Why should you care?

ShopNest history pages must not join heavy write tables during sales; projectors keep a ready-to-read store warm.

See it live — copy this example

Paste into a C# console or class library project and run dotnet run.

// Write side commits event; projector updates read model
Console.WriteLine("WRITE: OrderPlaced event committed");
Console.WriteLine("BUS: order_placed delivered");
Console.WriteLine("PROJECTOR: upsert OrdersByUser read row");
Console.WriteLine("QUERY: SELECT FROM OrdersByUser WHERE UserId=@u");

What happened?

• Unlike the basic CQRS intro, this lesson focuses on the async projection path and lag.
• Queries never call the write model for list screens.

Practice next

1. Draw write → outbox → bus → projector → read DB.
2. Define OrdersByUser columns.
3. Show UI messaging for brief lag.
4. Add OrderShipped projector update.
5. Metric projection lag seconds.

Remember

Events feed read models. Expect lag; design for it. Idempotent projectors.

ShopNest history projector

order_placed upserts user history rows.

Outcome: History API stays cheap under load.