This is fantastic news. What would you say were the hardest aspects of the design and implementation, in terms of effort and thinking that went into those accomplishments ?
I would say, building an scalable raft implementation has been by far the hardest thing. Mostly because we started from scratch and bypassed the page-cache with our read-ahead and write-behind strategies, etc. so building tiers of caches, etc and have a working raft system was hard.
Here is a post on building a dummy key-value db to test jepsen https://vectorized.io/validating-consistency/