Weak Voting Database Replication Protocols Providing Different Isolation Levels

Recently, several works have taken advantage of a database isolation notion suitable for replicated approaches, called Generalized Snapshot Isolation, that provides greater performance since read-only transactions are never blocked nor cause update transactions to block or abort. However, this concept has not been formally described for replicated environments where a logical copy of the system must be considered in order to study its behavior. In this work, we study formally the conditions that guarantee the one-copy equivalent Generalized Snapshot Isolation level in a database replicated systems using Snapshot Isolation replicas. Some recent protocols based on Snapshot Isolation replicas use a certifying technique, but this technique requires sending the readset in order to achieve a serializable level, what is prohibitive. Thus, we propose a basic replication protocol, and some possible enhancements for this protocol, based on a weak voting technique over replicas providing snapshot isolation level. This protocol and its enhanced versions are able to provide different isolation levels to transactions submitted by a database user only using one protocol. Each transaction submitted to the database can be executed ensuring a given isolation level, what provides a great flexibility to applications that may demand different isolation levels for their transactions.