Asynchronous version advancement in a distributed three version database

We present an efficient protocol for multi-version concurrency control in distributed databases. The protocol creates no more than three versions of any data item, while guaranteeing that: update transactions never interfere with read-only transactions; the version advancement mechanism is completely asynchronous with (both update and read-only) user transactions; and read-only transactions do not acquire locks and do not write control information into the data items being read. This is an improvement over existing multi-versioning schemes for distributed databases, which either require a potentially unlimited number of versions, or require coordination between version advancement and user transactions. Our protocol can be applied in a centralized system also, where the improvement over existing techniques is in reducing the number of versions from four to three. The proposed protocol is valuable in large applications that currently shut off access to the system while managing version advancement manually, but now have a need for automating this process and providing continuous access to the data.