Directory sync service
Design a directory-sync service that keeps user and group data synchronized from thousands of customers' source-of-truth directories (Active Directory, Workday, Google Workspace) into your platform, where each customer may have hundreds of thousands of users, syncs run continuously, and a person joining/leaving/changing groups in the source must reflect on your side reliably and in order — without a transient sync error leaving access in a wrong state. Walk through the sync model (push vs pull, full vs incremental), how you keep it correct under partial failures, and the consistency model you expose.
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.