POA Policies in Software

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114 POA Policies
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A key feature of the POA specification is that an application can contain multiple POA instances Each POA instance represents a grouping of objects that have similar characteristics These characteristics are controlled via POA policies that are specified when a POA is created All server applications have at least one POA, the Root POA, which has a standard set of policies
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Policies are objects that you use to define the characteristics of a POA and the objects created within it Like the POA and POAManager interfaces, the CORBA specification defines the POA policy interfaces in the standard PortableServer module As with all policy interfaces, POA policy types derive from the CORBA::Policy interface
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module CORBA { typedef unsigned long PolicyType; interface Policy { readonly attribute PolicyType policy_type; Policy copy(); void destroy(); }; typedef sequence<Policy> PolicyList; //
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The Policy interface and its associated types provide basic management operations The policy_type read-only attribute allows you to determine the actual derived type of a policy through the base Policy interface PolicyType is a tag value that is controlled by the OMG to ensure that all standard policy types have unique tags The copy operation allows you to clone a Policy object The returned reference refers to a completely new copy of the target Policy object The destroy operation allows you to destroy the target Policy object PolicyList allows you to group references to various derived Policy objects to form sets of policies The POA creation operation accepts an argument of type PolicyList that allows policies to be set for the new POA We cover POA creation in detail in Section 115 Policy objects are ocality-constrained objects This means that even though they look and act just like regular objects, any attempt to pass their references as arguments to normal CORBA operations or to convert them to strings via ORB::object_to_string will result in a CORBA::MARSHAL exception Such objects can be accessed only in the context of the local ORB under which they were created Some objects are locality-constrained because they supply access to fundamental services such as the ORB or the POA, whereas others are locality-constrained because allowing access to them from remote processes provides no benefit For example, allowing a process to register a local servant in a remote POA makes no sense because servants are not CORBA objects A number of POA-related interfaces, including the POA itself, are locality-constrained
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As we show in the next few sections, all POA policies have the same form: their values are specified using an enumerated type, and all the policy interfaces have a read-only attribute of that enumerated type that can be used to get the policy value 1141 CORBA Object Life Span One feature of CORBA that sets it apart from other distributed application development platforms is that it provides transparent and automatic activation of objects If a client application issues a request to a target object that is currently not running or not activated, the ORB implementation activates a server process for the object if necessary and then activates the object itself Any activation of server processes and target objects is transparent to the requesting client (See 14 for details concerning this transparent object location and activation process) CORBA objects that can live beyond any particular process in which they are created or activated are called persistent objects These objects are so named because they persist across the lifetimes of multiple server processes Despite the utility of persistent objects, application developers using CORBA before the adoption of the POA discovered that they also required another type of object that had a shorter lifetime Specifically, they found it valuable to use proprietary extensions provided by several ORB vendors to create objects whose lifetimes were bounded by that of the process or even the object adapter in which they were created For example, one application might send a reference to one of its objects to another application with the intent of having the second application eventually call it back However, if the first application exits, it may no longer want the callback information In that case, it does not want the callback to be delivered, and it does not want the ORB to reactivate the callback object As we explain in 9, the POA supports two types of CORBA objects: the persistent object originally specified by CORBA, and a new shorter-lived object called a transient object The lifetime of a transient object is bounded by the lifetime of the POA in which it is created Thus, transient objects are useful in situations requiring temporary objects, such as the callback scenario just described One additional benefit of transient objects is that they require less book-keeping by the ORB After you deactivate the POA used to create a transient object, the object cannot be reactivated This means that the ORB does not need to keep track of how to locate the object if it is not active when a request is made on it, nor how to activate it within a new server process This in turn typically means less overhead in administering the CORBA application itself A single POA must support either persistent objects or transient objects; it cannot support both If an object is created using a POA that supports persistent objects, that object will be persistent; otherwise, it will be transient To support both transient and persistent objects in a single server, the server must have at least two POAs: one for each kind of object One reason for this, as we explain in 14, is that persistent objects
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