We assume that each transaction is self-contained, meaning that it performs its computation without any direct communication with other transactions .Transactions do communicate indirectly, of course, by storing and retrieving data in the database. However, this is the only way they can affect each other's execution .To ensure transaction atomicity, the DBS must control all of the ways that transactions interact. This means that the DBS must mediate each transaction's operations that can affect other transactions. In our model, the only such operations are accesses to shared data. Since a transaction accesses shared data by issuing database operations to the DBS, the DBS can control all such actions, as required .In many systems, transactions are allowed to communicate by sending messages. We allow such message communication in our model, provided that those messages are stored in the database. A transaction sends or receives a message by writing or reading the data item that holds the message. This restriction on message communication only applies to messages between transactions. Two or more processes that are executing on behalf of the same transaction can freely exchange messages, and those messages need not be stored in the database. In general, a transaction is free to control its internal execution using any available mechanism. Only interactions between different transactions need to be controlled by the DBS.