Reducing the Latency of Non-Blocking Commitment using Optimism and Replication

Atomic commitment protocols are used to ensure the atomicity of atomic transactions. The best known and most widely used atomic commitment protocol is two phase commit (2PC). The main idea behind 2PC is to perform two rounds of voting under the guidance of a site acting as coordinator. The coordinator asks first for votes on whether to commit (yes vote) or abort (no vote) the transaction. The participants send their vote and the coordinator decides to commit if all participants voted yes. Otherwise the decision is to abort. The main drawback of 2PC is that it might block. This happens when the coordinator fails after having received yes votes from all participants but before sending the commit message. In that situation, the participants cannot make a decision among themselves because they do not know what the coordinator decided before it failed. By deciding to commit or abort, they could be doing exactly the opposite of what the coordinator did, thereby violating the atomicity of the transaction. Non-blocking commit protocols, e.g., 3 phase commit (3PC) have been proposed to overcome this drawback. However, and in spite of the work in the area, the standard atomic commitment protocol is still 2PC. The main reason is that most transactional systems pay as much attention to performance as they do to consistency. For instance, in most systems, if a transaction has not committed after a given period of time, it is summarily aborted regardless of whether the coordinator is up or down. There are good practical