Module 18: Microservices & Advanced Architecture

This module is the “Black Belt” level of .NET development. It moves away from how to write a single application and focuses on how to design a system of applications that can serve millions of users and scale across global teams.

1. Monolithic vs. Microservices

This is the fundamental architectural choice for an organization.

• Monolith: The entire application (User UI, Business Logic, Database Access) is one single codebase and one single deployment unit.
  • Pros: Simple to develop, test, and deploy.
  • Cons: Becomes a “Big Ball of Mud” as it grows; if one part fails, the whole site goes down.
• Microservices: The application is split into small, independent services (e.g., OrderingService, CatalogService, IdentityService).
  • Pros: Each service can use a different database, be written by a different team, and scale independently.
  • Cons: Extremely complex to manage, monitor, and debug.

2. Clean Architecture (Onion Architecture)

Popularized by Robert C. Martin (“Uncle Bob”), this is the gold standard for structuring .NET solutions. The goal is to keep the Business Logic (Domain) at the center, independent of UI, Databases, or External APIs.

• Domain: Entities and logic (no dependencies).
• Application: Interfaces and “Use Cases” (depends on Domain).
• Infrastructure: Database implementations, Email services (depends on Application).
• Web/API: The entry point (depends on Application).

3. Repository & Unit of Work Patterns

These patterns act as a wrapper around Entity Framework Core to further decouple your business logic from the database.

• Repository: A class that handles data logic for a specific entity (e.g., GetActiveProducts()). It hides the complexity of LINQ queries.
• Unit of Work: Ensures that multiple repositories share the same DbContext transaction. It allows you to save changes across multiple tables as a single “all-or-nothing” operation.

4. API Gateway

In a microservices world, the frontend shouldn’t have to talk to 20 different URLs.

• The Gateway: Acts as a single entry point (a “Traffic Cop”). The client talks to the Gateway, and the Gateway routes the request to the correct internal microservice.
• Functions: It also handles Authentication, Rate Limiting, and Response Aggregation. Common tools in .NET include Ocelot or YARP (Yet Another Reverse Proxy).

5. Message Queues (Asynchronous Communication)

In microservices, services shouldn’t usually talk to each other directly (Synchronous HTTP). If Service A calls Service B and Service B is down, Service A fails too.

• The Solution: Use a Message Queue (like RabbitMQ or Azure Service Bus).
• The Flow: Service A publishes a message (“OrderCreated”) to the queue. Service B picks it up whenever it’s ready. This is called Event-Driven Architecture.

6. Distributed Systems Concepts

When your app runs on 10 servers instead of 1, new problems emerge:

• Eventual Consistency: Since each microservice has its own database, data might not be updated everywhere instantly.
• Service Discovery: How does one service find the IP address of another? (e.g., using Consul or Kubernetes DNS).
• Circuit Breaker Pattern: A safety mechanism. If a service is failing repeatedly, the “Circuit Breaker” trips and stops sending requests to it for a while to allow it to recover (using a library like Polly).

7. Senior-Level Mindset: The “Trade-off”

A senior developer knows that Microservices are not always the answer. They are a solution to an organizational problem (too many developers on one team), not just a technical one. A senior dev evaluates “Complexity vs. Scalability” before making a choice.