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.