Google

Massively scalable web applications encounter a fundamental tension in computing between performance and correctness: whereas performance is often addressed by using a large and therefore distributed machine, correctness, especially of an interruptible and multi-threaded program, requires data invariants to be maintained with the assistance of an underlying transaction layer. We provide a Distributed Transaction algorithm assuming a strongly consistent layer providing Local Transactions. We give most of the detail for operation in optimistic mode; however the same infrastructure could be used for pessimistic transactions as well, even at the same time. The algorithm is general but we express it in the particular instance of Google App Engine.