Both Two-Phase Commit (2PC) protocol and Three-Phase Commit (3PC) protocol are popular with Distributed DBMS instances because all nodes must commit to a transaction or none of them will. It is an all-or-nothing proposition. Both protocols share a Prepare (Voting) and Commit/Abort phase, but 3PC adds an additional pre-Commit phase in which every participating node must vote yes to a commit before it is actually done. Compared to 3PC, Two-Phase Commit may be characterized as sending the command and hoping for the best, since the bulk of the transaction (the instructions for what to actually do) are transmitted with the commit phase. The return message after the transaction, from each participant, determines commit or abort status globally.
The 3PC extra step of pre-commit is intended to clear up any global commit/abort failure issues or blocking. This step polls for availability before anything is done and the nodes can “act independently in the event of a failure” (Connolly & Begg, 2015). This is an important distinction. In 2PC, a single abort vote or acknowledgement undoes the entire process. In 3PC, assuming the pre-commit phase came back with a global commit vote, even a timeout or network partition would not cause a global abort.
Terminating a process, according to Connolly & Begg (2015), is where the differences between these protocols are most critical. In 2PC it is possible to have a block because after the vote, the nodes are waiting on a commit or abort message from coordinator before making the global commit. If partition occurs, they are stuck until coordinator re-establishes communication. A power failure is more catastrophic, as it may involve multiple nodes and the controller. In both 2PC and 3PC, backup procedures are activated. 2PC participants remain in a blocked state. Of course, overall, there are tradeoffs. The major issue with 3PC is the communication overhead, which is to be expected with the extra phase (Kumar, 2016).
Connolly, T. & Begg, C. (2015). Database Systems: A Practical Approach to Design, Implementation, and Management (6th ed.). London, UK: Pearson.
Kumar, M. (2016). Commit protocols in distributed database system: A comparison. International Journal for Innovative Research in Science & Technology, 2(12), 277-281.