Architecture Decision

Systematically evaluate architecture decisions, document trade-offs, and select appropriate patterns for context. Provides frameworks for pattern selection, ADR creation, and technical debt management.

When to Use This Skill

Use this skill when:

• Making technology choices
• Evaluating architectural patterns
• Creating Architecture Decision Records
• Assessing technical debt
• Comparing design alternatives

Do NOT use this skill when:

• Writing implementation code
• Working on requirements (use requirements-analysis)
• Doing full system design (use system-design)

Core Principle

Context drives decisions. No pattern is universally good or bad. The best architecture is not the most elegant—it's the one that best serves its purpose while remaining maintainable and evolvable.

The Trade-off Triangle

Every architectural decision involves trade-offs:

Vertex	Maximized By	Cost
Simplicity	Monolith, sync communication, single DB	Scalability limits
Flexibility	Microservices, event-driven, plugins	Complexity overhead
Performance	Caching, denormalization, optimized code	Maintainability

Balance Strategies:

• Start simple, add complexity as needed
• Measure before optimizing
• Use abstractions to defer decisions
• Evolve incrementally

Quality Attributes

Performance

• Metrics: Response time (p50, p95, p99), throughput, resource utilization
• Tactics: Caching, load balancing, async processing

Scalability

• Dimensions: Horizontal, vertical, elastic
• Patterns: Stateless services, sharding, event streaming

Reliability

• Metrics: Uptime, MTBF, MTTR
• Patterns: Circuit breakers, retries, redundancy

Maintainability

• Factors: Readability, modularity, testability
• Patterns: Clean architecture, DDD, SOLID

Context-Pattern Mapping

Team Context

Context	Preferred Patterns	Avoid
Small team	Monolith, vertical slices, shared DB	Microservices, complex abstractions
Multiple teams	Service boundaries, API contracts	Shared state, tight coupling

Scale Context

Context	Preferred Patterns	Reasoning
Startup	Monolith first, vertical scaling	Optimize for development speed
Enterprise	Service mesh, horizontal scaling	Optimize for operational scale

Decision Matrix Template

Option	Consistency	Flexibility	Scalability	Complexity	Cost	Total
Option A	5	2	3	2	3	15
Option B	3	5	4	3	3	18
Option C	2	3	5	1	2	13

Weight factors based on context priorities.

Architecture Decision Record (ADR) Template

# ADR-[NUMBER]: [TITLE]

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

## Context
[What is the situation requiring a decision?]

### Requirements
- [Requirement 1]
- [Requirement 2]

### Constraints
- [Constraint 1]
- [Constraint 2]

## Decision
[What is the decision?]

### Justification
- [Reason 1]
- [Reason 2]

## Consequences

### Positive
- [Benefit 1]
- [Benefit 2]

### Negative
- [Drawback 1]
- [Drawback 2]

## Alternatives Considered

### [Alternative 1]
Reason rejected: [Why]

### [Alternative 2]
Reason rejected: [Why]

Architectural Refactoring Patterns

Branch by Abstraction

1. Create abstraction over current implementation
2. Implement new solution behind abstraction
3. Switch to new implementation
4. Remove old implementation

Strangler Fig

1. Identify boundary
2. Implement new solution for new features
3. Gradually migrate old features
4. Retire old system

Parallel Run

1. Implement new solution
2. Run both old and new
3. Compare results
4. Switch when confident

Technical Debt Management

Debt Categories

Type	Examples	Payment Strategy
Design	Missing abstractions, tight coupling	Refactoring sprints
Code	Duplication, complexity, poor naming	Continuous cleanup
Test	Missing tests, flaky tests	Test improvement
Documentation	Missing docs, outdated diagrams	Documentation sprints

Metrics

• Debt ratio: Debt work / Total work (target < 20%)
• Interest rate: Extra effort due to debt
• Debt ceiling: Maximum acceptable debt

Anti-Patterns

Big Ball of Mud

Symptoms: No clear structure, everything depends on everything Remedy: Identify boundaries, extract modules, establish interfaces

Distributed Monolith

Symptoms: Services must deploy together, sync chains, shared DBs Remedy: Merge related services, async communication, separate DBs

Golden Hammer

Symptoms: One solution for all problems, force-fitting patterns Remedy: Learn alternatives, evaluate objectively, prototype options

Related Skills

• system-design - Full system design with ADRs
• code-review - Implementation validation
• task-decomposition - Breaking down architectural work
• requirements-analysis - Understanding constraints