The broker runs a single group commit loop on behalf of all clients, so no one contends for the object. Critically, it doesn't acknowledge a write until the group commit has landed in object storage. No client moves on until its data is durably committed.
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Yea, the group commit is the real insight here.
I read this blog post and to help wrap my head around it I put together a simple TCP-based KV store with group commit, helped make it click for me.
https://github.com/a10y/group-commit/