#include <db_cxx.h> int DbTxn::prepare(u_int8_t gid[DB_XIDDATASIZE]);
         The DbTxn::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 DbTxn::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 DbTxn::prepare() 
            
            
                method either returns a non-zero error value or throws an
                exception that encapsulates a non-zero error value on
                failure, and returns 0 on success.
            
        
    
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 DbEnv::txn_recover() telling the application which global transactions must be resolved.