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CORFU and Tango are interesting research implementations in this general area, with emphasis on a dedicated "sequencer" - see https://rebeccabilbro.github.io/the-shared-log-abstraction/

Facebook's LogDevice had a similar sequencer component too https://engineering.fb.com/2017/08/31/core-data/logdevice-a-...

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Yeah, and the key idea here is that the sequencer is soft state that can be recovered from the log at any time. It acts to improve throughput while it's up, but if it's down the system can still make progress. The key to this is that the log is the ultimate ground source of truth, and individual entries in the log are write once. This means racing writers can detect and work around each other in the absence of a sequencer, via the hole filling protocol.

FaunaDB (fauna.com) is a commercial system based on Calvin.

Disclosure: I work on FaunaDB.

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