System design interview question
"Design S3. Users create buckets and PUT and GET objects by key. It must be extremely durable — you are promising to essentially never lose data — highly available, and it must scale to trillions of objects. Tell me how you achieve that durability."
durability engineering and the metadata-versus-data split at extreme scale. The distinguishing content is: erasure coding versus replication, how you actually justify a durability number, the fact that the key namespace is flat (not a filesystem tree), and the consistency model. It is one of the few questions where the honest answer involves arithmetic about disk failure rates.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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