Architecture Patterns

Architecture Patterns

Reference for architectural design and documentation. Use this skill when:

• Architect reviews system design
• Planner considers architectural constraints
• Analyst investigates integration approaches
• Critic evaluates architectural alignment

Architecture Decision Records (ADR)

ADR Format

Every significant architectural decision should be documented:

# ADR-[NNN]: [Decision Title]

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[Proposed | Accepted | Deprecated | Superseded by ADR-XXX]

## Context
[What is the situation? What forces are at play?]

## Decision
[What is the change being proposed or decided?]

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

### Negative
- [Tradeoff 1]
- [Tradeoff 2]

### Neutral
- [Side effect]

## Alternatives Considered
1. [Alternative 1]: [Why rejected]
2. [Alternative 2]: [Why rejected]

## Related
- ADR-XXX: [Related decision]
- [External reference]

When to Write ADRs

Common Patterns

Layered Architecture

┌─────────────────────────────────┐
│         Presentation            │  UI, API endpoints
├─────────────────────────────────┤
│          Application            │  Use cases, orchestration
├─────────────────────────────────┤
│           Domain                │  Business logic, entities
├─────────────────────────────────┤
│        Infrastructure           │  DB, external services
└─────────────────────────────────┘

Rules:

• Dependencies point downward only
• Lower layers never import from higher
• Domain has no external dependencies

Use when: Enterprise apps, clear separation needed

Repository Pattern

Purpose: Abstract data access, enable testability

// Interface in domain layer
interface UserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
}

// Implementation in infrastructure layer
class PostgresUserRepository implements UserRepository {
  async findById(id: string): Promise<User | null> {
    const row = await db.query('SELECT * FROM users WHERE id = $1', [id]);
    return row ? mapToUser(row) : null;
  }
}

Use when:

• Need to swap data stores
• Testing without real database
• Multiple data sources

Service Layer

Purpose: Encapsulate business operations

class OrderService {
  constructor(
    private orderRepo: OrderRepository,
    private paymentGateway: PaymentGateway,
    private notifier: Notifier
  ) {}

  async placeOrder(cart: Cart, payment: PaymentInfo): Promise<Order> {
    const order = Order.fromCart(cart);
    await this.paymentGateway.charge(payment, order.total);
    await this.orderRepo.save(order);
    await this.notifier.sendConfirmation(order);
    return order;
  }
}

Use when:

• Multiple steps in operation
• Transaction coordination
• Cross-cutting concerns

Event-Driven Architecture

┌─────────┐    Event    ┌─────────┐
│ Service │───────────► │  Queue  │
│    A    │             │         │
└─────────┘             └────┬────┘
                             │
              ┌──────────────┼──────────────┐
              ▼              ▼              ▼
         ┌─────────┐   ┌─────────┐   ┌─────────┐
         │ Handler │   │ Handler │   │ Handler │
         │    1    │   │    2    │   │    3    │
         └─────────┘   └─────────┘   └─────────┘

Use when:

• Loose coupling between components
• Asynchronous processing
• Multiple consumers of same event
• Audit trail needed

Dependency Injection

Purpose: Invert control, enable testing

// Without DI (hard to test)
class OrderService {
  private db = new PostgresDatabase();
}

// With DI (testable)
class OrderService {
  constructor(private db: Database) {}
}

// Production
new OrderService(new PostgresDatabase());

// Test
new OrderService(new MockDatabase());

Anti-Patterns to Detect

Detection Commands

# Find large files (potential God objects)
find . -name "*.ts" -exec wc -l {} \; | sort -rn | head -10

# Find circular dependencies (TypeScript)
npx madge --circular src/

# Find files with many imports
grep -c "^import" src/**/*.ts | sort -t: -k2 -rn | head -10

System Architecture Documentation

Required Sections

For system-architecture.md:

1. Purpose: What does this system do?
2. High-Level Architecture: Diagram, major components
3. Components: Each component's responsibility
4. Data Flow: How data moves through system
5. Dependencies: External services, libraries
6. Quality Attributes: Performance, security, scalability goals
7. Decisions: ADRs or decision log
8. Known Issues: Technical debt, problem areas

Diagram Standards

Use Mermaid for version-controlled diagrams:

graph TB
    subgraph Presentation
        API[API Gateway]
        UI[Web UI]
    end
    
    subgraph Application
        Auth[Auth Service]
        Orders[Order Service]
    end
    
    subgraph Data
        DB[(PostgreSQL)]
        Cache[(Redis)]
    end
    
    UI --> API
    API --> Auth
    API --> Orders
    Orders --> DB
    Orders --> Cache

Reconciliation Changelog Template

When the Architect reconciles architecture docs after implementations, use this format in the system-architecture.md changelog:

| Date | Change | Rationale | Source |
|------|--------|-----------|--------|
| 2024-12-20 | Added memory retrieval caching layer | Reconciled from Plan-015 implementation | Plan-015-memory-caching |
| 2024-12-18 | Updated API boundary diagram | Implementation added new endpoint | Post-implementation audit |
| 2024-12-15 | Documented Cognee integration pattern | Previously undocumented, discovered during health audit | Health audit |

Reconciliation Entry Format:

• Date: When reconciliation occurred
• Change: What was updated in architecture docs
• Rationale: "Reconciled from Plan-NNN" or "Post-implementation audit" or "Health audit discovery"
• Source: Reference to plan, implementation, or audit that triggered reconciliation

Design Debt Registry Template

Track architectural improvements in the Problem Areas section of system-architecture.md:

## Problem Areas / Design Debt Registry

### Active Design Debt

| ID | Area | Current State | Optimal State | Priority | Discovered | Last Reviewed |
|----|------|---------------|---------------|----------|------------|---------------|
| DD-001 | Memory Subsystem | Direct Cognee calls scattered | Unified memory service facade | Medium | 2024-12-15 | 2024-12-20 |
| DD-002 | Error Handling | Inconsistent error types | Typed error hierarchy | Low | 2024-12-18 | 2024-12-18 |

### Resolved Design Debt

| ID | Resolution | Resolved Date | Related Plan |
|----|------------|---------------|--------------|
| DD-000 | Extracted shared utilities | 2024-12-10 | Plan-012 |

Design Debt Entry Fields:

• ID: Sequential identifier (DD-NNN)
• Area: Component or subsystem affected
• Current State: What exists now (brief)
• Optimal State: What would be better (brief)
• Priority: Critical / High / Medium / Low
• Discovered: When architect identified the debt
• Last Reviewed: When last evaluated (may affect priority)

Priority Guidelines:

• Critical: Blocking other improvements or causing active issues
• High: Should address in next 1-2 releases
• Medium: Address when touching related code
• Low: Nice-to-have, address opportunistically

Agent Responsibilities

Architect Agent

• Maintain system-architecture.md as single source of truth
• Document ADRs within master doc
• Challenge plans violating architectural constraints
• Provide verdicts: APPROVED / APPROVED_WITH_CHANGES / REJECTED

Analyst Agent

• Reference architecture when investigating integration points
• Consult Architect for systemic pattern questions
• Document architectural context in analysis findings

Planner Agent

• Read system-architecture.md before planning
• Ensure plans respect documented patterns
• Flag architectural impact in plan scope

Critic Agent

• Reference system-architecture.md during plan review
• Verify architectural alignment
• Flag plans that violate documented decisions

See references/diagram-templates.md for diagram examples.

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