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Unlike a POA whose POAManager has been transitioned into the inactive state, which cannot be reactivated, a previously destroyed POA can be re-created in the same process This is because a POA is essentially a container for object-to-servant associations and normally does not encapsulate network resources as a POAManager can Destroying a POA therefore destroys object-to-servant associations without necessarily shutting down or invalidating communications resources used by the application POA destruction can cause problems for applications that have poorly configured POA hierarchies For example, if a parent POA has a ServantActivator that is an object registered with one of its child POAs, servant etherealization will be unable to complete correctly Because the child POA hosting the ServantActivator is destroyed before its parent, the parent becomes unable to use the ServantActivator to etherealize its servants POA implementations cannot detect this problem, so it is up to you to avoid creating this type of situation in your applications
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1114 Applying POA Policies
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The number of POAs in your application and the policies you choose for each one depend on several factors Some of them are as follows: The number of objects your application intends to support Expected rates and durations of requests The underlying persistent store, if any, required by your objects The level of resources and services supplied by the computer and operating system hosting your application Any non-CORBA software your application must wrap or otherwise interact with Some aspects of the distributed domain in which your application runs, especially if the ability to relocate objects into other servers in that domain is desired We have left some of these factors vague for now, but we discuss details concerning each of them in the following sections Note that we initially ignore the differences between persistent and transient CORBA objects because many POA issues do not depend on the value of the LifespanPolicy We focus on issues related to POAs for persistent and transient objects in Section 11145 11141 Multithreading Issues A fundamental choice you must make for your applications is whether they are singlethreaded or multithreaded This choice depends on several details, including the following: Whether the underlying operating system and C++ language run time provide adequate multithreading support The threading requirements of your ORB implementation The tools you have available for debugging multithreaded applications
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Your levels of expertise and experience in creating and maintaining multithreaded applications The capacity of any third-party libraries used in your application to work properly in a multithreaded environment If your operating system, C++ language run time, or ORB does not support applications running in multithreaded environments, you must choose to make your applications single-threaded Beware, however, that not all ORB implementations support both singleand multithreaded operation; some of them support only one or the other but not both Also, not all ORBs adequately support applications that simultaneously act as both client and server Such ORBs do not listen for incoming requests while the application is waiting for a response to a request it has made on another server You must consult with your ORB documentation to determine the level of support your ORB provides for singlethreaded and multithreaded applications The threading choice you make for the whole application determines the values of the ThreadPolicy that you can meaningfully apply to your POAs For example, making an application single-threaded disallows concurrent request processing even when a POA is created with the ORB_CTRL_MODEL value for its ThreadPolicy Even if multithreading support is available, you might still wish to use a POA with the SINGLE_THREAD_MODEL value for the ThreadPolicy If your servant implementations are based on third-party software that is not thread-safe and if you do not wish to implement code to serialize all calls to it, using the SINGLE_THREAD_MODEL guarantees that your servant invocations are serialized by the POA In general, we recommend using the ORB_CTRL_MODEL value for the ThreadPolicy As explained in Section 1147, this is the default if you do not specify a ThreadPolicy value at POA creation time Specify the SINGLE_THREAD_MODEL only if you know that your ORB does not support multithreading and you are not concerned with porting your application to another ORB that does, or if your servants are not designed to support concurrent invocations In 21 we explain the POA threading models in much more detail We also explore how your choice of whether your program is single- or multithreaded affects its throughput, performance, and scalability 11142 ObjectId Assignment A simple rule for deciding whether a POA should have the USER_ID or SYSTEM_ID value for the IdAssignmentPolicy is to use system-assigned object identifiers for transient objects and use user-assigned identifiers for persistent objects You typically use the USER_ID value for the IdAssignmentPolicy together with the PERSISTENT value for the LifespanPolicy because ObjectIds for persistent objects normally
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