A Probabilistic Analysis of Snapshot Isolation with Partial Replication

Abstract

Snapshot isolation has received a considerable amount of attention in the context of full database replication. Such popularity is mainly because read-only transactions executing under snapshot isolation are never blocked or aborted. In partial replication, where each replica holds only a part of the database, transactions may require access to remote databases. Each remote read operation of the transaction must execute in a consistent global database snapshot as the local operations; if such a snapshot is not available, the transaction must be aborted. In this paper we are interested in the effects of distributed transactions on the abort rate of partially replicated snapshot isolation systems. We present a simple probabilistic analysis of transaction abort rates for two different concurrency control mechanisms: lock- and version-based. The former models the behavior of a replication protocol providing one-copy-serializability; the latter models snapshot isolation. Our analysis reveals that in the version-based system the execution abort rate decreases exponentially as the number of data versions available increases. As a consequence, in all cases considered, two versions of each data item were sufficient to eliminate aborts due to distributed transactions.