(WarpStream co-founder) We use S3 for data replication, availability, and durability, but not for the log ordering guarantees. For that, we have our own custom metadata store that "imposes" ordering guarantees on top of the data that is written to S3.
We don't rely on any kind of timestamp-based last write wins policy for concurrent writes. In fact the agents will never write a file to S3 with the same name more than once or overwrite an existing file.
Its stateless from the customer's perspective since they don't have to manage the consensus layer (which is only possible because we separated data from metadata).
Obviously there is state somewhere since the cluster is storing data, its just offloaded to S3 + our control plane, while still keeping all the data in the customer's cloud account.
We don't rely on any kind of timestamp-based last write wins policy for concurrent writes. In fact the agents will never write a file to S3 with the same name more than once or overwrite an existing file.