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Content-addressed chunks, delta sync, a journal-shaped metadata DB, long-poll notifications to fifty million idle clients, and why last-writer-wins is a lie users notice.
Design a service that keeps a folder of files synchronized across a user's devices and with the cloud: edit a document on your laptop and it appears on your phone seconds later; work offline and changes reconcile when you reconnect; share a folder and teammates see each other's edits.
This is a classic that rewards systems thinking over API sketching. The naive design - upload the whole file on every save - dies immediately on bandwidth and storage math. The real design is built from chunking, content-addressed storage, and a metadata journal that turns "sync" into "replicate an ordered log of small facts." The distinguishing conversations are delta sync, offline conflict resolution (where the honest answer is not a clever merge), and how millions of idle clients learn about changes without polling the service to death.
Asking these before diving into a solution is the difference between a "hire" and a "no signal" rating. Pick the questions whose answers would change your design.
Capacity estimation · architecture with all 7 components explained · 6 deep dives · trade-off analysis · 8 common follow-up questions
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Namespace vs bytes, erasure coding vs replication, the eleven-nines durability math, multipart upload, and why LIST is the hardest API in the system.
CRDTs vs OT, presence, cursor broadcasting, and conflict-free merging when 50 people edit the same doc at once.
Fan-out at write vs read, at-least-once vs exactly-once, dead-letter queues, and the multi-channel delivery problem - one message, ten failure modes.
Long-lived connections, ordering guarantees, presence, and the difference between 1:1 chat and a 50K-member group.
Reading is the floor. The interview signal is in walking through this live with someone probing follow-ups. Use the AI mock interview to practice talking through requirements, architecture, and trade-offs out loud.
Mock interview: design File Sync →