File Storage & Sync

A file-sync service like Dropbox looks simple from the outside: drop a file in a folder and it appears on every device. Underneath it is one of the most storage-heavy and bandwidth-sensitive systems you can be asked to design. You are juggling exabytes of durable blobs, hundreds of terabytes of metadata, a long tail of huge binary uploads that arrive in bursts, and a sync problem that fans changes out to hundreds of millions of devices without melting your servers. The trick that makes all of it tractable is the one the grader rewards: stop treating files as files. Treat them as content-addressed chunks, dedup aggressively, and only ever move the bytes that actually changed. The simulator drives a representative metadata-and-block cell at 12,000 operations per second, ramps it through 18,000 and on to 24,000 during a bulk-upload storm, and — overlapping those ramps — kills a service replica, blips the object store for six seconds, and slows the gateway. Your job is to hold availability at or above 98 percent, p99 latency under 120 milliseconds, throughput at or above 12,000 ops per second, peak saturation under 90 percent, and the footprint at nine instances or fewer. Every decision below serves those five numbers.