Implementation based in GMU, a Cloud-TM project research (www.cloudtm.eu).
GMU is a muti-version scheme that provides Update Serializability. It allows the read-only transaction to proceed without blocking, but they can observe an old value (but always a consistent value).
This scheme relies in Vector Clock to manage the snapshots to read and the transaction dependencies in each node in the cluster.
Also, it is a genuine scheme, i.e., only the nodes involved in the transaction (that contains data read or write) are involved in the transaction commit.
A paper describing GMU can be found here: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=6258018