Generalized snapshot isolation and a prefix-consistent implementation

Generalized snapshot isolation extends snapshot isolation as used in Oracle and other databases in a manner suitable for replicated databases. While (conventional) snapshot isolation requires that transactions observe the “latest ” snapshot of the database, gener-alized snapshot isolation allows the use of “older ” snapshots, fa-cilitating a replicated implementation. We show that many of the desirable properties of snapshot isolation remain. In particular, un-der certain assumptions on the transaction workload the execution is serializable. An implementation of generalized snapshot isolation can choose which past snapshot it uses. An interesting choice for a replicated database is prefix-consistent snapshot isolation, in which the snap-shot contains at least all the writes of locally committed transac-tions. As an instance of generalized snapshot isolation, it inherits all of its properties. In addition, read-only transactions never block, and consecutive transactions submitted in a single workflow on a particular replica observe the updates of their predecessors in the workflow. We present two implementation strategies of prefix-consistent snap-shot isolation. We conclude with an analytical performance model of one of the implementations, bringing out the benefits, in particu-lar reduced latency for read-only transactions, and showing that the potential downsides, in particular the change in abort rate of update transactions, are limited