Efficient Snapshot Isolation in Paxos-Replicated Database Systems

Modern database systems are increasingly deployed in a cluster of commodity machines with Paxos-based replication technique to offer better performance, higher availability and fault-tolerance. The widely adopted implementation is that one database replica is elected to be a leader and to be responsible for transaction requests. After the transaction execution is completed, the leader generates transaction log and commit this transaction until the log has been replicated to a majority of replicas. The state of the leader is always ahead of that of the follower replicas since the leader commits the transactions firstly and then notifies other replicas of the latest committed log entries in the later communication. As the follower replica can’t immediately provide the latest snapshot, both read-write and read-only transactions would be executed at the leader to guarantee the strong snapshot isolation semantic. In this work, we design and implement an efficient snapshot isolation scheme. This scheme uses adaptive timestamp allocation to avoid frequently requesting the leader to assign transaction timestamps. Furthermore, we design an early log replay mechanism for follower replicas. It allows the follower replica to execute a read operation without waiting to replay log to generate the required snapshot. Comparing with the conventional implementation, we experimentally show that the optimized snapshot isolation for Paxos-replicated database systems has better performance in terms of scalability and throughput.