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Handling failures

The Transaction Manager is responsible for performing the two-phase commit protocol. It must maintain the current state of this protocol for every transaction it manages. It should also be able to deal with failures in any component, including itself.

A problematic situation occurs if the contact between the TM and the RM is lost when the protocol is between phase 1 and 2. In this case, the RM has promised to be able to commit a certain update, but it does not yet know whether it should actually do so. This is determined in phase 2. Because of this uncertainty, the RM does not know what value to return to other transactions that asks for the information that was updated. Should the old or the new value be returned? A Mimer database server will typically abort transactions that request data which was updated by a transaction that is in doubt.

The situation is automatically resolved when the contact between the TM and the RM is reestablished. Since both TM and RM save all information on disk, they may both crash between phase 1 and 2, and still be able to carry through with the two phase commit protocol.

However, if the TM somehow fails to reconnect to a Mimer database server that has prepared transactions in doubt, there is another option. The operator may perform a heuristic commit or a heuristic rollback. By doing this, the operator does the role that the TM normally does and resolves the state of the transaction that is in doubt. This can be done by using the TRANSACTIONS command, described in Mimer SQL User's Manual, TRANSACTIONS.

Note that if the TM has already instructed some RMs to (for example) commit, while the operator does a heuristic rollback on another RM, a transactional inconsistency has been introduced. This must be resolved manually. Because of this risk, heuristic operations should be used with due care.

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