Abstract-One of the weaknesses of database replication protocols, compared to centralized DBMSs, is that they are unable to manage concurrent execution of transactions at different isolation levels. In the last years, some theoretical works related to this research line have appeared but none of them has proposed and implemented a real replication protocol with support to multiple isolation levels. This paper takes advantage of our MADIS middleware and one of its implemented Snapshot Isolation protocols to design and implement SIRC, a protocol that is able to execute concurrently both Generalized Snapshot Isolation (GSI) and Generalized Loose Read Committed (GLRC) transactions. We have also made a performance analysis to show how this kind of protocols can improve the system performance and decrease the transaction abortion rate in applications that do not require the strictest isolation level in every transaction.

Many database replication protocols have been designed for guaranteeing a serialisable isolation level, since it is appropriate for almost all applications. However, it also requires a tight coordination among replicas and might generate high abortion rates with some workloads. So, other isolation levels have also been considered, such as snapshot isolation and cursor stability, but none of the previous works has proposed an overall support for more than one isolation level at the same time. This paper explores such a research line.

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