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This section describes the architecture of the abstract model implied by the POA, and the interactions between various components.
The ORB is an abstraction visible to both the client and server. The POA is an object visible to the server. User-supplied
implementations are registered with the POA (this statement is a simplification; more detail is provided below). Clients hold
references upon which they can make requests. The ORB, POA, and implementation all cooperate to determine which servant the
operation should be invoked on, and to perform the invocation.
shows the detail of the relationship between the POA and the implementation.
Ultimately, a POA deals with an Object Id and an active servant. By active servant, we mean a programming object that exists
in memory and has been presented to the POA with one or more associated object identities. There are several ways for this
association to be made.
Figure 11-1 Abstract POA Model
If the POA supports the RETAIN policy, it maintains a map, labeled Active Object Map, that associates Object Ids with active
servants, each association constituting an active object. If the POA has the USE_DEFAULT_SERVANT policy, a default servant
may be registered with the POA. Alternatively, if the POA has the USE_SERVANT_MANAGER policy, a user-written servant manager
may be registered with the POA. If the Active Object Map is not used, or a request arrives for an
Object Reference
Client
object not present in the Active Object Map, the POA either uses the default servant to perform the request or it invokes
the servant manager to obtain a servant to perform the request. If the RETAIN policy is used, the servant returned by a servant
manager is retained in the Active Object Map. Otherwise, the servant is used only to process the one request.
In this specification, the term active is applied equally to servants, Object Ids, and objects. An object is active in a POA
if the POA’s Active Object Map contains an entry that associates an Object Id with an existing servant. When this specification
refers to active Object Ids and active servants, it means that the Object Id value or servant in question is part of an entry
in the Active Object Map. An Object Id can appear in a POA's Active Object Map only once.
A c t i v a t o r
Servant pointer
Figure 11-2 POA Architecture