Architecture Decision Records (ADRs)

Documenting Architectural Choices

Introduction

Architecture Decision Records (ADRs) capture significant architectural decisions made for a system, including the context, decision, and consequences. For integration workloads that evolve over time, ADRs provide a historical record of why decisions were made, enabling better future decisions and onboarding of new team members.

This comprehensive guide covers:

ADR format — Structure and content
Workflow — Creating and managing ADRs
Integration examples — Common architectural decisions
Maintenance — Keeping ADRs current

ADR Format

Standard Template

# ADR-XXX: [Title]

## Status
[Proposed | Accepted | Deprecated | Superseded by ADR-XXX]

## Context
[Describe the issue driving this decision]

## Decision
[Describe the change being proposed/decided]

## Consequences
[Describe resulting changes - positive, negative, neutral]

### Positive
-

### Negative
-

### Neutral
-

## Related Decisions
[Links to related ADRs]

## Notes
[Additional context, alternatives considered, etc.]

Integration-Specific ADRs

Example: Event Delivery Pattern

# ADR-015: Use Service Bus over Event Grid for Order Processing

## Status
Accepted

## Context
We need to decouple our order processing system from downstream services. 
We evaluated Azure Event Grid and Service Bus for message delivery.

## Decision
Use Azure Service Bus for order processing with the following configuration:
- Standard tier with topics and subscriptions
- Dead letter queue for failed messages
- Session support for ordered processing

## Consequences
### Positive
- Guaranteed delivery with peek-lock
- Supports complex routing via topics
- Enables ordered processing via sessions
- Mature SDK and tooling

### Negative
- Higher cost than Event Grid for simple scenarios
- Requires more operational management
- Additional complexity for sessions

### Neutral
- Different monitoring approach than Event Grid

## Related Decisions
- ADR-012: Use managed identities for service authentication
- ADR-018: Implement outbox pattern for data consistency

ADR Workflow

ADR Lifecycle

┌─────────────────────────────────────────────────────────────────────┐
│                  ADR LIFECYCLE                                      │
├─────────────────────────────────────────────────────────────────────┤
│                                                                     │
│   PROPOSED                                                          │
│   ├── Write initial ADR with context and options                    │
│   ├── Share with team for review                                    │
│   └── Gather feedback                                               │
│                                                                     │
│   REVIEWED                                                          │
│   ├── Discuss in architecture review (optional)                     │
│   ├── Update based on feedback                                      │
│   └── Document alternatives considered                              │
│                                                                     │
│   ACCEPTED                                                          │
│   ├── Mark as accepted                                              │
│   ├── Implement the decision                                        │
│   └── Reference in code/docs                                        │
│                                                                     │
│   AMENDED (if needed)                                               │
│   ├── Changes to implementation                                     │
│   └── Update ADR to reflect changes                                 │
│                                                                     │
│   DEPRECATED (when superseded)                                      │
│   ├── Link to new ADR that supersedes this one                      │
│   └── Document why the old approach was replaced                    │
│                                                                     │
└─────────────────────────────────────────────────────────────────────┘

ADR Repository Structure

docs/
├── adr/
│   ├── README.md                    # Index of ADRs
│   ├── ADR-001-use-typescript.md    # First ADR
│   ├── ADR-002-implement-auth.md    # Second ADR
│   └── ...

ADR Index Example

# Architecture Decision Records

## Index

| ADR | Title | Status | Date |
|-----|-------|--------|------|
| ADR-001 | Use TypeScript for Functions | Accepted | 2024-01-15 |
| ADR-002 | Implement Azure AD authentication | Accepted | 2024-02-01 |
| ADR-003 | Use Service Bus for order processing | Accepted | 2024-02-15 |
| ADR-004 | Implement CI/CD with GitHub Actions | Accepted | 2024-03-01 |
| ADR-005 | Use managed identities | Proposed | 2024-03-15 |

Common Integration ADRs

Sample Decisions

{
  "commonIntegrationADRs": {
    "messaging": [
      "Use Service Bus vs Event Grid vs Event Hubs",
      "Implement dead letter handling strategy",
      "Use sessions for ordered processing"
    ],
    "identity": [
      "Use managed identities vs service principals",
      "Authentication mechanism for APIs",
      "Implement RBAC for integration services"
    ],
    "compute": [
      "Use Functions vs Logic Apps vs Container Apps",
      "Configure scaling strategy",
      "Hosting plan selection"
    ],
    "storage": [
      "Use Cosmos DB vs SQL vs Table storage",
      "Data retention and archiving strategy",
      "Caching strategy"
    ]
  }
}

Best Practices

Writing Effective ADRs

┌─────────────────────────────────────────────────────────────────────┐
│                  ADR BEST PRACTICES                                 │
├─────────────────────────────────────────────────────────────────────┤
│                                                                     │
│   ✓ Write when making significant decisions                         │
│   ✓ Include context - what was the problem?                         │
│   ✓ Document alternatives considered                                │
│   ✓ Be specific about the decision                                  │
│   ✓ List consequences - what's the impact?                          │
│   ✓ Update when decisions change                                    │
│   ✓ Link related ADRs                                               │
│   ✓ Keep ADRs focused on single decision                            │
│                                                                     │
│   FORMAT:                                                           │
│   • Number sequentially                                             │
│   • Use clear, descriptive titles                                   │
│   • Keep concise - few hundred words max                            │
│   • Use tables for structured information                           │
│                                                                     │
└─────────────────────────────────────────────────────────────────────┘