Design a Distribution-Center Inventory Update Service

Design a service that receives stock updates from distribution centers and maintains the latest quantity, availability, and last-update time for each location–item pair. Support fast lookups, thousands of locations, millions of items, and tens of thousands of updates per second. ### Constraints & Assumptions Preserve the distinction between an update's event time and its authoritative ordering/version. Clarify whether updates are absolute snapshots or deltas before defining deduplication and out-of-order behavior. Historical data must remain available for analytics. ### Clarifying Questions Who assigns versions for a location–item pair? Are negative quantities valid? How fresh must reads be? Does availability mean quantity greater than zero or include reservations and other rules? How long is history retained? ### What a Strong Answer Covers Idempotent ingestion, per-key ordering, partitioning, latest-state storage, lookup consistency, hot-key handling, analytics history, and recovery/reconciliation. ### Follow-up Questions What if two sources claim the same version? How would you handle a late delta versus a late absolute snapshot? What happens when caches lag or a partition is replayed after failure?