File sync & storage (Dropbox)

Keep files synchronised across a user's devices without re-uploading an entire 1 GB file when one paragraph changes. Files are split into content-addressed chunks (~4 MB or Rabin-fingerprinted variable chunks) stored in an immutable object store, while a transactional metadata database tracks versioned chunk lists and notifies linked devices.

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Duration 45:00

Requirements checklist

Upload, download, and automatically synchronise files and folders across multiple devices per user.Delta sync and deduplication: upload and download only modified chunks, and skip chunks whose SHA-256 hash already exists.Maintain file version history and handle concurrent offline edits without silently losing work (conflict copies).Zero silent data loss or corruption; 99.999999999% (11 nines) block durability via replication / Reed-Solomon erasure coding.Low-latency change propagation: linked online devices learn of a file change within a couple of seconds.Strict separation between small transactional metadata and multi-exabyte immutable block payloads.

Clarifying questions

Maximum file size and typical file size distribution?
How should concurrent offline edits to the same file be resolved?
Do we deduplicate blocks within a single user's account or globally across accounts?
How long do we retain deleted files and old versions?

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requirements

Suggested time: about 6 minutes. Separates metadata ACID requirements from immutable block storage.

Notes for requirements