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Berkeley DB XA Implementation
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## Berkeley DB XA Implementation

Berkeley DB provides support for distributed transactions using the two-phase commit protocol via its DB_TXN->prepare() interfaces. The DB_TXN->prepare() method performs the first phase of a two-phase commit, flushing the log to disk, and associating a global transaction ID with the underlying Berkeley DB transaction. This global transaction ID is used by the global transaction manager to identify the Berkeley DB transaction, and will be returned by the DB_ENV->txn_recover() method when it is called during recovery.

However, Berkeley DB does not perform distributed deadlock detection. Instead, when being used as an XA resource manager, Berkeley DB acquires all locks in a non-blocking mode. This results in pre-emptive abort of transactions that have the potential to deadlock. While this can lead to more transactions being aborted than is strictly necessary, it avoids system-wide hanging due to distributed deadlocks.

When using distributed transactions, there is no way to perform hot backups of multiple environments and guarantee that the backups are globally transaction-consistent across these multiple environments. If backups are desired, then all write transactions should be suspended; that is, active write transactions must be allowed to complete and no new write transactions should be begun. Once there are no active write transactions, the logs may be copied for backup purposes and the backup will be consistent across the multiple environments.

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