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12.4. Transaction Outcomes
12.4.1. About Transaction Outcomes
There are three possible outcomes for a transaction.
- Roll-back
- If any transaction participant cannot commit, or the transaction coordinator cannot direct participants to commit, the transaction is rolled back. See Section 12.4.3, “About Transaction Roll-Back” for more information.
- Commit
- If every transaction participant can commit, the transaction coordinator directs them to do so. See Section 12.4.2, “About Transaction Commit” for more information.
- Heuristic outcome
- If some transaction participants commit and others roll back. it is termed a heuristic outcome. Heuristic outcomes require human intervention. See Section 12.4.4, “About Heuristic Outcomes” for more information.
12.4.2. About Transaction Commit
When a transaction participant commits, it makes its new state durable. The new state is created by the participant doing the work involved in the transaction. The most common example is when a transaction member writes records to a database.
After commit, information about the transaction is removed from the transaction coordinator, and the newly-written state is now the durable state.
12.4.3. About Transaction Roll-Back
A transaction participant rolls back by restoring its state to reflect the state before the transaction began. After a roll-back, the state is the same as if the transaction had never been started.
12.4.4. About Heuristic Outcomes
A heuristic outcome, or non-atomic outcome, is a transaction anomaly. It refers to a situation where some transaction participants committed their state, and others rolled back. A heuristic outcome causes state to be inconsistent.
Heuristic outcomes typically happen during the second phase of the 2-phase commit (2PC) protocol. They are often caused by failures to the underlying hardware or communications subsystems of the underlying servers.
There are four different types of heuristic outcome.
- Heuristic rollback
- The commit operation failed because some or all of the participants unilaterally rolled back the transaction.
- Heuristic commit
- An attempted rollback operation failed because all of the participants unilaterally committed. This may happen if, for example, the coordinator is able to successfully prepare the transaction but then decides to roll it back because of a failure on its side, such as a failure to update its log. In the interim, the participants may decide to commit.
- Heuristic mixed
- Some participants committed and others rolled back.
- Heuristic hazard
- The outcome of some of the updates is unknown. For the ones that are known, they have either all committed or all rolled back.
Heuristic outcomes can cause loss of integrity to the system, and usually require human intervention to resolve. Do not write code which relies on them.
12.4.5. JBoss Transactions Errors and Exceptions
For details about exceptions thrown by methods of the
UserTransaction class, see the UserTransaction API specification at http://docs.oracle.com/javaee/6/api/javax/transaction/UserTransaction.html.
