Why should you care?

You care about this when a checkout fails and you must trace it across Gateway → Order → Payment → RabbitMQ in minutes, not days.

See it live — copy this example

Create a Web API project (dotnet new webapi), paste the code, then run dotnet run.

[Fact]
public async Task PlaceOrder_Publishes_OrderPlacedEvent()
{
    await using var factory = new WebApplicationFactory<Program>();
    // assert message published or HTTP 201
}

Run Example »

Edit the code and click Run — like W3Schools Try it Yourself.

What happened?

• The example shows Contract Testing wired into User Service.
• Read each line, run it locally, then change one setting and observe what breaks or improves.
• That is how teams learn in production too — small experiments, not big bang rewrites.

Try it yourself

1. Open or create the ShopNest project area for User Service.
2. Apply the Contract Testing pattern from the lesson example.
3. Run dotnet build && dotnet run (or docker compose up when the lesson uses containers).
4. Change a string or number in the example and run again — predict the output first.
5. Break the code on purpose (remove a semicolon), read the compiler error, then fix it.

Remember

Contract Testing connects to User Service in ShopNest Cloud-Native. Practice by editing the example yourself — do not only read. Move on when you can explain this topic in your own words without looking.

Real-world: HDFC-style fund transfer

Transfer, ledger, fraud check, and SMS run as separate services with idempotency keys. Saga compensates if fraud blocks after debit.

Outcome: Salary-day load handled without duplicate debits or shared-database locks.