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 a parent transaction
causes all unresolved children of the parent transaction to be prepared.
The txn_prepare function returns a non-zero error value on failure and 0 on success.
Errors
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.