Advanced Topics in .NET framework

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The TransactionIsolationLevel.Any setting is special and should be used only with com ponents that aren t the root of a transaction. When this setting is used, the component doesn t care for the isolation level and uses whatever isolation level has been set up by the component that created the transaction. The Any setting is often useful with transac tional components that might not be the root of a transaction namely, components marked with TransactionOption.Required rather than with TransactionOp tion.RequiresNew. (The latter components are always the root of a new transaction.) Let s see how you can use optional arguments of the Transaction attribute to change the isolation level and set the transaction timeout:
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<Transaction(TransactionOption.Required, _ Isolation:=TransactionIsolationLevel.ReadUncommitted, Timeout:=10) _ Public Class MoneyMover
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The Timeout value is in seconds and overrides the machine-level default value (60 sec onds) specified on the Options tab of the My Computer node Properties window in the Component Services MMC snap-in. You need to work with Windows XP or Windows Server 2003 to change the default isolation level and timeout. The Transaction attribute, like most of the attributes that interact with the COM+ catalog, is effective only if the COM+ application hasn t been created yet. Therefore, you must switch to the MMC snap-in and delete the BankMoneyMover application to force .NET to create a brand-new application with the transaction settings that you ve specified. Or you can change these values after installing the COM+ application from the Transactions tab of the component s Properties dialog box. (See the left portion of Figure 31-5.)
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Note The only attributes that are always read from the metadata in the component and that supersede the attributes in the COM+ catalog are: JustInTimeActivation, AutoComplete, and ObjectPooling, plus the SecurityRole attribute when used at the method level. The ObjectPool ing attribute in source code can enable or disable object pooling, but it always uses the pool size defined in the COM+ catalog.
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The isolation level applies to all the methods in the transactional component: if a method opens a database connection or interacts with another resource managed by the MS DTC, a transaction with the specified isolation level is automatically created (and your code must vote for its outcome, either by invoking the SetComplete or SetAbort method or implicitly by means of an AutoComplete attribute). It often happens that different methods in a component require different isolation levels, but unfortu nately you can t apply the Transaction attribute at the method level.
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31:
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Figure 31-5
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The Transaction and Activation tabs of the Properties window of a COM+ component
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For example, if one method updates a database and requires the RepeatableRead level, whereas another method performs a read operation for which a ReadCommitted level would be enough, the best you can do is use a Transaction attribute at the class level that specifies a RepeatableRead isolation level and accept the unnecessary overhead that results when the latter method is invoked. To avoid this unnecessary overhead, you must create two additional classes: one that performs all the write operations at the RepeatableRead level, and one that performs all read operations at the ReadCommitted level. The original component would have no Transaction attribute and would be responsible only for dispatching calls to one of the two components, depending on whether it s a write or a read operation. A component that behaves in this way is known as a facade component.
Enabling Object Pooling
Object pooling can help improve your COM+ applications in many ways. For example, if your objects take a relatively long time to initialize themselves, it makes sense not to release them when the first client has completed its job, so that they can serve requests from other clients. Object pooling is also useful to limit the number of objects that can be alive at any given moment, so that they don t take too much memory on the server. Another good occasion for adopting pooling is when you have a well-defined number of resources be they database connections, parallel or serial ports, or other peripheral devices and each resource is managed by a single instance of your component. As with most serviced component features, you can enable object pooling from the MMC snap-in, as shown in the right portion of Figure 31-5, or by means of an attribute in source code at the class level:
<Transaction(TransactionOption.Required), JustInTimeActivation(), _ ObjectPooling(MinPoolSize:=2, MaxPoolSize:=10, CreationTimeout:=10000)> _ Public Class MoneyMover
Part VII:
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