Introduction to Architectural Design Principles & Patterns
Why Principles Matter Beyond SOLID
When most developers first learn software design, they encounter class-level principles like SOLID, DRY, KISS, and YAGNI.
These guidelines are vital: they help us write clean, readable, and maintainable classes and methods.
But here’s the reality: once systems grow beyond a few classes, SOLID alone is not enough.
Real-world software involves:
- Dozens of modules (packages, libraries).
- Hundreds of services in distributed systems.
- Multiple databases, APIs, and frameworks.
At this scale, we face architectural questions like:
- How do we prevent business logic from depending on frameworks?
- How should services communicate without creating fragile dependencies?
- How do we group modules for maximum stability and reuse?
- How can we evolve the system without a full rewrite?
These questions require architectural design principles and architectural patterns.
Principles vs Patterns
It’s common for developers to mix up principles and patterns. Let’s clarify:
- Principles: Broad, timeless guidelines that shape software structure.
- Example: High cohesion, low coupling.
- Example: Dependency Rule.
- Patterns: Reusable, named solutions that apply those principles in practice.
- Example: Layered Architecture.
- Example: Hexagonal Architecture.
Principles are the "why"; patterns are the "how."
Ignoring principles leads to fragile architectures, while ignoring patterns means reinventing the wheel.
Core Architectural Principles (The “Why”)
Here are some of the foundational architectural design principles we’ll study in depth later:
Dependency Rule (Clean Architecture)
Dependencies should always point inward, towards business logic.
Frameworks, UIs, and databases must not dictate the core.Cohesion & Coupling Principles
- Group classes and modules so they change together (cohesion).
- Minimize unnecessary dependencies (coupling).
Information Hiding
Only expose contracts (APIs); hide implementation details.Separation of Concerns
Distinct boundaries between presentation, domain logic, and infrastructure.Domain-Driven Design (DDD Lite)
Use bounded contexts and ubiquitous language to model complex domains.Scalability & Modularity
Design systems that can grow horizontally (scale-out) or evolve incrementally.Evolutionary Architecture
Systems should adapt through controlled change (strangler fig pattern, continuous delivery).
Core Architectural Patterns (The “How”)
Once principles are understood, we turn them into concrete architectures via patterns:
Layered Architecture (N-Tier)
Presentation, business, and data layers.Hexagonal (Ports & Adapters) Architecture
Keep business logic independent of frameworks, UIs, and databases.Microservices vs Modular Monolith
How to split or consolidate based on Conway’s Law.CQRS (Command Query Responsibility Segregation)
Separate read and write models for clarity and scalability.Event Sourcing
Capture all state changes as immutable events.
These patterns provide proven ways to implement principles in practice.
Example: Principle + Pattern in Action
Imagine building an e-commerce platform.
Without Principles/Patterns
- Business logic directly calls Stripe SDK.
- Domain entities have JPA annotations (
@Entity), tightly binding them to persistence. - Services share the same database schema.
- Any change to the DB schema breaks multiple services.
This system is fragile, hard to test, and tightly coupled.
With Principles & Patterns
- Dependency Rule: Core business logic independent of frameworks.
- High Cohesion: Order-related logic lives in Order module only.
- Low Coupling: Payment integration abstracted behind interface.
- Pattern Applied: Hexagonal Architecture (business logic at the center, adapters outside).
Java Example
// Business Rule (inner layer)
public class OrderService {
private final PaymentProcessor processor;
public OrderService(PaymentProcessor processor) {
this.processor = processor;
}
public void placeOrder(Order order) {
processor.charge(order);
// core business rules
}
}
// Interface (boundary)
public interface PaymentProcessor {
void charge(Order order);
}
// Framework Integration (outer layer)
public class StripeProcessor implements PaymentProcessor {
public void charge(Order order) {
// call Stripe API
}
}✅ Testable: OrderService can be tested without Stripe.
✅ Flexible: Swap Stripe with PayPal without rewriting business logic.
✅ Cohesive: OrderService only manages orders, not unrelated domains.
Real-World Case Studies
1. Netflix Microservices
- Principles Applied: High cohesion (each microservice owns one domain), low coupling (services communicate via APIs, not shared DBs).
- Patterns Used: Microservices + API Gateway pattern.
- Result: Scalability, but requires strong observability and deployment automation.
2. Amazon’s Order & Payment Systems
- Principles Applied: Dependency Rule (payment gateways abstracted), information hiding (internal APIs).
- Patterns Used: Hexagonal architecture around critical domains.
- Result: Ability to swap third-party payment providers with minimal disruption.
3. Banking Systems
- Principles Applied: Stable dependencies (core domain doesn’t depend on unstable UIs).
- Patterns Used: Layered + CQRS in transaction processing.
- Result: Regulatory compliance + predictable evolution.
Interview Prep: Common Questions
Q1: How do architectural principles differ from design patterns?
Answer:
- Principles are broad guidelines (timeless).
- Patterns are concrete solutions (context-specific).
- Example: Dependency Inversion Principle → applied via Clean Architecture.
Q2: Why does the Dependency Rule matter in large-scale systems?
Answer:
- Keeps business rules free from frameworks.
- Improves testability, portability, and adaptability.
Q3: How would you explain high cohesion and low coupling at the service level?
Answer:
- Cohesion: All order-related logic lives inside Order service.
- Coupling: Order service doesn’t depend on Inventory DB; it uses Inventory API.
Q4: When would you choose microservices over a modular monolith?
Answer:
- Choose microservices when scaling teams/org structure.
- Stay with modular monolith until complexity demands distributed systems.
Q5: What happens if you ignore architectural principles?
Answer:
- Systems become rigid, fragile, and costly to evolve.
- Example: Shared DB across services → schema change breaks multiple services.
Common Pitfalls to Avoid
- Putting framework annotations in core domain classes.
- Creating God-services that own multiple domains (low cohesion).
- Cyclic dependencies between services/modules.
- Premature microservices adoption without automation maturity.
Summary & Key Takeaways
- SOLID isn’t enough for large-scale systems — we need architectural principles.
- Principles guide direction (Dependency Rule, cohesion, modularity).
- Patterns provide reusable implementations (layered, hexagonal, microservices).
- Together, they ensure systems are scalable, maintainable, and resilient.
Next Steps
Now that you understand the difference between principles and patterns, the next lesson explores the Dependency Rule — the foundation of Clean Architecture.