time - **Outcome-focused**: Document what was decided and why - **Alternative-aware**: Show what options were considered and rejected ### Why Document Decisions? **Context Preservation:** - Future team members understand *why* things are the way they are - Avoid "we've always done it this way" without knowing the reason - Preserve institutional knowledge across team changes **Onboarding:** - New developers can read ADR history to understand system evolution - Reduces repetitive explanations of architectural choices - Provides learning path through project decisions **Avoiding Repeat Mistakes:** - Document why certain approaches were rejected - Prevent re-proposing failed solutions - Learn from past trade-offs **Alignment:** - Create shared understanding across team - Resolve disagreements with documented rationale - Enable asynchronous decision-making ### When to Write an ADR Write an ADR when: - **Significant architectural choices**: Microservices vs monolith, event-driven vs request/response - **Technology selections**: Database choice, framework adoption, language decisions - **Pattern decisions**: State management approach, authentication strategy, error handling patterns - **Breaking changes**: API redesigns, data model changes, migration strategies - **Performance-critical decisions**: Caching strategies, optimization approaches, scalability patterns - **Security-related choices**: Authentication methods, encryption strategies, access control models - **Infrastructure decisions**: Deployment strategy, hosting platform, CI/CD approach - **Dependency selections**: Major library choices, third-party service integrations **Don't write an ADR for:** - Minor implementation details - Temporary workarounds - Personal coding style preferences - Reversible low-impact choices ## ADR Structure ### Standard Format ```markdown # ADR-XXXX: [Short Title] ## Status [Proposed | Accepted | Deprecated | Superseded by ADR-YYYY] ## Date YYYY-MM-DD ## Context [Problem statement and environment] ## Decision [What was decided] ## Consequences [What results from this decision] ## Alternatives Considered [Other options evaluated] ## Related ADRs [Links to related decisions] ## References [External links and documentation] ``` ### Section Guidelines **Title:** - Format: `ADR-XXXX: Short Decision Title` - Be specific: "ADR-0012: Use PostgreSQL for Primary Database" - Not generic: "ADR-0012: Database Choice" **Status:** - **Proposed**: Under discussion, not yet accepted - **Accepted**: Decision is final and being implemented - **Deprecated**: No longer recommended but not replaced - **Superseded by ADR-YYYY**: Replaced by a newer decision **Date:** - Use ISO format: YYYY-MM-DD - Date of status change, not original proposal **Context:** - Describe the forces at play - Technical constraints - Business requirements - Team capabilities - Timeline pressures - Budget considerations - Example: "We need to store structured data with complex relationships. Team has limited database expertise. Must support 100k+ users within 6 months." **Decision:** - State clearly what was decided - Include scope and boundaries - Specify what is NOT included - Example: "We will use PostgreSQL as our primary database for all structured data. NoSQL solutions may be used for specific use cases (caching, session storage) but PostgreSQL is the default." **Consequences:** - **Positive**: What benefits we gain - **Negative**: What drawbacks we accept - **Neutral**: What trade-offs we're making Example: ```markdown ### Positive - Strong ACID guarantees for transactions - Rich query capabilities with SQL - Mature ecosystem and tooling ### Negative - Requires careful index management for performance - Scaling horizontally is more complex than NoSQL - Team needs PostgreSQL training ### Neutral - Need to establish backup/restore procedures - Must define connection pooling strategy ``` **Alternatives Considered:** - List 2-4 alternatives seriously evaluated - For each: Pros, Cons, Why rejected - Be fair to alternatives (avoid strawman arguments) **Related ADRs:** - Link to decisions that influenced this one - Link to decisions this one influences - Create a decision graph over time **References:** - Documentation links - Blog posts or articles that influenced decision - Benchmark results - Proof of concepts ## ADR Numbering and Naming ### Sequential Numbering Use zero-padded sequential numbers: - `ADR-0001`, `ADR-0002`, ..., `ADR-0010`, ..., `ADR-0100` - Four digits handles up to 9,999 ADRs (enough for most projects) - Don't skip numbers when superseding (new ADR gets next number) ### File Naming Format: `ADR-XXXX-short-title.md` Examples: - `ADR-0001-use-postgresql-database.md` - `ADR-0002-adopt-react-19-frontend.md` - `ADR-0003-implement-jwt-authentication.md` **Naming guidelines:** - Use lowercase - Use hyphens (not underscores or spaces) - Keep title short but descriptive - Include key technology/pattern name ### Location Recommended locations: - **Option 1**: `ai-docs/decisions/` (alongside other AI-focused docs) - **Option 2**: `docs/adr/` (standard documentation location) - **Option 3**: Root `adr/` directory (if ADRs are primary docs) **Choose consistently across project.** For Claude Code plugins: - Use `ai-docs/decisions/` to co-locate with CLAUDE.md and other AI docs - Makes ADRs easily discoverable by Claude ## When to Write an ADR ### Technology Adoption **Write an ADR when:** - Adopting a new framework or major library - Choosing between competing technologies - Upgrading to a new major version with breaking changes **Examples:** - ADR-0015: Migrate from Webpack to Vite - ADR-0023: Adopt TypeScript for Backend Services - ADR-0031: Use Bun Runtime Instead of Node.js ### Architectural Pattern Selection **Write an ADR when:** - Choosing architectural style (monolith, microservices, serverless) - Selecting state management approach - Defining API design patterns - Establishing error handling strategies **Examples:** - ADR-0008: Use Event-Driven Architecture for Order Processing - ADR-0012: Implement Repository Pattern for Data Access - ADR-0019: Adopt REST over GraphQL for Public API ### Breaking Changes **Write an ADR when:** - Making changes that break backward compatibility - Migrating data models - Redesigning core APIs - Changing authentication mechanisms **Examples:** - ADR-0025: Remove Legacy API v1 Endpoints - ADR-0033: Migrate from Sessions to JWT Tokens - ADR-0041: Change User ID from Integer to UUID ### Performance-Critical Decisions **Write an ADR when:** - Implementing caching strategies - Making database optimization choices - Selecting algorithms for hot paths - Deciding on async vs sync processing **Examples:** - ADR-0017: Implement Redis for Session Caching - ADR-0022: Use WebSockets for Real-Time Updates - ADR-0028: Adopt Background Job Queue for Heavy Processing ### Security-Related Choices **Write an ADR when:** - Choosing authentication methods - Implementing authorization strategies - Selecting encryption approaches - Defining security policies **Examples:** - ADR-0010: Use OAuth 2.0 with PKCE for Mobile Auth - ADR-0014: Implement Row-Level Security in PostgreSQL - ADR-0020: Adopt OWASP Guidelines for API Security ## ADR Lifecycle ### Proposal Phase **Creating a Proposal:** 1. Draft ADR with status "Proposed" 2. Share with team for feedback 3. Discuss in team meeting or async comments 4. Iterate on Context and Alternatives sections **Review Process:** - Ensure all alternatives are fairly represented - Verify consequences are realistic - Check that context captures current constraints - Validate decision aligns with project goals ### Acceptance **When to Accept:** - Team consensus reached (or decision maker approves) - Decision is being implemented - No major unresolved concerns **Accepting an ADR:** 1. Change status to "Accepted" 2. Update date to acceptance date 3. Commit to repository 4. Communicate to team **Post-Acceptance:** - ADR becomes immutable (don't edit decision or alternatives) - May add notes or references sections if new info emerges - Use comments to clarify, don't rewrite history ### Deprecation Process **When to Deprecate:** - Decision is no longer recommended but not replaced - Technology or pattern is obsolete - Security vulnerability makes approach unsafe **Deprecating an ADR:** 1. Change status to "Deprecated" 2. Add "Deprecation Note" section explaining why 3. Update date to deprecation date 4. Don't remove ADR from repository Example deprecation note: ```markdown ## Deprecation Note (2026-01-15) This decision has been deprecated due to security vulnerabilities discovered in the XYZ library. See ADR-0045 for the replacement approach using the ABC library. ``` ### Superseding Old ADRs **When to Supersede:** - Making a decision that replaces an old one - Reversing a previous decision - Evolving a decision with significant changes **Superseding Process:** 1. Create new ADR with next sequential number 2. Reference old ADR in "Context" section 3. Update old ADR status to "Superseded by ADR-XXXX" 4. Explain why old decision is being replaced **Example:** Old ADR-0012: ```markdown ## Status Superseded by ADR-0034 ``` New ADR-0034: ```markdown ## Context ADR-0012 chose MongoDB for our primary database. After 2 years of production use, we've encountered scaling issues with our relational data model. This ADR supersedes ADR-0012. ``` ## Integration with Claude Code ### Guiding Agent Behavior ADRs help Claude understand project conventions: **In CLAUDE.md:** ```markdown ## Architectural Decisions Key decisions documented in ai-docs/decisions/: - ADR-0001: Use PostgreSQL for all structured data - ADR-0005: Adopt React 19 with TypeScript strict mode - ADR-0012: Implement repository pattern for data access Claude should follow these patterns in all code generation. ``` ### Referencing ADRs **In skill files:** ```markdown ## Data Access Patterns Follow the repository pattern (ADR-0012). All database queries must go through repository classes in the `repositories/` directory. ``` **In agent instructions:** ```markdown When generating database code: 1. Use PostgreSQL (ADR-0001) 2. Implement via repository pattern (ADR-0012) 3. Use TypeORM for ORM layer (ADR-0018) ``` ### ADR Compliance Checking Create a validation agent to check ADR compliance: **Example agent task:** ```markdown Review the code changes and check compliance with ADRs: - ADR-0001: All database access uses PostgreSQL - ADR-0012: All queries go through repository classes - ADR-0022: Real-time features use WebSockets Report any violations. ``` ## Best Practices ### Do **✓ Write ADRs for significant decisions** - Technology choices with long-term impact - Architectural patterns affecting multiple components - Security or performance-critical decisions **✓ Document alternatives fairly** - Present real alternatives, not strawmen - Show pros/cons objectively - Explain rejection reasons clearly **✓ Keep ADRs immutable after acceptance** - Don't rewrite history - Use new ADRs to supersede old ones - Preserve original context and rationale **✓ Link related ADRs** - Show decision dependencies - Create decision graph over time - Help readers understand evolution **✓ Update status promptly** - Accept when implementation begins - Deprecate when no longer valid - Supersede when replaced ### Don't **✗ Don't over-document** - Skip ADRs for trivial choices - Don't write ADRs for personal style preferences - Avoid ADRs for easily reversible decisions **✗ Don't write vague context** - Bad: "We needed a database" - Good: "We need ACID transactions for financial data, team has SQL experience, must scale to 1M users" **✗ Don't ignore alternatives** - Always document what else was considered - Explain why alternatives were rejected - Be honest about trade-offs **✗ Don't let ADRs get stale** - Update status when decisions change - Deprecate obsolete ADRs - Don't leave "Proposed" ADRs open indefinitely **✗ Don't make ADRs too long** - Keep to 1-2 pages maximum - Link to detailed docs instead of including them - Focus on decision and rationale, not implementation details ### Writing Good Context **Include:** - Technical constraints (performance, security, scalability) - Business requirements (timeline, budget, features) - Team capabilities (skills, experience, size) - Environmental factors (existing systems, integrations) **Example - Poor Context:** ```markdown ## Context We need a database for our application. ``` **Example - Good Context:** ```markdown ## Context Our e-commerce platform needs to store product catalogs, orders, and user data. Key requirements: - ACID transactions for order processing (financial accuracy) - Support for complex queries (product search, filtering) - Scale to 100k daily active users within 6 months - Team has 3 years SQL experience, no NoSQL experience - Must integrate with existing PostgreSQL warehouse system - Budget allows managed database hosting Performance requirements: - 95th percentile query time < 100ms - Support 1000 concurrent connections - 99.9% uptime SLA ``` ### Documenting Alternatives Fairly **Structure for each alternative:** ```markdown ### Alternative X: [Technology/Pattern Name] **Description**: [Brief overview] **Pros**: - [Benefit 1] - [Benefit 2] - [Benefit 3] **Cons**: - [Drawback 1] - [Drawback 2] **Why Rejected**: [Honest explanation of why this wasn't chosen] ``` **Example:** ```markdown ### Alternative 2: MongoDB **Description**: Document-oriented NoSQL database **Pros**: - Flexible schema allows rapid iteration - Built-in horizontal scaling - Good for unstructured data **Cons**: - No ACID transactions across collections (deal-breaker for orders) - Team has no MongoDB experience (6-month learning curve) - Query language less powerful for complex joins **Why Rejected**: Lack of multi-document transactions is critical for our order processing workflow where we need to atomically update inventory, orders, and user balances. The 6-month learning curve also conflicts with our launch timeline. ``` ## Examples ### Example 1: Database Selection ```markdown # ADR-0001: Use PostgreSQL for Primary Database ## Status Accepted ## Date 2026-01-28 ## Context Our SaaS application needs a database for user accounts, subscriptions, and billing data. Requirements: **Functional:** - Store structured relational data (users, plans, invoices) - Support complex queries (reporting, analytics) - ACID transactions for billing operations **Non-Functional:** - Scale to 500k users in year one - 99.9% uptime requirement - Response time < 100ms for 95th percentile - Point-in-time recovery for data safety **Team & Environment:** - Team has 5+ years PostgreSQL experience - Existing monitoring tools support PostgreSQL - Running on AWS with RDS available - $500/month database budget ## Decision We will use PostgreSQL (version 15+) as our primary database for all structured data. Deployment via AWS RDS for managed service. **Scope:** - All user data, subscriptions, billing - Application-level caching with Redis - Logs stored in CloudWatch, not database ## Consequences ### Positive - Strong ACID guarantees protect billing integrity - Rich SQL query support for complex reports - Mature ecosystem (ORMs, monitoring, tools) - Team expertise minimizes ramp-up time - AWS RDS provides automated backups and high availability ### Negative - Horizontal scaling requires careful sharding strategy - More expensive than serverless options at low volume - Requires connection pool management ### Neutral - Need to establish index management process - Must define migration strategy for schema changes - Need to set up automated backup testing ## Alternatives Considered ### Alternative 1: MongoDB **Pros**: - Flexible schema for rapid iteration - Built-in horizontal scaling (sharding) - JSON-native data model **Cons**: