Solution Architecture

Solution Architecture - Structuring Code and Use Cases

Organize code by business boundaries so change stays local, testable, and easy to reason about.

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Quick Overview

• Clean Architecture: Layered design with dependencies pointing inward; stable core, replaceable outer layers.
• DDD (Domain-Driven Design): Model the business with bounded contexts, aggregates, and ubiquitous language.
• Vertical Slices: Organize by feature/use case instead of technical layers to minimize cross-cutting changes.
• MVVM: UI architecture for desktop/mobile apps (WPF/MAUI); separates view, state, and behavior.
• You can combine them: DDD inside Clean Architecture, vertical slices inside the Application layer, MVVM in the UI.

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Detailed Explanation

Clean Architecture (Layered, Dependencies Inward)

What it is: A layered architecture where the Domain has zero dependencies. Application orchestrates use cases. Infrastructure and UI sit on the outside.

+-------------------------------+
|     Presentation / UI         |
+-------------------------------+
|       Application             |  Use cases, CQRS handlers
+-------------------------------+
|          Domain               |  Entities, Value Objects
+-------------------------------+
|       Infrastructure          |  DBs, brokers, APIs
+-------------------------------+

Use it when: You have long-lived business rules and want infrastructure swaps (SQL, Kafka, MT5 bridges) without touching domain logic.

Trade-offs: More projects and abstractions; can be overkill for tiny apps.

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Domain-Driven Design (DDD)

What it is: A modeling approach that focuses on the business language and boundaries.

Core elements:

• Bounded Contexts: Clear ownership boundaries (e.g., Orders, Risk, Pricing).
• Aggregates: Consistency boundaries with invariants enforced in one place.
• Ubiquitous Language: Shared vocabulary with business + engineering.

Use it when: The domain is complex and business rules change often (trading rules, risk limits, compliance).

Trade-offs: Requires strong collaboration and discipline; adds modeling overhead.

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Vertical Slice Architecture

What it is: Organize code by feature/use case so each slice includes its handler, validation, and DTOs.

Application/
  Orders/
    PlaceOrder/
      PlaceOrderCommand.cs
      PlaceOrderHandler.cs
      PlaceOrderValidator.cs
      PlaceOrderDto.cs

Benefits:

• Change stays in one folder (low cognitive load).
• Tests and handlers stay close to their use case.
• Avoids over-abstraction by feature.

Trade-offs: Can duplicate infrastructure concerns if you skip shared abstractions.

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MVVM (Model-View-ViewModel)

What it is: UI pattern for desktop/mobile apps where the View binds to a ViewModel (state + commands).

View  <->  ViewModel  <->  Model
XAML       C# state        Domain/DTOs

Use it when: You need testable UI logic and clean separation in WPF/MAUI apps.

Trade-offs: Extra plumbing for bindings and property change notifications.

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Example - Vertical Slice Use Case

// Application/Orders/PlaceOrder/PlaceOrderCommand.cs
public sealed record PlaceOrderCommand(string Symbol, decimal Quantity) : IRequest<Guid>;

// Application/Orders/PlaceOrder/PlaceOrderHandler.cs
public sealed class PlaceOrderHandler : IRequestHandler<PlaceOrderCommand, Guid>
{
    private readonly IOrderRepository _orders;

    public PlaceOrderHandler(IOrderRepository orders)
    {
        _orders = orders;
    }

    public async Task<Guid> Handle(PlaceOrderCommand request, CancellationToken ct)
    {
        var order = Order.Create(request.Symbol, request.Quantity);
        await _orders.AddAsync(order, ct);
        return order.Id;
    }
}

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Choosing the Right Approach

• Clean Architecture: Long-lived backend systems with multiple integrations.
• DDD: Complex domains that need strict language and boundaries.
• Vertical Slices: Fast-moving teams, feature-driven development.
• MVVM: Desktop/mobile UI with heavy interaction and binding.

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Why It Matters for Interviews

• You can justify how you structure a solution and why it scales with team size.
• You can explain trade-offs (complexity vs flexibility, speed vs rigor).
• You can show how architecture supports testability and maintainability.

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Common Pitfalls

• Treating Clean Architecture as a strict template instead of a guide.
• Building a massive shared kernel that becomes a dependency sink.
• Splitting into micro-layers too early without proven complexity.
• Putting UI logic in Views instead of ViewModels (MVVM violations).

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Quick Reference

• Clean Architecture: Dependencies flow inward; domain has zero dependencies.
• DDD: Bounded contexts + aggregates + ubiquitous language.
• Vertical Slice: Organize by feature, not technical layer.
• MVVM: View binds to ViewModel; Model stays pure.

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Sample Interview Q&A

• Q: Can Clean Architecture and DDD coexist?
• A: Yes. DDD provides the modeling approach inside the Domain layer, while Clean Architecture provides the dependency and layering rules around it.

• Q: Why choose vertical slices over layered folders?
• A: Vertical slices keep related code together, reduce navigation overhead, and keep changes localized to a feature.

• Q: When would MVVM be a bad fit?
• A: For simple screens with minimal state; MVVM overhead may outweigh benefits. For complex UI, it pays off quickly.