The Sequencer:

- does not have a persistent/disk-backed state

- It is a singleton process

- it and only it does order, no logs do ordering

... if the singleton sequencer crashes, I do not see on this high level description how the system recovers, if the sequencer is the only one that knows write order but has no persistent write "log".

What am I missing?

This... does not appear to be something you run outside of a dedicated datacenter, AWS with its awful networking and slow/silently throttling storage would probably muck this thing up under any substantive scale?