Middleware-based database replication protocols require few or no changes in the database engine. As a consequence, they are more portable and flexible than kernel-based protocols, but have coarser-grain information about transaction access data, resulting in reduced concurrency and increased aborts. This paper proposes conflict-aware load-balancing techniques to increase the concurrency and reduce the abort rate of middleware-based replication protocols. Our algorithms strive to assign transactions to replicas so that the number of conflicting transactions executing on distinct servers is reduced and the load over the servers is equitably distributed. A performance evaluation using a prototype of our system running the TPC-C benchmark shows that aborts can be reduced with no penalty in response time.
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