Begin a transaction in the current fiber.
A transaction is attached to caller fiber, therefore one fiber can have only one active transaction. See also box.begin().
Returns: 0 on success Returns: -1 on error. Perhaps a transaction has already been started.
Commit the current transaction. See also box.commit().
Returns: 0 on success Returns: -1 on error. Perhaps a disk write failure
Roll back the current transaction. See also box.rollback().
Roll back the current transaction as far as the specified savepoint.