Designing Architecture

Principles and patterns for designing maintainable, scalable, and robust software systems.

When to Use This Skill

• Designing new systems or features
• Choosing between architectural patterns
• Making technology decisions
• Reviewing system design
• Planning for scalability
• Refactoring legacy systems

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Core Architecture Principles

SOLID Principles

Principle	Summary	Violation Sign
Single Responsibility	One reason to change	Class does too many things
Open/Closed	Open for extension, closed for modification	Modifying existing code for new features
Liskov Substitution	Subtypes replaceable for base types	Overrides break parent behavior
Interface Segregation	Small, focused interfaces	Classes implement unused methods
Dependency Inversion	Depend on abstractions	High-level modules depend on low-level

The Dependency Rule

Outer layers depend on inner layers, NEVER the reverse.

┌─────────────────────────────────────┐
│     Frameworks & Drivers           │  ← Database, Web, UI
├─────────────────────────────────────┤
│     Interface Adapters             │  ← Controllers, Presenters, Gateways
├─────────────────────────────────────┤
│     Application Business Rules     │  ← Use Cases
├─────────────────────────────────────┤
│     Enterprise Business Rules      │  ← Entities
└─────────────────────────────────────┘

Dependencies point INWARD only.

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Architectural Patterns

Layered Architecture

┌─────────────────────────────────────┐
│     Presentation Layer             │  ← UI, API Controllers
├─────────────────────────────────────┤
│     Application Layer              │  ← Use Cases, Services
├─────────────────────────────────────┤
│     Domain Layer                   │  ← Business Logic, Entities
├─────────────────────────────────────┤
│     Infrastructure Layer           │  ← Database, External APIs
└─────────────────────────────────────┘

Use when: Traditional applications, clear separation needed Avoid when: High-performance needs, event-driven systems

Hexagonal Architecture (Ports & Adapters)

           ┌───────────────┐
           │   Primary     │
           │   Adapters    │  ← REST API, CLI, GraphQL
           └───────┬───────┘
                   │
        ┌──────────▼──────────┐
        │                     │
        │   ┌───────────┐     │
        │   │   Core    │     │
Primary │   │  Domain   │     │ Secondary
Ports   │   │  Logic    │     │ Ports
        │   └───────────┘     │
        │                     │
        └──────────┬──────────┘
                   │
           ┌───────▼───────┐
           │   Secondary   │
           │   Adapters    │  ← Database, Message Queue, External API
           └───────────────┘

Use when: Testability is critical, multiple interfaces needed Avoid when: Simple CRUD applications

Microservices Architecture

┌─────────┐  ┌─────────┐  ┌─────────┐
│ Service │  │ Service │  │ Service │
│    A    │  │    B    │  │    C    │
└────┬────┘  └────┬────┘  └────┬────┘
     │            │            │
     └────────────┴────────────┘
                  │
           ┌──────▼──────┐
           │   Message   │
           │    Bus      │
           └─────────────┘

Use when: Independent scaling, team autonomy, polyglot needs Avoid when: Small teams, simple domains, tight coupling required

Event-Driven Architecture

Event Source → Event Bus → Event Handlers
     │              │              │
     ▼              ▼              ▼
  Produces    Routes Events    Consumes
  Events      (Kafka, RabbitMQ)  Events

Use when: Async processing, decoupling, audit trails Avoid when: Immediate consistency required, simple workflows

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Design Patterns

Creational Patterns

Pattern	Purpose	When to Use
Factory	Create objects without specifying class	Object creation logic is complex
Builder	Construct complex objects step-by-step	Many optional parameters
Singleton	Single instance globally	Shared resource (use sparingly)
Dependency Injection	Inject dependencies externally	Testability, loose coupling

Structural Patterns

Pattern	Purpose	When to Use
Adapter	Convert interface to another	Integrating incompatible systems
Decorator	Add behavior dynamically	Extending functionality without inheritance
Facade	Simplified interface to complex system	Hiding complexity
Repository	Abstract data access	Separating domain from persistence

Behavioral Patterns

Pattern	Purpose	When to Use
Strategy	Interchangeable algorithms	Multiple ways to do something
Observer	Notify dependents of changes	Event systems, reactive updates
Command	Encapsulate actions as objects	Undo/redo, queuing, logging
Chain of Responsibility	Pass request along handlers	Middleware, validation chains

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Domain-Driven Design Concepts

Strategic Design

Concept	Definition	Example
Bounded Context	Explicit boundary for a domain model	Order context, Shipping context
Ubiquitous Language	Shared vocabulary between devs and domain experts	"Order", "Line Item", "Fulfillment"
Context Map	How bounded contexts relate	Customer shared between Sales and Support

Tactical Patterns

Pattern	Purpose	Example
Entity	Object with identity	User, Order
Value Object	Object without identity	Money, Address
Aggregate	Cluster of entities with root	Order + LineItems
Domain Event	Something that happened	OrderPlaced, PaymentReceived
Repository	Collection-like access to aggregates	OrderRepository
Domain Service	Logic that doesn't fit entities	PricingService

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System Design Considerations

Scalability Patterns

Pattern	Description	Trade-off
Horizontal Scaling	Add more instances	Statelessness required
Vertical Scaling	Bigger machines	Hardware limits
Caching	Store computed results	Cache invalidation
Database Sharding	Split data across DBs	Query complexity
Read Replicas	Separate read/write	Eventual consistency
CDN	Edge content delivery	Static content only

Resilience Patterns

Pattern	Purpose	Implementation
Circuit Breaker	Prevent cascade failures	Fail fast when downstream is down
Retry with Backoff	Handle transient failures	Exponential delay between retries
Bulkhead	Isolate failures	Separate thread pools per dependency
Timeout	Bound waiting time	Max wait for responses
Fallback	Graceful degradation	Default behavior when service unavailable

Data Consistency Patterns

Pattern	Consistency	Use When
ACID Transactions	Strong	Financial data, critical operations
Saga	Eventual	Distributed transactions
Event Sourcing	Eventual	Audit trails, complex state
CQRS	Eventual	Different read/write models

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Technology Decision Framework

When to Use a Database

Need	Recommended	Avoid
Relational data, ACID	PostgreSQL, MySQL	MongoDB
Document storage, flexible schema	MongoDB, DynamoDB	Relational
Key-value, high speed	Redis, Memcached	Relational
Time series	InfluxDB, TimescaleDB	Generic SQL
Graph relationships	Neo4j, Neptune	Relational (for complex)
Search	Elasticsearch, Meilisearch	Full table scans

When to Use Message Queues

Need	Pattern
Async processing	Queue (SQS, RabbitMQ)
Event broadcasting	Pub/Sub (SNS, Kafka)
Task scheduling	Delayed queues
Load leveling	Queue with workers
Event sourcing	Log-based (Kafka)

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Architecture Decision Records (ADR)

Template

# ADR-001: [Title]

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

## Context
[Why is this decision needed?]

## Decision
[What is the decision?]

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

### Negative
- [Trade-off 1]
- [Trade-off 2]

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

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Anti-Patterns to Avoid

1. Big Ball of Mud - No clear structure, everything depends on everything
2. Golden Hammer - Using one pattern for all problems
3. Premature Optimization - Designing for scale before proving need
4. Analysis Paralysis - Over-designing, never shipping
5. Distributed Monolith - Microservices with tight coupling
6. Anemic Domain Model - Entities with only getters/setters
7. God Object - One class that does everything
8. Leaky Abstraction - Implementation details leak through interfaces

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Decision Checklist

Before finalizing an architecture decision, verify:

• Does it solve the actual problem?
• Is it the simplest solution that works?
• Can the team maintain it?
• Does it align with existing patterns?
• Is it testable?
• Can it evolve as requirements change?
• Are the trade-offs acceptable?
• Is the decision documented?

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Quick Reference

SOLID:
  S - Single Responsibility
  O - Open/Closed
  L - Liskov Substitution
  I - Interface Segregation
  D - Dependency Inversion

PATTERNS:
  Layered     → Simple, clear separation
  Hexagonal   → Testable, adaptable
  Microservices → Scalable, independent
  Event-Driven  → Decoupled, async

DDD BUILDING BLOCKS:
  Entity, Value Object, Aggregate
  Repository, Domain Event, Domain Service

SCALABILITY:
  Horizontal scaling, Caching, Sharding, CDN

RESILIENCE:
  Circuit Breaker, Retry, Bulkhead, Timeout