Ganymed: scalable replication for transactional web applications

Data grids, large scale web applications generating dynamic content and database service providing pose significant scalability challenges to database engines. Replication is the most common solution but it involves difficult trade-offs. The most difficult one is the choice between scalability and consistency. Commercial systems give up consistency. Research solutions typically either offer a compromise (limited scalability in exchange for consistency) or impose limitations on the data schema and the workload. In this paper we introduce Ganymed, a database replication middleware intended to provide scalability without sacrificing consistency and avoiding the limitations of existing approaches. The main idea is to use a novel transaction scheduling algorithm that separates update and read-only transactions. Transactions can be submitted to Ganymed through a special JDBC driver. Ganymed then routes updates to a main server and queries to a potentially unlimited number of read-only copies. The system guarantees that all transactions see a consistent data state (snapshot isolation). In the paper we describe the scheduling algorithm, the architecture of Ganymed, and present an extensive performance evaluation that proves the potential of the system.