One example of #4 that is making headway in production systems is the Calvin protocol (a RAFT based write-ahead-log with lock-free deterministic transaction app

One example of #4 that is making headway in production systems is the Calvin protocol (a RAFT based write-ahead-log with lock-free deterministic transaction application). It's implemented by my employer FaunaDB. You can learn more here https://fauna.com/blog/consistency-without-clocks-faunadb-transaction-protocol https://fauna.com/blog/consistency-without-clocks-faunadb-tr...