int txn_prepare(DB_TXN *tid, u_int8_t gid[DB_XIDDATASIZE]);
The txn_prepare function initiates the beginning of a two-phase commit.
In a distributed transaction environment, Berkeley DB can be used as a local transaction manager. In this case, the distributed transaction manager must send prepare messages to each local manager. The local manager must then issue a txn_prepare and await its successful return before responding to the distributed transaction manager. Only after the distributed transaction manager receives successful responses from all of its prepare messages should it issue any commit messages.
In the case of nested transactions, preparing the parent causes all unresolved children of the parent transaction to be committed. Child transactions should never be explicitly prepared. Their fate will be resolved along with their parent's during global recovery.
The gid parameter specifies the global transaction ID by which this transaction will be known. This global transaction ID will be returned in calls to txn_recover, telling the application which global transactions must be resolved.
The txn_prepare function returns a non-zero error value on failure and 0 on success.
The txn_prepare function may fail and return a non-zero error for errors specified for other Berkeley DB and C library or system functions. If a catastrophic error has occurred, the txn_prepare function may fail and return DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.
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