I searched the site a bit and saw no information whatsoever on what is behind it, so I'm going to guess: For the local store, probably something like an appen

I searched the site a bit and saw no information whatsoever on what is behind it, so I'm going to guess: For the local store, probably something like an append-only (aka copy on write aka persistent) B-tree-ish data structure. This guarantees good behaviour on SSDs, and cheap/easy transactions. It can also be scaled linearly over multiple spindles. For the distributed aspect, I guess they use some paxos variation (I guess this from the separation between logging the transaction and durably applying this) But again: I am merely guessing. With regard to flow: Is it just me, or does it look like a poor man's concurrency monad.