This is great. I sketched out a similar design for the Delicious backend, but the team that built it went in a completely different and stupid direction. Oh well.

Mine also had:

- some way to do server locality (I don't think FF needs this)

- some way to do table locality (I don't think FF needs this either)

- per blob versioning and type

- a virtual shard layer, so new machines can be added inexpensively

Sorry for any inadvertent whining, incidentally.