The three-phase commit protocol is a method used in distributed computing to ensure that all parts of a system reach a consensus on whether to commit or abort a transaction. It is an extension of the two-phase commit protocol, which consists of a prepare phase followed by a commit phase. The three-phase commit introduces an additional phase to improve fault tolerance and reduce the likelihood of uncertainty during network failures or crashes. By dividing the transaction process into three distinct stages—prepare, pre-commit, and commit—the system can handle various scenarios more effectively.
In the first phase, known as "prepare," one coordinator node sends a request to all participant nodes to determine if they are ready to commit the transaction. Each participant must respond with either a "yes" (indicating readiness) or "no" (indicating a problem). In the second phase, called "pre-commit," if all participants signal readiness, the coordinator instructs them to enter a state where they temporarily "pre-commit" the transaction. This phase allows nodes to acknowledge that they are prepared to finalize the transaction without actually committing it yet. The coordinator waits for confirmation from all participants before proceeding to the final phase.
The last phase, "commit," occurs when the coordinator receives all pre-commit confirmations from the participants. At this point, the coordinator sends a commit message to all nodes, allowing them to finalize the transaction. If any participant fails to respond or encounters an issue during the pre-commit phase, the coordinator can decide to abort the transaction, preventing any inconsistent states. A practical example can be seen in a banking application where funds are transferred between accounts. Using the three-phase commit ensures that both accounts are either updated together or not at all, reducing the risk of incomplete transactions due to system failures.