DB_TXN->prepare()
#include <db.h>
int
DB_TXN->prepare(DB_TXN *tid, u_int8_t gid[DB_GID_SIZE]); The DB_TXN->prepare() method 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 DB_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.
All open cursors in the transaction are closed and the first cursor close error will be returned.
A transaction running under serializable snapshot isolation — one begun with the DB_TXN_SNAPSHOT flag, or begun in an environment configured with DB_TXN_SNAPSHOT — cannot be prepared: DB_TXN->prepare() returns EINVAL for such a transaction. SSI's serializability check runs at commit time and can still abort the transaction after prepare would have returned, so the combination is refused rather than allowing a prepared transaction to become uncommittable. Applications that need two-phase commit must run those transactions without DB_TXN_SNAPSHOT.
The DB_TXN->prepare() method returns a non-zero error value on failure and 0 on success. The errors that this method returns include the error values of DBcursor->close() and the following:
DB_LOCK_DEADLOCK
A transactional database environment operation was selected to resolve a deadlock.
DB_LOCK_NOTGRANTED
A Berkeley DB Concurrent Data Store database environment configured for lock timeouts was unable to grant a lock in the allowed time.
You attempted to open a database handle that is configured for no waiting exclusive locking, but the exclusive lock could not be immediately obtained. See DB->set_lk_exclusive() for more information.
EINVAL
If the transaction was begun with DB_TXN_SNAPSHOT (serializable snapshot isolation), which cannot be prepared for two-phase commit; if the cursor is already closed; or if an invalid flag value or parameter was specified.
Parameters
gid
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 DB_ENV->txn_recover() telling the application which global transactions must be resolved.