Capstone: a multi-tenant enterprise document platform, in 45 minutes

A full mock interview: clarify 500 tenants across three regions, estimate 50 TB, 1,000 requests a second and a 3 TB index, sketch regional data planes under a metadata-only control plane, then deep-dive into five isolation layers, quotas and fair queues, residency and export, ordered deletion, and layered security, and finish with 10x scaling, edge cases, per-tenant monitoring and alerts. Every Week 9 idea in one answer.