Practice — Reliability, Resilience & Observability Patterns (5 questions)

An order-placement request flows through: CDN/edge (99.99%) → API gateway (99.99%) → order service (99.9%) → inventory service (99.9%) → primary database (99.95%) → a single third-party fraud-check API (99.0%). All calls are synchronous and every one must succeed for the order to be placed.

1. Compute the end-to-end availability and the expected downtime per year.
2. The team can either (a) add a second, independent fraud-check vendor with automatic failover, or (b) raise the order and inventory services from 99.9% to 99.99% each. Which option improves end-to-end availability more? Show the arithmetic.
3. What assumption does your answer to (2) depend on, and how could it be violated in practice?