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The JustInTimeActivation attribute enables an important feature of COM+ components. A JIT-activated component isn t created when the client calls the New operator and is actually instantiated only later, when the client makes the first call to it. COM+ compo nents can be instantiated only by means of a parameterless constructor; therefore, no code needs to actually run when the client uses the New keyword. JIT-activated com ponents make a distributed application more scalable, because they don t take memory until one of their methods is actually invoked. JIT activation is a prerequisite for transactional objects and is a desired feature for poolable objects, as I ll explain in the section Enabling Object Pooling, later in this chap ter. As a matter of fact, JIT activation is automatically enabled if you use the Transaction attribute to specify that the component is transactional, even though I have added it explicitly to the MoneyMover class for clarity. If a method in a transactional object votes for the transaction outcome, the object is imme diately deactivated when the method returns. The client continues to keep an object ref erence, but the actual object doesn t exist any longer. If the client makes a new call into the object, COM+ transparently creates a fresh new instance of the object. This mechanism ensures that fewer resources are taken on the computer where the object runs and, more important, guarantees that the semantics of the transaction are satisfied. To understand why this is necessary, consider this scenario. Object A votes commit and then returns to the client. At this point in time, the state of the object (its fields and prop erties) reflects the state of the database. Object B, which is enrolled in the same transac tion as Object A, votes abort and forces the transaction to roll back. If Object A weren t a JIT-activated object, clients could mistakenly use its public fields and properties and make wrong assumptions about values stored in the database. By requiring that transac tional objects be JIT-activated and by destroying them immediately after they vote for the transaction outcome, COM+ prevents clients from using the previous state of the object. Transactional objects manage two internal bits that reflect the state of the object: the consistent bit and the done bit. The done bit tells COM+ whether a JIT-activated object can be deactivated; the consistent bit tells COM+ whether the transaction can be com mitted (if True) or must be rolled back (if False). When the method returns, COM+ checks these bits and decides what to do. If a method in a transactional object performs an action on a database and leaves the database in a consistent state for example, it takes money from one account and successfully moves it to another account it should set the done bit to True as well as set the consis tent bit to True to inform COM+ that the transaction can be committed. If the database update attempted by the object can t be completed correctly for example, there isn t enough money in the first account or the second account doesn t exist the object should set the consistent bit to False to tell COM+ to immediately roll back the transaction the object lives in. The exact behavior depends on whether the object is the root of the
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Part VII:
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transaction that is, whether the object initiated a new transaction. (The transaction root is any object whose Transaction attribute is set to RequiresNew or the first object in a call chain whose Transaction attribute is set to Required.) If the object is the root of the transaction and the done bit is True, the transaction is committed or rolled back, depending on the consistent bit. The transaction is also rolled back if the transaction timeout expires. Regardless of the transaction outcome, if the done bit is True when a method call returns, the JIT activation feature ensures that, even if the client maintains an object ref erence, the object is destroyed and a new object instance is silently created at the next method invocation. It bears repeating that a JIT-activated object should never rely on values assigned to class-level fields and properties, because these values are gone when the object is deactivated and reactivated at the next method call. For all practical purposes, JIT-activated objects should be regarded as stateless objects. If the transaction can t be completed in a single method call, the object should leave the done bit set to False. In this case, COM+ keeps the object instance alive, and the cli ent must invoke another method to complete the transaction. If the client never calls such a method, the transaction will stay active and you can see it in the Transaction List node of the MMC snap-in. If the transaction has a timeout, COM+ will roll it back auto matically when the timeout expires. An object that hasn t set its done bit to True can reliably save values in its private fields and properties between calls. If the client releases the object that hasn t set the done bit, the current value of the consistent bit determines whether the transaction is com mitted or aborted when the garbage collection actually releases the object. The transaction is actually committed or rolled back only when the GC releases the object, which might happen at a random time. For this reason, a client should never release a reference to a transaction COM+ object whose done bit is True. The SetComplete and SetAbort methods let you affect the consistent and done bits by executing a single statement. Both these methods set the done bit to True but they dif fer for value assigned to the consistent bit (True for SetComplete; False for SetAbort). The ContextUtil class exposes other members that let you affect the consistent and the done bit individually, as I explain later in this chapter.
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Note The transactional features of COM+ really shine when multiple databases are involved. Without a coordinator such as COM+ it is extremely difficult if not impossible to correctly implement a distributed transaction between different databases. COM+ transac tions use the two-phase transactional support from the Microsoft Distributed Transaction Coordinator (MS DTC), which adds significant overhead. If you are working with a single database and don t require other COM+ features such as object pooling and synchronization you might want to implement standard transactions by using the BeginTransaction method of the ADO.NET Connection object or by manually sending SQL commands to your database.
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31:
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