Fix: The Checkout That Waits for Everyone

When a customer pays, the Checkout API synchronously calls the Email service, then Invoicing, then Analytics — in a chain. Yesterday the email provider had a slow afternoon and checkout timed out for 3 hours: customers could not buy because a marketing email was slow. The canvas shows the current chain. Decouple checkout from everything that does not need to happen before the customer sees "order confirmed".

Requirements

•Only payment-critical work stays on the synchronous path; everything else becomes asynchronous
•Email, invoicing and analytics consume work via queue/pub-sub with workers
•Failed deliveries have a destination and a retry story (DLQ)
•A slow or dead downstream service must not affect checkout latency or success

Non-Functional Requirements

•Checkout p99 must be independent of email/invoice/analytics provider latency

Constraints

•Do not redesign payment processing itself — the problem is the post-payment chain

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