One of the important premises of object storage is that if your PutObject or multipart upload succeed, the entire object is atomically replaced. It is eventually consistent, so you may not immediately retrieve the just-uploaded object with GetObject, but you should see the new version eventually, and never see part of one version mixed with part of another. This should natively support compare-and-swap: "hey if the existing etag is what I expect, apply my change, otherwise ignore my change and tell me so". This has nothing to do with DynamoDB and is not reimplementing its feature set. It is just a natural extension of how the service already works (from an API consumer perspective, not necessarily an implementation perspective).
