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1.1.2 Two-Phase Commit Protocol

  • Article

The two phases of the two-phase commit protocol as described in [GRAY] are Phase One and Phase Two. These phases can be described informally as "are you ready" and "go / no go," respectively.

Phase One (are you ready) begins when all the required changes to the resource state have been made, and the root application asks the transaction manager to complete the transaction. Phase One ends when the transaction manager determines the outcome of the transaction; that is, whether all the changes are to be made or whether no changes are to be made.

When the root application asks the root transaction manager to complete the transaction, it makes either a commit request, asking that all the changes are to be made, or an abort request, asking that no changes are to be made. A commit request causes the root transaction manager to ask each of the subordinate participants that are involved in the transaction whether they are prepared to commit the changes that were made. This process is called voting on the transaction outcome. Each subordinate participant votes for one of three outcomes:

  • Read-Only

  • Prepared

  • Aborted

Read-Only and Prepared are positive votes. Aborted is a negative vote. If every subordinate participant votes positively, then the final outcome of the transaction as a whole is to make all the changes; that is a commit outcome.

If any subordinate participant votes negatively, the root transaction manager determines that the final outcome of the transaction as a whole is to make no changes; that is an abort outcome. An abort request causes the root transaction manager to notify each subordinate participant to make no changes and to notify each of their respective subordinate participants if there are any to abort the transaction.

A subordinate transaction manager determines its vote by aggregating the votes of its subordinate participants. If a subordinate transaction manager has no subordinate participants, or if all of its subordinate participants vote Read-Only, then the subordinate transaction manager votes Read-Only. If at least one subordinate participant votes Prepared, and after collecting all votes no subordinate participant votes Aborted, then the subordinate transaction manager votes Prepared. In all other cases, the subordinate transaction manager votes Aborted, in which case it also notifies any subordinate participants that had voted Prepared that the transaction has been aborted.

Until a participant votes on the outcome of a transaction, that participant can unilaterally abort the transaction by issuing an abort request to its transaction manager. This request is called a unilateral abort. Further details of unilateral abort are described in [MS-DTCO] section 1.3.2.1.

Phase Two begins after the root transaction manager determines the outcome of the transaction. In this phase, each subordinate participant that voted Prepared is sent either a request to commit the changes if the outcome was the commit outcome or a request to undo (rollback) the changes if the outcome was the abort outcome. The root transaction manager also sends the outcome of the transaction to the root application. A subordinate participant that voted Read-Only is not notified of the outcome of the transaction. For example, a resource manager might vote Read-Only if it made no changes as part of the transaction. A subordinate participant that voted Abort is also not notified of the transaction outcome.

Phase Two ends after the root transaction manager communicates to the participants what the outcome is (commit or abort), and participants have notified the transaction manager that the operation is successfully completed.

The two-phase commit protocol is described in [GRAY]. The DTCO protocol adds Phase Zero (section 1.1.3), which expands the beginning of Phase One.