Implementing CQRS and Event Sourcing in .NET Core

Let’s face it — monolithic applications and tightly coupled logic may have been the norm for decades, but they don’t cut it in today’s data-driven, scalable-first world.

If you’re building systems that must handle high loads, offer snappy responsiveness, and stay maintainable over time, it’s time to rethink your approach.

Enter CQRS (Command Query Responsibility Segregation) and Event Sourcing — two modern, powerful architectural patterns that bring clarity, traceability, and scalability to your .NET Core applications.

In this guide, we’ll explore how to implement CQRS and Event Sourcing in .NET Core — with no chaos, just clean architecture and scalable success.

Environment Setup: Your Development Foundation

Before diving into implementation, ensure your development environment is set up for success.

Tools and Requirements:

1. .NET Core SDK (Latest LTS)
   Use the latest LTS version to ensure long-term support and stability.
2. IDE
   • Visual Studio (rich enterprise tooling)
   • VS Code (lightweight and extensible)
3. Recommended Project Structure
   • Api — Your Web API entry point.
   • Application — CQRS handlers and interfaces.
   • Domain — Core business logic and event definitions.
   • Infrastructure — Persistence, integrations, and event store.
   • EventStore — In-memory, relational (append-only table), or tools like EventStoreDB.
4. Essential Libraries
   • MediatR for CQRS implementation.
   • Entity Framework Core or custom repository pattern for persistence.
   • Newtonsoft.Json for event serialization.
   • FluentValidation for validating commands.

What is CQRS (Command Query Responsibility Segregation)?

CQRS is the practice of separating read and write responsibilities into distinct models.

• Commands modify state.
• Queries return data.

Why CQRS?

• Clean separation of concerns.
• Independent optimization for reads and writes.
• Prevents unintended side effects.
• Improved scalability, especially under load.
• Enables use of different data stores for read/write sides.

What is Event Sourcing?

Instead of saving only the current state of your entities, Event Sourcing stores every state change as a distinct event.

Example:

Instead of a User object with current name/email:

• UserCreated
• EmailChanged
• NameUpdated

Replaying these events reconstructs the current state.

Benefits:

• Built-in audit trail
• “Time travel” to any point in history
• Perfect for high-change or regulated systems
• Improved debugging and recovery

When combined with CQRS, Event Sourcing allows your command handlers to generate events that asynchronously update your read models.

How CQRS and Event Sourcing Work Together in .NET Core

1. Commands

• Sent from the API layer as simple DTOs (e.g., RegisterUserCommand).
• Handled by IRequestHandler<T> via MediatR.
• Result in domain events after validation and business logic.

2. Event Storage

• Events are serialized (JSON) and saved in an event store.
• Use a relational table or purpose-built store like EventStoreDB.
• Store metadata: timestamp, correlation ID, event type.

3. Aggregates and Projections

• Aggregates rebuild their state by replaying stored events.
• Projections (read models) update asynchronously to serve queries.
• Choose optimized data stores like SQL, NoSQL, or Elasticsearch.

4. Queries

• Queries directly hit read models.
• No dependency on domain logic = blazing fast.

5. Event Handlers

• Perform asynchronous side effects (e.g., send emails, update projections).
• Clean, decoupled processing via event bus or background services.

Best Practices for CQRS + Event Sourcing

Embrace Immutability
Once stored, an event is never altered. Introduce new event types when business rules evolve.

Keep Commands Lean
Each command should do one thing — avoid bloated, orchestrating handlers.

Use Separate Data Stores
Separate databases for reads and writes improve performance and prevent contention.

Make Event Handling Idempotent
Handle failures gracefully. Retrying should not break consistency or duplicate side effects.

Snapshot for Performance
When aggregates grow large, use snapshots to reduce replay time.

Invest in Observability
CQRS and Event Sourcing bring visibility — use structured logs, tracing, and dashboards to harness that.

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Conclusion

Implementing CQRS and Event Sourcing in .NET Core is more than a technical upgrade — it’s a paradigm shift.

Instead of managing just what the state is, you focus on how it got there — making systems more resilient, scalable, and explainable.

With tools like MediatR, EF Core, and EventStoreDB, plus the native async-first design of .NET Core, you’re ready to build architectures that scale and last.

So if you’re done patching monoliths and want to embrace robust patterns for the future — it’s time to give CQRS and Event Sourcing a seat at your architecture table.