Tips and Tricks #182: Use CQRS for Complex Domain Logic

Separate read and write operations for better scalability and simpler code.

Code Snippet

// Commands (Write side)
public record CreateOrderCommand(string UserId, List Items);

public class CreateOrderHandler
{
    private readonly IOrderRepository _repo;
    
    public async Task Handle(CreateOrderCommand cmd)
    {
        var order = Order.Create(cmd.UserId, cmd.Items);
        await _repo.SaveAsync(order);
        return order.Id;
    }
}

// Queries (Read side)
public record GetOrderSummaryQuery(string OrderId);

public class GetOrderSummaryHandler
{
    private readonly IReadOnlyDbContext _db;
    
    public async Task Handle(GetOrderSummaryQuery query)
    {
        return await _db.OrderSummaries
            .Where(o => o.Id == query.OrderId)
            .FirstOrDefaultAsync();
    }
}

Why This Helps

• Optimized read models for queries
• Simpler command handlers with single responsibility
• Independent scaling of read/write workloads

How to Test

• Test commands and queries independently
• Verify read model consistency

When to Use

Complex domains, high-read workloads, event-sourced systems.

Performance/Security Notes

Start simple. Full CQRS with separate databases adds complexity. Use when justified.

References

• https://martinfowler.com/bliki/CQRS.html

Try this tip in your next project and share your results in the comments!