Designing and Debugging CQRS Mediator Pattern Applications

I’ve definitely spent too many late nights hunting bugs in 2,000-line service classes where a "quick fix" to a GET request somehow managed to break the entire database write logic. It’s a nightmare we’ve all seen as projects scale and the "fat service" layer starts to rot.

This article walks through moving away from that "monolithic service hell" by implementing CQRS and the Mediator pattern in .NET 9. It uses the CloudPallet WMS (a production warehouse system) as a case study for keeping enterprise codebases modular and actually debuggable.

• Separation of concerns across 80+ unique command files and 50+ query files, each with a single, isolated handler.
• Swapping out bloated WarehouseService implementations for MediatR handlers to eliminate cascading regressions.
• Leveraging .NET 9 and ASP.NET Core to build read-optimized projections that stay completely decoupled from domain entities.
• Enforcing strict return types like Unit, Guid, or specific DTOs to ensure side-effect purity in every operation.
• Designing denormalized read models for performance while keeping the write-side (Commands) strictly normalized.
• Streamlining the debugging workflow within Visual Studio to navigate the decoupled architecture without getting lost.

At the end of the day, the biggest win isn't just the clean architecture—it’s finally being able to open a PR without worrying about breaking a feature three folders away.

Read the full article here:
https://erwinwilsonceniza.qzz.io/blogs/designing-and-debugging-cqrs-mediator-pattern-applications