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TEXTBOX: TEXTBOX_HEAD: Make room for JavaSpaces: Read the whole series!
In general, transactional systems let you group together a set of operations so that they are performed atomically -- that is, either all of the operations complete, or none of them do. Without this transactional ability, the states of systems can easily become inconsistent -- especially distributed systems in which participants can crash the network or leave the network before an operation has completed. By using transactions, you can ensure that the operations do complete, or if they don't, that the state of the entire distributed system remains unchanged.
Systems that support transactions, such as a database management system, typically build transactions into the system's core. Jini takes a more lightweight and flexible approach: It provides a transaction service that manages a set of participants through a transaction process. The transaction service leads the participants through a "two-phase commit protocol," a fairly simple and standard protocol that ensures that either all participants complete their respective operations in the transaction, or none of them do. If you're interested, you can find out more about this protocol in the Jini Transaction Specification (see Resources below).
The participants in a Jini transaction are typically Jini services and devices. If you are developing a Jini service, you
can enable it to participate in Jini transactions by implementing the
TransactionParticipant interface. You can find out more about this interface in the Jini Transaction Specification.
Now we'll give you a better idea of how you can use transactions with JavaSpaces.
The JavaSpaces application programming interface integrates Jini transactions in a clean and well-thought-out manner. As a result, introducing transactional security into your JavaSpaces applications is usually fairly painless. To use transactions with space-based operations, typically you first ask a transaction manager to create a transaction and manage it for a specified lease time. Then, you pass the transaction to each space operation you'd like to occur under the transaction (which may include operations over more than one space). Assuming there are no problems along the way, you then explicitly commit the transaction, which results in all operations completing. If any problems occur, you can abort the transaction, which will leave the space unchanged. The transaction might also be aborted by the transaction manager if, for instance, the transaction's lease expires.