Cross-region blob replication

Design cross-region replication for a large-media object store where multi-GB objects (video masters, datasets) written in one region must be asynchronously replicated to 2–3 other regions for durability and low-latency local reads, at a rate of 100 GB/s of new writes globally. Replication must be efficient (don't re-ship unchanged data on overwrite), must not lose objects if a region is destroyed, and readers in a remote region need clear semantics about whether they'll see the latest version yet.