Request-reply, done properly

The pattern everyone starts with, because it maps onto how people think: ask the billing system a question, wait for the answer. This lesson builds the callout correctly the first time — Named Credential, explicit timeout, and an error type the caller can actually act on — and then breaks it on purpose to show what each safeguard was for.

The part most tutorials skip: the retry. A timeout does not mean the request failed; it means you stopped waiting. Retrying a charge that actually went through is how a customer gets billed twice, which is why the retry sends an idempotency key and the lesson spends ten minutes on that one header.