🔐 SOLID Principles in Software Design¶

SOLID is a set of foundational principles for object-oriented design that help build systems that are maintainable, extensible, and testable. These principles were introduced by Robert C. Martin (Uncle Bob) and are central to ConnectSoft's architectural best practices.

🌟 Goals of SOLID¶

✅ Maintainability — Enable quick, safe changes and bug fixes.
📈 Scalability — Allow new functionality to be added with minimal friction.
🧩 Modularity — Encourage decoupling for reusability and substitution.
🔍 Testability — Enable unit testing with isolation and mocks.
🧡 Collaboration — Produce readable and understandable code for teams.

🧠 What is SOLID?¶

Principle	Name	Definition
S	Single Responsibility	A class should have one reason to change.
O	Open/Closed	Software should be open for extension but closed for modification.
L	Liskov Substitution	Subtypes must be substitutable for their base types.
I	Interface Segregation	Interfaces should be client-specific, not general-purpose.
D	Dependency Inversion	Depend on abstractions, not concretions.

🔔 Why SOLID Principles?¶

Info

SOLID is not just a set of abstract ideals — it’s a practical discipline that solves everyday problems in architecture, development, and maintenance.

1. Reduce Fragility: Without SOLID, changes in one part of the system can cascade into others.

2. Improve Collaboration: Developers can work on separate parts of the system without stepping on each other’s toes.

3. Enhance Readability: Clear separation of concerns helps developers onboard and debug faster.

4. Promote Testability: SOLID encourages isolation, which is essential for writing reliable unit tests.

5. Future-Proof Systems: With extensible and modular code, adapting to new requirements or technologies is seamless.

6. Encourage Clean Design: Applying SOLID regularly leads to natural adoption of Clean Architecture and DDD principles.

In ConnectSoft: SOLID is enforced across all templates, service layers, and domain models to ensure that code remains robust, extensible, and aligned with business goals.

🔎 Principle-by-Principle Deep Dive¶

🛡️ Single Responsibility Principle (SRP)¶

Definition: Each class or module should have a single responsibility and only one reason to change.

Example: UserService handles user operations; delegate DB and email logic to other components.

public class UserService {
  private readonly IUserRepository _repo;
  private readonly IEmailService _email;

  public void CreateUser(UserDto user) {
    _repo.Save(user);
    _email.SendWelcome(user.Email);
  }
}

Real-World Scenario:

In an appointment system, split user registration logic from notification sending and analytics logging.

Diagram:

graph LR
  API[API Request] --> UserService --> Repo[Repository]
  UserService --> EmailService

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Benefits:

Easier maintenance.
Clear ownership of logic.
Faster onboarding for developers.

🚪 Open/Closed Principle (OCP)¶

Definition: Entities should be open for extension but closed for modification.

Example: Use inheritance or composition to support new payment methods.

public interface IPaymentMethod {
  void Process();
}

public class PayPalPayment : IPaymentMethod {
  public void Process() => /* logic */;
}

Diagram:

graph TD
  PaymentService -->|uses| IPaymentMethod
  IPaymentMethod --> CreditCard
  IPaymentMethod --> PayPal

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Real-World Scenario:

Adding Apple Pay without touching existing logic.

Benefits:

Fewer regressions.
Supports plugin architecture.
Encourages encapsulation.

🔄 Liskov Substitution Principle (LSP)¶

Definition: Subclasses should be substitutable for their base classes.

Anti-Example:

public class Square : Rectangle {
  public override void SetWidth(int w) {
    base.SetWidth(w);
    base.SetHeight(w); // breaks expected Rectangle behavior
  }
}

Correct Example:

public interface IShape {
  int Area();
}

public class Rectangle : IShape {
  public int Width { get; set; }
  public int Height { get; set; }
  public int Area() => Width * Height;
}

Benefits:

Safer polymorphism.
Predictable behavior.
Enables refactoring without fear.

📚 Interface Segregation Principle (ISP)¶

Definition: Avoid forcing clients to depend on methods they don't use.

Bad Example:

public interface IMultiFunctionDevice {
  void Print(); void Scan(); void Fax();
}

Good Example:

public interface IPrinter { void Print(); }
public interface IScanner { void Scan(); }
public interface IFax { void Fax(); }

Real-World Scenario:

A mobile app only prints documents; it should not be forced to implement scanning or faxing logic.

Benefits:

Focused interfaces.
Greater flexibility.
Simplifies testing.

🔄 Dependency Inversion Principle (DIP)¶

Definition: High-level modules should not depend on low-level modules. Both should depend on abstractions.

Example:

public interface IMessageSender {
  void Send(string message);
}

public class NotificationService {
  private readonly IMessageSender _sender;

  public NotificationService(IMessageSender sender) {
    _sender = sender;
  }

  public void Notify(string msg) => _sender.Send(msg);
}

Real-World Scenario:

Easily swap EmailSender with SmsSender or PushSender.

Benefits:

Inversion of control.
Enables DI frameworks.
Reduces tight coupling.

🔧 Patterns Supporting SOLID¶

graph TD
  FactoryPattern --> SRP
  FactoryPattern --> OCP
  StrategyPattern --> OCP
  StrategyPattern --> LSP
  ObserverPattern --> DIP
  AdapterPattern --> ISP
  DependencyInjection --> DIP

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🔄 Factory Pattern¶

Aligned Principle:

Single Responsibility Principle (SRP)
Open/Closed Principle (OCP)

Overview:

Provides a way to create objects without exposing the instantiation logic to the client.

Example Use Case:

Creating instances of payment processors without modifying client code.

public interface IPaymentProcessor { void ProcessPayment(); }
public class CreditCardProcessor : IPaymentProcessor { /* Implementation */ }
public class PayPalProcessor : IPaymentProcessor { /* Implementation */ }

public class PaymentProcessorFactory
{
    public static IPaymentProcessor Create(string type) =>
        type switch
        {
            "CreditCard" => new CreditCardProcessor(),
            "PayPal" => new PayPalProcessor(),
            _ => throw new ArgumentException("Invalid type")
        };
}

🧠 Strategy Pattern¶

Aligned Principle:

Open/Closed Principle (OCP)
Liskov Substitution Principle (LSP)

Overview:

Encapsulates algorithms and behaviors as interchangeable strategies.

Example Use Case:

Switching between pricing strategies (e.g., discount vs. seasonal).

public interface IPricingStrategy { decimal CalculatePrice(decimal basePrice); }

public class DiscountStrategy : IPricingStrategy {
    public decimal CalculatePrice(decimal basePrice) => basePrice * 0.9m;
}

public class SeasonalStrategy : IPricingStrategy {
    public decimal CalculatePrice(decimal basePrice) => basePrice * 0.8m;
}

public class PricingContext {
    private readonly IPricingStrategy _strategy;
    public PricingContext(IPricingStrategy strategy) { _strategy = strategy; }
    public decimal GetFinalPrice(decimal basePrice) => _strategy.CalculatePrice(basePrice);
}

📣 Observer Pattern¶

Aligned Principle:

Dependency Inversion Principle (DIP)

Overview:

Establishes a publish-subscribe relationship between objects.

Example Use Case:

Notify multiple subsystems when an order is placed.

public interface IObserver { void Update(string status); }
public class EmailNotifier : IObserver { public void Update(string status) { /* Email logic */ } }
public class SmsNotifier : IObserver { public void Update(string status) { /* SMS logic */ } }

public class Order {
    private readonly List<IObserver> _observers = new();

    public void Attach(IObserver observer) => _observers.Add(observer);
    public void Notify(string status) => _observers.ForEach(o => o.Update(status));
}

🔌 Adapter Pattern¶

Aligned Principle:

Interface Segregation Principle (ISP)

Overview:

Translates one interface into another expected by the client.

Example Use Case:

Wrapping a third-party API to match your internal interface.

public interface IStorage { void Save(string data); }

public class CloudStorageAdapter : IStorage {
    private readonly ExternalCloudStorage _external;
    public CloudStorageAdapter(ExternalCloudStorage external) { _external = external; }
    public void Save(string data) => _external.Upload(data);
}

🧩 Dependency Injection (DI)¶

Aligned Principle:

Dependency Inversion Principle (DIP)

Overview:

Inverts control by injecting dependencies rather than constructing them internally.

Example Use Case:

Decoupling notification services using constructor injection.

public interface INotifier { void Notify(string message); }
public class EmailNotifier : INotifier { public void Notify(string message) { /* Email logic */ } }

public class NotificationService {
    private readonly INotifier _notifier;
    public NotificationService(INotifier notifier) { _notifier = notifier; }
    public void Send(string message) => _notifier.Notify(message);
}

📘 Advanced SOLID Applications in Modern Architecture¶

This section extends the foundational SOLID document to cover best practices, applied strategies in modern system styles (Microservices, Cloud-Native, Event-Driven, and API-Driven), design patterns, team practices, and architecture-level outcomes.

💡 Best Practices¶

✅ Apply SRP when refactoring large service classes.
✅ Use abstract interfaces and dependency injection.
✅ Write use cases with no external dependencies.
✅ Use factory/strategy when OCP needs to be enforced.
✅ Create DTOs for cross-layer communication.

❌ Common Pitfalls¶

❌ Overengineering with premature abstractions.
❌ Creating interfaces for classes unlikely to change.
❌ Mixing domain and infrastructure in one class.
❌ Reusing entities across bounded contexts without isolation.

💼 Best Practices in Teams¶

✅ Code reviews to catch violations early.
✅ Knowledge sharing sessions on SOLID.
✅ Enforce DIP with CI linting or architectural rules.
✅ Refactor constantly to maintain SRP.

🏢 Applying SOLID to Microservices¶

SOLID Principle	Application in Microservices
SRP	One microservice = one business capability
OCP	Add new behavior via new services or handlers
LSP	Each microservice is replaceable by another version with the same contract
ISP	APIs focused on specific clients or responsibilities
DIP	Use interfaces, queues, or event contracts for inter-service communication

Diagram:

graph TD
  Microservice1[Order Service] --> EventBus
  Microservice2[Invoice Service] --> EventBus
  EventBus --> NotificationService

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☁️ SOLID in Cloud-Native Systems¶

ISP: API surface tailored for each consumer.
DIP: External services abstracted (e.g., via Service Interfaces).
SRP: Serverless functions focused on single tasks.

Example: A BlobUploader service writes to Azure Blob, but uses IStorageWriter abstraction. A Kubernetes volume or S3 implementation can be swapped without touching core logic.

🔄 Event-Driven Systems and SOLID¶

Principle	Application
SRP	Each event handler processes a single type of event
OCP	Add new event types without altering existing handlers
DIP	Publish/subscribe abstractions decouple producers and consumers

Diagram:

graph TD
  OrderCreated --> EmailSender
  OrderCreated --> InventoryAdjuster
  OrderShipped --> NotificationService

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🔗 API-Driven Architectures and SOLID¶

ISP: Build client-specific endpoints to reduce overfetching/underfetching
OCP: Add versioned endpoints without breaking existing ones
DIP: Abstract HTTP layers or clients for external API integrations

Example: Create separate /v1/orders, /v1/orders/summary, and /v1/orders/details to keep endpoints focused per use case.

🚀 Success Stories¶

SaaS Payment Platform: Used OCP and DIP to add crypto payments with zero changes to existing services.
Healthcare Scheduling System: Applied SRP to separate availability, booking, and notification flows.
E-Commerce Checkout: Used Strategy pattern to switch between pricing algorithms for different countries.
Telehealth Consultation Workflow: Leveraged DIP and ISP to build loosely coupled integrations between scheduling, patient portal, and video conferencing services.
Cloud Migration at Scale: Applied SRP and DIP to decouple domain services from their original on-prem infrastructure, enabling container-based cloud deployments.
AI Model Serving Platform: Used OCP to support new machine learning models without modifying the orchestration or monitoring services. Each model handler adhered to the same processing contract.: Used OCP and DIP to add crypto payments with zero changes to existing services.
Healthcare Scheduling System: Applied SRP to separate availability, booking, and notification flows.
E-Commerce Checkout: Used Strategy pattern to switch between pricing algorithms for different countries.

🔄 Key Takeaways¶

SOLID helps bridge code and architecture — from class design to service boundaries.
Apply SOLID across layers: code, domain, service, integration.
Combine with DDD, patterns, and observability to build future-proof platforms.

🧠 SOLID Benefits Across System Types¶

System Type	Primary Benefit of SOLID
Microservices	Scalability and modularity
Cloud-Native	Resilience and provider independence
Event-Driven	Decoupling and asynchronous fault tolerance
API-Centric	Interface clarity and backward compatibility

Diagram: System Alignment

graph TD
  SOLID --> Microservices
  SOLID --> CloudNative
  SOLID --> EventDriven
  SOLID --> APIArchitecture

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📊 Summary Table¶

Principle	Benefit	Pattern Used
SRP	Maintainable classes	Application Layering
OCP	Easy extension	Strategy, Factory
LSP	Safe substitution	Interface contracts
ISP	Clean API design	Interface refinement
DIP	Decoupling, flexibility	Dependency Injection

📈 Conclusion¶

SOLID is not just for classes — it powers composable services, flexible APIs, resilient cloud functions, and decoupled messaging. Adopting SOLID in architectural thinking leads to:

🔧 Maintainable ecosystems
🚀 Agile delivery with reduced regressions
☁️ Portability across environments
🧪 Testable and observable systems

ConnectSoft templates and frameworks embrace SOLID as the foundation of all scalable, domain-aligned software solutions.

🔐 SOLID Principles in Software Design¶

🌟 Goals of SOLID¶

🧠 What is SOLID?¶

🔔 Why SOLID Principles?¶

🔎 Principle-by-Principle Deep Dive¶

🛡️ Single Responsibility Principle (SRP)¶

🚪 Open/Closed Principle (OCP)¶

🔄 Liskov Substitution Principle (LSP)¶

📚 Interface Segregation Principle (ISP)¶

🔄 Dependency Inversion Principle (DIP)¶

🔧 Patterns Supporting SOLID¶

🔄 Factory Pattern¶

🧠 Strategy Pattern¶

📣 Observer Pattern¶

🔌 Adapter Pattern¶

🧩 Dependency Injection (DI)¶

📘 Advanced SOLID Applications in Modern Architecture¶

💡 Best Practices¶

❌ Common Pitfalls¶

💼 Best Practices in Teams¶

🏢 Applying SOLID to Microservices¶

☁️ SOLID in Cloud-Native Systems¶

🔄 Event-Driven Systems and SOLID¶

🔗 API-Driven Architectures and SOLID¶

🚀 Success Stories¶

🔄 Key Takeaways¶

🧠 SOLID Benefits Across System Types¶

📊 Summary Table¶

📈 Conclusion¶

🔹 References¶