That's right: You upload an object, and the HTTP 200 response includes a header that is the object ID. Your imagination is almost certainly correct: creating a mapping from user-defined key to storage location is really best done by a metadata store so that data doesn't need to be reshuffled with topology changes.

A classic distributed key-value store (such as Dynamo[1]) spends a lot of its design on how to consistently map user-keys to storage locations. Most of these solutions are focused on performance (so avoiding a middle-layer lookup) and accept the tradeoff that data will need to be re-balanced when doing things like expanding the cluster. This isn't an acceptable tradeoff for object stores that have much higher data density (100+ TBs per NIC)

[1]: https://www.allthingsdistributed.com/files/amazon-dynamo-sos...