EJB Design Models in .NET framework

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The EJB specification offers three component types for use in EJB applications Typically, you'll see all three in use: Q No State - represented by the stateless session beans They do not keep state on behalf of the client between calls of their methods, and so can be shared between many clients They are relatively short-lived and can be viewed as providing a single-use service An example is a bean with a method that calculates the Value Added Tax of an item; the method just returns its calculation and is ready for the next request, no matter which client that comes from Session Oriented - represented by stateful session beans, which maintain state across methods calls and transactions Their life is determined by the life of their client A typical example is a shopping cart at a web site because the output depends very much on its caller
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Persistent - known as entity beans, they are a representation or view of data from a data store (a relational database, an object-oriented database, a file, etc) Because of this they are transactional As examples, entity beans can represent the information associated with a driver's license, an order, or a resultset from a query We'll leave detailed explanations of entity beans until the next chapter
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In this chapter we'll cover session beans Firstly we'll be interested in replacing the functionality of the Oracle PL/SQL stored procedures by stateless session beans Then we'll look at stateful session beans
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Transaction Support in EJBs
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Transactions are everywhere and often need to be mapped to software - they can be defined as any workflow that is 'all or nothing', that is, it must all succeed or be returned to its original state (or 'rolled back') There are four very important properties that must be guaranteed by systems that support transactions: atomicity, consistency, isolation, and durability These four properties are generally known better by the 'ACID' acronym: Q Q Atomicity - all the actions that make up a transaction form a logical unit As such they must all complete successfully or the transaction is undone and no actions complete Consistency - the data store used by for the transaction must always be in a consistent state That is, if a transaction fails half way through, the data store must not retain partially modified data Uncommitted changes to the data store must be capable of being rolled back Isolation - the state of the transaction is not available outside itself until it has been successfully completed, so it can execute without interference
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Q Durability - once the transaction has been completed successfully, the state is stored permanently The EJB specification has declarative support for various modes of transactional management As we mentioned in the previous section, the deployment descriptor tells the container how to manage transactions when methods on a bean are invoked The modes are illustrated in the following table: Transaction Support Mandatory Required NotSupported RequiresNew Supports Never Description The caller must start the transaction before it uses the bean An error occurs if no transaction is available The bean requires a transaction; the container will start one if the caller hasn't already done so The container suspends the caller's transaction before calling the bean The container starts a new transaction for every call, even if there is already a transaction in progress, in which case it will suspend it The container just passes the caller's transaction along If there is no transaction in progress, no new one is created The bean does not participate within a transaction context
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We'll see these in action later in the chapter, when we deploy our beans In the next chapter, we'll look at further transaction support in deployment descriptors As you can see, the transactional support offered by the EJB specification makes it pretty easy to develop transactional components without having to get into the low-level stuff
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