Architecture Decision Records

Architecture Decision Records

Capture architectural decisions systematically so your team understands not just what was decided, but why — and what alternatives were considered.

Core Principles

1. Decisions Are More Important Than Diagrams

A diagram shows the current architecture. An ADR explains why it is that way. When someone asks "why did we do it this way?" the ADR is the answer.

2. Capture Context, Not Just Conclusions

Every architectural decision exists in a web of constraints, tradeoffs, and alternatives. If you only record the conclusion, future engineers will wonder if you considered the obvious alternative — and they might reverse it without understanding why the original choice was made.

3. Lightweight Is Sustainable

An ADR doesn't need to be a 10-page document. A structured 1-page record is infinitely better than nothing. If the process is heavy, people won't follow it.

4. Accept and Track Superseded Decisions

Architecture evolves. An ADR that gets superseded is a success — it means the system adapted. Old ADRs remain valuable as historical records of the team's thinking.

ADR Maturity Model

Target: Level 3 for most teams. Level 4+ for regulated or long-lived systems.

Actionable Guidance

The Standard ADR Template

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# ADR-{NNN}: {Title}

## Status
[Proposed | Accepted | Deprecated | Superseded]

*If Superseded, list the replacing ADR: Superseded by ADR-{NNN}*

## Context
{Describe the problem, constraints, and forces at play. 
 What is the business or technical need? 
 What are the non-negotiable constraints?
 What options were considered?}

## Decision
{State the decision clearly. 
 What are we doing? What are we NOT doing?}

## Consequences
{List the positive and negative consequences of this decision.
 What tradeoffs are we accepting?
 What becomes easier? What becomes harder?}

## Alternatives Considered
{List alternatives and why they were rejected. This is the most 
 important section for future readers.}

### Option A: {Name}
- **Pros**: ...
- **Cons**: ...
- **Why rejected**: ...

### Option B: {Name}
- **Pros**: ...
- **Cons**: ...
- **Why rejected**: ...

## Compliance
{How will we verify this decision is followed?
 Automated checks? Manual review? Linting rules?}

The Lightweight ADR Template

For quick decisions that still need recording:

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# ADR-042: Use PostgreSQL for Analytics Store

**Status**: Accepted  
**Date**: 2024-03-15  
**Author**: Alice Chen  
**Deciders**: Alice Chen, Bob Smith, Carol Davis

## Context
We need a store for aggregated analytics data. Requirements: 
JSON support, time-series optimized, managed service preferred.

## Decision
Use PostgreSQL with TimescaleDB extension on RDS.

## Rationale
- JSONB for flexible event schemas
- TimescaleDB hypertables for time-series queries
- RDS for managed operations
- Team already familiar with PostgreSQL

## Alternatives
- **MongoDB**: Better for unstructured data, but adds operational complexity 
  and team lacks expertise → rejected
- **ClickHouse**: Excellent for analytics but overkill for our volume (100k events/day) → rejected

## Consequences
+ Existing PostgreSQL expertise applies
+ Single database reduces operational burden
- Need to learn TimescaleDB syntax
- JSONB queries are less performant than dedicated document store

ADR Workflow

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┌────────────┐   ┌──────────────┐   ┌────────────┐   ┌───────────────┐
│  Identify  │   │   Draft     │   │  Review   │   │   Accept &   │
│ Decision  ─┼─► │   ADR      ─┼─► │  & Discuss│──►│   Commit    │
│ Needed    │   │  (Proposed) │   │           │   │  (Accepted)  │
└────────────┘   └──────────────┘   └────────────┘   └───────────────┘
                                           │                  │
                                           │  Rejected        │  Later
                                           ▼                  ▼
                                     ┌──────────┐     ┌──────────────┐
                                     │  Revise  │     │  Superseded  │
                                     │  or File │     │  by New ADR  │
                                     └──────────┘     └──────────────┘

Step 1: When to Write an ADR

Write an ADR when the decision:

• Is significant: Changes the architecture, not just implementation
• Is irreversible: Hard to undo (database choice, framework, cloud provider)
• Has tradeoffs: There's no obvious "right" answer
• Will be referenced later: Someone will ask "why?"
• Involves cost: Financial, operational, or opportunity cost

Examples of ADR-worthy decisions:

• Choosing a database, message queue, or cache
• Adopting a new framework or major library
• API design decisions (REST vs GraphQL vs gRPC)
• Deployment strategy (Kubernetes vs serverless)
• Data model changes (schema design, migration strategy)
• Security architecture (auth flows, encryption approach)

Examples of non-ADR decisions:

• Renaming a variable or function
• Adding a minor dependency with no architectural impact
• Bug fixes or minor refactoring
• Configuration changes (environment variables, feature flags)

Step 2: Draft the ADR

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# Create the ADR file
mkdir -p docs/adr/
cp templates/adr-template.md docs/adr/ADR-043-use-graphql-for-public-api.md

# ADR naming convention
# ADR-{NNN}-{short-descriptive-slug}.md
# Use leading zeros for sorting: ADR-001, ADR-002, ..., ADR-043

Step 3: Review

Include the ADR in the same PR as the implementation, or as a standalone PR for purely architectural decisions. Reviewers should check:

• Is the context clear? Can a new team member understand the problem?
• Are alternatives fairly represented? Not straw-man arguments?
• Are consequences honestly assessed? Both positive and negative?
• Is the decision aligned with existing architecture?
• Are there better alternatives we haven't considered?

Step 4: Accept and Maintain

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# After acceptance, the ADR status changes to "Accepted"
# If the decision is later revisited:

## Status
Superseded by ADR-052

## Rationale for Deprecation
In 2024, a managed Kafka service became available that eliminates
the operational overhead that motivated our original SQS choice.
The scale of our event processing has also grown 10x since ADR-021.

Storing ADRs with Code

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project/
├── docs/
│   └── adr/
│       ├── index.md              # Catalog of all ADRs
│       ├── ADR-001-initial-project-structure.md
│       ├── ADR-002-database-selection.md
│       ├── ADR-003-api-protocol.md
│       ├── ADR-004-deprecated-by-008.md
│       ├── ...
│       └── ADR-052-event-stream-architecture.md
└── .adr-dir                      # Points to the ADR directory

Why store ADRs in the repository:

• Version controlled alongside the code they describe
• Visible in the same PRs as the implementation
• Found by new team members exploring the codebase
• Branch-specific ADRs for experiment documentation

ADR Index Template

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# Architecture Decision Records

## Active (Accepted)

| ADR | Title | Date | Area |
|-----|-------|------|------|
| ADR-003 | API Protocol: GraphQL | 2024-01-20 | API |
| ADR-002 | Database: PostgreSQL | 2024-01-15 | Data |
| ADR-008 | Event Bus: RabbitMQ | 2024-02-10 | Infrastructure |

## Proposed

| ADR | Title | Date | Author |
|-----|-------|------|--------|
| ADR-009 | Cache Strategy: Redis with write-through | 2024-03-01 | Alice |

## Deprecated / Superseded

| ADR | Title | Superseded By | Date |
|-----|-------|---------------|------|
| ADR-001 | Initial: SQLite | ADR-002 | 2024-01-15 |
| ADR-004 | Event Bus: SQS | ADR-008 | 2024-02-10 |

Advanced ADR Patterns

Combining Multiple Decisions

Sometimes one PR involves several related decisions. Handle with care:

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# ADR-030: Order Service Decomposition

**Status**: Accepted

## This ADR covers three decisions:
1. Extract order management from the monolith
2. Use event-driven communication between order and inventory services
3. Adopt PostgreSQL for the order service database

## Decision
Extract the Order Service as a standalone service...

Alternative: Write one ADR per decision and reference them:

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ADR-031: Extract Order Service from Monolith
ADR-032: Event-Driven Communication for Order Service
ADR-033: Database Selection for Order Service

Y-Statements

A concise format for decisions with clear tradeoffs:

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## Decision (Y-Statement)

In the context of {situation/need},
facing {constraint/force},
we decided for {option A} over {option B}
to achieve {positive consequence},
accepting {negative consequence}.

---

**Example:**
In the context of needing real-time notifications across services,
facing the constraint of not wanting to manage a dedicated messaging infrastructure,
we decided for AWS SNS over RabbitMQ
to achieve zero operational overhead for pub/sub messaging,
accepting vendor lock-in to AWS and higher per-message costs at scale.

Capturing Rejected Decisions

Sometimes the most valuable ADR is the one about a decision you didn't take:

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# ADR-017: Rejected — Migrate to Microservices

**Status**: Rejected  
**Date**: 2024-02-01

## Context
Proposal to break the monolith into microservices for better scalability.

## Decision
We decided NOT to pursue microservice decomposition at this time.

## Rationale
- Team size (6 engineers) is too small to manage N services
- Current monolith handles 10k RPM comfortably
- Deployment frequency is satisfactory (daily)
- Distributed transactions would add complexity without clear benefit
- We'll revisit this when:
  a) Team grows to 15+
  b) Monolith deployment takes >30 minutes
  c) Two or more features need different scaling policies

Architecture Governance Patterns

Architecture Review Board (ARB)

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# Architecture Review Board Charter

## Purpose
Ensure architectural consistency and quality across all products.

## Composition
- 1 Staff Engineer (permanent)
- 2 Senior Engineers (rotating, 6-month term)
- 1 Product Manager (non-voting)

## When to Escalate
- Cross-team architectural decisions
- Technology stack additions
- Major refactoring or migrations
- Decisions with significant cost implications

## Process
1. Author drafts ADR → send to ARB
2. ARB reviews within 1 week
3. ARB meeting to discuss (if needed)
4. Decision documented in ADR status

Automated ADR Linting

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# .adr-lint.yml
rules:
  required-sections:
    - Status
    - Context
    - Decision
    - Consequences
    - Alternatives Considered
  
  status-values:
    allowed:
      - Proposed
      - Accepted
      - Deprecated
      - Superseded
      - Rejected
  
  naming:
    pattern: '^ADR-\d{3}-[a-z0-9-]+\.md$'
    message: "ADR files must follow ADR-{NNN}-{slug}.md naming"
  
  no-duplicate-numbers: true
  
  index-required: true
  index-path: 'docs/adr/index.md'

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# Run ADR linting in CI
npx adr-lint docs/adr/

# Example output:
# ✓ ADR-001: All required sections present
# ✓ ADR-002: All required sections present
# ✗ ADR-003: Missing "Alternatives Considered" section
# ✓ ADR-004: Valid status "Accepted"
# ✗ ADR-005: Invalid naming — use ADR-005-{slug}.md

Linking ADRs to Code

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# In code comments, reference the ADR that explains the design choice

# Uses Redis-backed rate limiting (see ADR-022)
# Rationale: We need distributed rate limiting across 10 instances
# and in-memory approaches won't work with horizontal scaling.
from ratelimit import RateLimiter

# SQLite for local dev, PostgreSQL in production (see ADR-002)
if config.ENV == "production":
    db = PostgresDatabase(config.DATABASE_URL)
else:
    db = SQLiteDatabase(":memory:")

# Using UUID v4 instead of auto-increment IDs (see ADR-015)
# Rationale: Prevents ID enumeration and simplifies sharding
order_id = uuid.uuid4()

ADR Change Tracking

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# ADR-010: Authentication Architecture

## Status
Accepted (Updated 2024-03-01)

## Changelog
| Date | Change | Author |
|------|--------|--------|
| 2024-01-15 | Initial draft | Alice |
| 2024-01-20 | Added SSO requirement | Bob |
| 2024-02-01 | Accepted after ARB review | Carol |
| 2024-03-01 | Updated token expiry from 1h to 24h based on UX feedback | Alice |

ADR Tools

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# adr-tools (command-line)
# https://github.com/npryce/adr-tools

# Install
brew install adr-tools

# Create a new ADR
adr new Use PostgreSQL for analytics store
# Creates: doc/adr/0001-use-postgresql-for-analytics-store.md

# List all ADRs
adr list

# Mark as superseded
adr supersede 0001 0008  # ADR-001 is superseded by ADR-008

# Link ADRs
adr link 0001 "Amends" 0003

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# Log4brains (modern ADR manager with UI)
# https://github.com/thomvaill/log4brains

# Install
npm install -g @log4brains/cli

# Initialize
log4brains init

# Create ADR
log4brains adr:new

# Preview the knowledge base
log4brains preview

# Build static site
log4brains build

Common Mistakes

1. Writing ADRs after implementation: The decision should be captured before or during implementation, not months later when nobody remembers the tradeoffs.
2. Too much detail, too little signal: An ADR isn't a design document. It shouldn't describe the implementation, just the architectural decision and why.
3. No alternatives section: This is the most valuable part of an ADR. Future engineers need to know what else was considered and why it was rejected.
4. Status never updated: An accepted ADR that is no longer true is misleading. Keep status current — "Superseded" or "Deprecated" are valid statuses.
5. ADRs stored outside the repository: Wiki pages and shared drives get lost. Keep ADRs with the code they describe.
6. Too many ADRs for trivial decisions: Reserve ADRs for meaningful architectural choices. Not every npm package addition needs an ADR.
7. No review process: An ADR that nobody reads might as well not exist. Ensure ADRs are reviewed as part of your standard workflow.
8. ADRs as blame documents: The purpose is understanding, not accountability. An ADR should never be used to say "see, I told you so" when a decision ages poorly.