Architecture Decision Records (ADRs)

ADRs capture important architectural decisions along with their context and consequences. They serve as a decision log that helps teams understand why certain choices were made.

Why ADRs Matter

• Institutional Memory: New team members understand past decisions
• Avoid Repeated Discussions: Document why alternatives were rejected
• Accountability: Track who made decisions and when
• Learning: Review past decisions to improve future ones

ADR Template

# ADR-001: Use Event Sourcing for Order Management

## Status
Accepted

## Date
2024-01-15

## Context
The order management system needs to:
- Track complete order history for audit and compliance
- Support temporal queries ("what was the order state on date X?")
- Enable event-driven integration with other services
- Handle high write throughput during peak sales

Current CRUD-based approach loses historical state and requires 
separate audit logging.

## Decision
We will implement Event Sourcing for the Order aggregate using 
EventStoreDB as the event store.

## Consequences

### Positive
- Complete audit trail without separate logging
- Natural fit for event-driven architecture
- Temporal queries supported out of the box
- High write performance with append-only storage

### Negative
- Increased complexity for developers unfamiliar with ES
- Need to implement projections for read models
- Event schema evolution requires careful planning
- Higher initial development cost

### Risks
- Team learning curve may slow initial delivery
- EventStoreDB operational expertise needed

## Alternatives Considered

### 1. CRUD with Audit Tables
- Simpler implementation
- Rejected: Doesn't support temporal queries, audit tables add complexity

### 2. Temporal Tables (SQL Server)
- Built-in SQL Server feature
- Rejected: Vendor lock-in, limited event-driven capabilities

### 3. Kafka as Event Store
- Team has Kafka experience
- Rejected: Not designed as event store, lacks optimistic concurrency

## References
- [Event Sourcing pattern](https://docs.microsoft.com/en-us/azure/architecture/patterns/event-sourcing)
- [EventStoreDB documentation](https://www.eventstore.com/docs/)

ADR Lifecycle

Organizing ADRs

docs/
└── architecture/
    └── decisions/
        ├── README.md           # Index of all ADRs
        ├── 0001-use-event-sourcing.md
        ├── 0002-api-gateway-selection.md
        ├── 0003-database-per-service.md
        └── 0004-authentication-strategy.md

<!-- README.md - ADR Index -->
# Architecture Decision Records

| ADR | Title | Status | Date |
|-----|-------|--------|------|
| [ADR-0001](0001-use-event-sourcing.md) | Use Event Sourcing | Accepted | 2024-01-15 |
| [ADR-0002](0002-api-gateway-selection.md) | API Gateway Selection | Accepted | 2024-01-20 |
| [ADR-0003](0003-database-per-service.md) | Database per Service | Accepted | 2024-02-01 |
| [ADR-0004](0004-authentication-strategy.md) | Authentication Strategy | Proposed | 2024-02-10 |

Real-World ADR Examples

ADR: API Gateway Selection

# ADR-002: Use YARP for API Gateway

## Status
Accepted

## Context
We need an API Gateway to:
- Route requests to microservices
- Handle authentication/authorization
- Implement rate limiting
- Provide request aggregation

Options evaluated:
1. YARP (Yet Another Reverse Proxy)
2. Ocelot
3. Kong
4. AWS API Gateway

## Decision
Use YARP as our API Gateway.

## Rationale
- Native .NET integration (team expertise)
- High performance (benchmarks show 2x Ocelot throughput)
- Extensible middleware pipeline
- Active Microsoft support
- No licensing costs

## Consequences
- Need to implement some features ourselves (admin UI)
- Less ecosystem than Kong
- Team needs to learn YARP configuration

## Decision Makers
- @john.smith (Tech Lead)
- @jane.doe (Principal Engineer)

ADR: Messaging Infrastructure

# ADR-003: Use RabbitMQ with MassTransit

## Status
Accepted

## Context
Microservices need asynchronous communication for:
- Event publishing
- Command distribution
- Saga orchestration

Requirements:
- At-least-once delivery
- Message ordering per partition
- Dead letter handling
- Saga state management

## Decision
Use RabbitMQ as message broker with MassTransit as abstraction layer.

## Rationale
- MassTransit provides saga support out of the box
- RabbitMQ is proven, well-understood
- Team has operational experience with RabbitMQ
- Can migrate to different broker later via MassTransit

## Alternatives Rejected

### Kafka
- Overkill for our message volume
- More complex operations
- Team lacks expertise

### Azure Service Bus
- Higher cost at our scale
- Vendor lock-in concerns

## Consequences
- Need RabbitMQ cluster for HA
- MassTransit learning curve
- Additional infrastructure to manage

ADR Review Process

// Example: ADR as code validation
public class AdrValidator
{
    public ValidationResult Validate(string adrContent)
    {
        var errors = new List<string>();
        
        // Required sections
        var requiredSections = new[] 
        { 
            "## Status", 
            "## Context", 
            "## Decision", 
            "## Consequences" 
        };
        
        foreach (var section in requiredSections)
        {
            if (!adrContent.Contains(section))
            {
                errors.Add($"Missing required section: {section}");
            }
        }
        
        // Status must be valid
        var validStatuses = new[] 
        { 
            "Proposed", 
            "Accepted", 
            "Deprecated", 
            "Superseded", 
            "Rejected" 
        };
        
        var statusMatch = Regex.Match(adrContent, @"## Status\s+(\w+)");
        if (statusMatch.Success)
        {
            var status = statusMatch.Groups[1].Value;
            if (!validStatuses.Contains(status))
            {
                errors.Add($"Invalid status: {status}");
            }
        }
        
        return new ValidationResult(errors);
    }
}

Tips for Effective ADRs

1. Write ADRs at decision time - Not weeks later when context is forgotten
2. Keep them concise - One decision per ADR
3. Include rejected alternatives - Prevents revisiting the same options
4. Link related ADRs - Show how decisions connect
5. Review periodically - Mark outdated ADRs as deprecated

Key Takeaways

1. Document Decisions: ADRs preserve architectural knowledge
2. Include Context: Future readers need to understand the situation
3. List Alternatives: Show what was considered and why rejected
4. Track Status: Keep ADRs updated as decisions evolve
5. Make Discoverable: Index and organize ADRs for easy access

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💡 Flashcard

What are the essential sections of an Architecture Decision Record?

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✅ Answer

Status (Proposed/Accepted/Deprecated/Superseded), Context (why the decision was needed), Decision (what was chosen), Consequences (positive and negative impacts), and optionally Alternatives Considered.

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💡 Flashcard

Why should ADRs include rejected alternatives?

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✅ Answer

Including rejected alternatives prevents teams from repeatedly discussing the same options, documents why certain approaches weren't suitable, and helps future team members understand the full decision context.

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