2-phase Commit Protocol

DEFINITION The 2-phase commit (2PC) protocol is a distributed algorithm to ensure the consistent termination of a transaction in a distributed environment. Thus, via 2PC an unanimous decision is reached and enforced among multiple participating servers whether to commit or abort a given transaction, thereby guaranteeing atomicity. The protocol proceeds in two phases, namely the prepare (or voting) and the commit (or decision) phase, which explains the protocol’s name. The protocol is executed by a coordinator process, while the participating servers are called participants. When the transaction’s initiator issues a request to commit the transaction, the coordinator starts the first phase of the 2PC protocol by querying—via prepare messages—all participants whether to abort or to commit the transaction. If all participants vote to commit then in the second phase the coordinator informs all participants to commit their share of the transaction by sending a commit message. Otherwise, the coordinator instructs all participants to abort their share of the transaction by sending an abort message. Appropriate log entries are written by coordinator as well as participants to enable restart procedures in case of failures.