TL;DR: Downtime monitoring for a multi-service SaaS checkout is a proof chain, not a single ping. Start an external transaction, record each dependency boundary, and classify the result by the customer-visible step that failed. Keep liveness, readiness, latency, and business failure separate. Then attach the affected checkout operation to a cost center without putting payment data into logs.

The operational constraint changes the design: a green load balancer does not prove that a buyer can complete checkout. A checkout can fail after the edge responds, while catalog, inventory, tax, payment, and order services disagree about what happened. The useful mental model is a map. The probe is the traveler, each service boundary is a checkpoint, and the final assertion is “an order can be created without exposing sensitive data.”

This article uses a five-service e-commerce checkout as the concrete example.…