You still need something close to ArcSwap with your persistent data structure, because once you’ve built your new tree (possibly using transients) you need to o

You still need something close to ArcSwap with your persistent data structure, because once you’ve built your new tree (possibly using transients) you need to overwrite the old root with the new, atomically (and probably in a CAS loop really). Left-right is an option which iirc avoids the CAS loop, as it’s a single writer primitive.