Portable Object Adapter (POA)

What is the Portable Object Adapter (POA)?

An object adapter is the mechanism that connects a request using an object reference with the proper code to service that request. The Portable Object Adapter, or POA, is a particular type of object adapter that is defined by the CORBA specification. The POA is designed to meet the following goals:

Allow programmers to construct object implementations that are portable between different ORB products.
Provide support for objects with persistent identities.
Provide support for transparent activation of objects.
Allow a single servant to support multiple object identities simultaneously.

This document presents an introduction to using the POA with the Java 2 Platform, Standard Edition. For a more complete description of the POA, see Chapter 11 of the CORBA 2.3.1 Specification.

Creating and Using the POA

The steps for creating and using a POA will vary according to the specific application being developed. The following steps generally occur during the POA life cycle:

Get the root POA
Define the POA's policies
Create the POA
Activate the POAManager
Activate the servants, which may include activating the Tie
Create the object reference from the POA

Each step is described in more detail in the sections that follow.

Step 1: Get the root POA

The first step is to get the first POA, which is called the rootPOA. The root POA is managed by the ORB and provided to the application using the ORB initialization interface under the initial object name "RootPOA".

An example of code that will get the root POA object and cast it to a POA is:

      ORB orb = ORB.init( args, null );
      POA rootPOA = POAHelper.narrow(orb.resolve_initial_references("RootPOA"));

Step 2: Define the POA's Policies

The Portable Object Adapter (POA) is designed to provide an object adapter that can be used with multiple ORB implementations with no rewriting needed to deal with different vendors' implementations.

The POA is also intended to allow persistent objects -- at least, from the client's perspective. That is, as far as the client is concerned, these objects are always alive, and maintain data values stored in them, even though physically, the server may have been restarted many times.

The POA allows the object implementer a lot more control over the object's identity, state, storage, and life cycle. You can create a POA without defining any policies and the default values will be used. The root POA has the following policies by default:

Thread Policy: ORB_CTRL_MODEL
Lifespan Policy: TRANSIENT
Object Id Uniqueness Policy: UNIQUE_ID
Id Assignment Policy: SYSTEM_ID
Servant Retention Policy: RETAIN
Request Processing Policy: USE_ACTIVE_OBJECT_MAP_ONLY
Implicit Activation Policy: IMPLICIT_ACTIVATION

The following code snippet shows how policies are set in the RMI-IIOP (with POA) example:

      Policy[] tpolicy = new Policy[3];
      tpolicy[0] = rootPOA.create_lifespan_policy(
        LifespanPolicyValue.TRANSIENT );
      tpolicy[1] = rootPOA.create_request_processing_policy(
        RequestProcessingPolicyValue.USE_ACTIVE_OBJECT_MAP_ONLY );
      tpolicy[2] = rootPOA.create_servant_retention_policy(
        ServantRetentionPolicyValue.RETAIN);

Each policy is discussed briefly in the following topics. For more information on POA policies, refer to Chapter 11, Portable Object Adapter of the CORBA/IIOP 2.3.1 Specification.

Thread Policy

This policy specifies the threading model used with the created POA. The default is ORB_CTRL_MODEL.

The ThreadPolicyValue can have the following values:

ORB_CTRL_MODEL - The ORB is responsible for assigning requests for an ORB-controlled POA to threads.
SINGLE_THREAD_MODEL - Requests for a single-threaded POA are processed sequentially. (NOTE: This policy is not supported in the ORB shipped with Sun's J2SE v.1.4.1 or higher).

Lifespan Policy

This policy specifies the lifespan of the objects implemented in the created POA. The default is TRANSIENT.

The LifespanPolicyValue can have the following values:

TRANSIENT - The objects implemented in the POA cannot outlive the POA instance in which they are first created.
PERSISTENT - The objects implemented in the POA can outlive the process in which they are first created.

Object Id Uniqueness Policy

This policy specifies whether the servants activated in the created POA must have unique object identities. The default is UNIQUE_ID.

The IdUniquenessPolicyValue can have the following values:

UNIQUE_ID - Servants activated with that POA support exactly one Object Id.
MULTIPLE_ID - A servant activated with that POA may support one or more Object Ids.

Id Assignment Policy

This policy specifies whether Object Ids in the created POA are generated by the application or by the ORB. The default is SYSTEM_ID.

The IdAssignmentPolicyValue can have the following values:

USER_ID - Objects created with that POA are assigned Object Ids only by the application.
SYSTEM_ID - Objects created with that POA are assigned a unique object id by the POA. If the POA also has the PERSISTENT policy, assigned Object Ids must be unique across all instantiations of the same POA.

Servant Retention Policy

This policy specifies whether the created POA retains active servants in an Active Object Map. The default is RETAIN.

The ServantRetentionPolicyValue can have the following values.

RETAIN - to indicate that the POA will retain active servants in its Active Object Map.
NON_RETAIN - to indicate Servants are not retained by the POA.

Request Processing Policy

This policy specifies how requests are processed by the created POA. The default is USE_ACTIVE_OBJECT_MAP_ONLY.

The RequestProcessingPolicyValue can have the following values:

USE_ACTIVE_OBJECT_MAP_ONLY - If the object ID is not found in the Active Object Map, an OBJECT_NOT_EXIST exception is returned to the client. The RETAIN policy is also required.
USE_DEFAULT_SERVANT - If the object ID is not found in the Active Object Map or the NON_RETAIN policy is present, and a default servant has been registered with the POA using the set_servant operation, the request is dispatched to the default servant.
USE_SERVANT_MANAGER - If the object ID is not found in the Active Object Map or the NON_RETAIN policy is present, and a servant manager has been registered with the POA using the set_servant_manager operation, the servant manager is given the opportunity to locate or activate a servant or raise an exception.

Implicit Activation Policy

This policy specifies whether implicit activation of servants is supported in the created POA. The default value is IMPLICIT_ACTIVATION.

The ImplicitActivationPolicyValue can have the following values:

IMPLICIT_ACTIVATION - Indicates implicit activation of servants. This requires SYSTEM_ID and RETAIN policies to be set.
NO_IMPLICIT_ACTIVATION - Indicates no implicit servant activation.

Step 3: Create the POA

Creating a new POA allows the application developer to declare specific policy choices for the new POA and to provide a different adapter activator and servant manager (these are callback objects used by the POA to activate POAs on demand and activate servants). Creating new POAs also allows the application developer to partition the name space of objects, as Object Ids are interpreted relative to a POA. Finally, by creating new POAs, the developer can independently control request processing for multiple sets of objects.

A POA is created as a child of an existing POA using the create_POA operation on the parent POA. To create a new POA, pass in the following information:

Name of the POA. The POA is given a name that must be unique with respect to all other POAs with the same parent. In the following example, the POA is named childPOA.
POA Manager. Specify the POA Manager to be associated with the new POA. If, as is shown in the following example, null is passed for this parameter, a new POA Manager will be created. The user can also choose to pass the POA Manager of another POA.
Policy List. Specify the policy list to be associated with the POA to control its behavior. In the following example, a persistent lifespan policy has already been defined for this POA.

The following code snippet shows how the POA is created in the Hello World: Persistent Server example.

// Create a POA by passing the Persistent Policy
POA persistentPOA = rootPOA.create_POA("childPOA", null, 
   persistentPolicy );

Step 4: Activate the POAManager

Each POA object has an associated POAManager object that controls the processing state of the POAs with which it is associated, such as whether requests to the POA are queued or discarded. The POAManager can also deactivate the POA. A POA Manager may be associated with one or more POA objects.

The POAManager can have the following states:

Holding - In this state, associated POAs will queue incoming requests.
Active - In this state, associated POAs will start processing requests.
Discarding - In this state, associated POAs will discard incoming requests.
Inactive - In this state, associated POAs will reject the requests that have not begun executing as well as any new requests.

The POAManagerOperations javadocs contain more information on these states.

POA Managers are not automatically activated when they are created. The following code snippet shows how the POAManager is activated in the Hello World: Persistent Server example. If the POA Manager is not activated in this way, all calls to the Servant will hang because, by default, the POA Manager is in the HOLD state.

            // Activate PersistentPOA's POAManager. Without this step,
            // all calls to Persistent Server will hang because POAManager
            // will be in the 'HOLD' state.
            persistentPOA.the_POAManager().activate( );

Step 5: Activate the servants

The following information is quoted from section 11.2.5 of the CORBA Specification.

At any point in time, a CORBA object may or may not be associated with an active servant.

If the POA has the RETAIN policy, the servant and its associated Object Id are entered into the Active Object Map of the appropriate POA. This type of activation can be accomplished in one of the following ways.

The server application itself explicitly activates individual objects (via the activate_object or activate_object_with_id operations).
The server application instructs the POA to activate objects on demand by having the POA invoke a user-supplied servant manager. The server application registers this servant manager with set_servant_manager.
Under some circumstances (when the IMPLICIT_ACTIVATION policy is also in effect and the language binding allows such an operation), the POA may implicitly activate an object when the server application attempts to obtain a reference for a servant that is not already active (that is, not associated with an Object Id).

If the USE_DEFAULT_SERVANT policy is also in effect, the server application instructs the POA to activate unknown objects by having the POA invoke a single servant no matter what the Object Id is. The server application registers this servant with set_servant.

If the POA has the NON_RETAIN policy, for every request, the POA may use either a default servant or a servant manager to locate an active servant. From the POA's point of view, the servant is active only for the duration of that one request. The POA does not enter the servant-object association into the Active Object Map.

When using RMI-IIOP technology, your implementations use delegation (known as the Tie model) to associate your implementation with the interface. When you create an instance of your implementation, you also need to create a Tie object to associate it with a CORBA interface. The following code snippet shows how to activate the Tie, if the POA policy is USE_ACTIVE_OBJECT_MAP_ONLY. This sample code is from the RMI-IIOP with POA example.

_HelloImpl_Tie tie = (_HelloImpl_Tie)Util.getTie( helloImpl );
String helloId = "hello";
byte[] id = helloId.getBytes();
tPOA.activate_object_with_id( id, tie );

The CORBA Specification discusses creating object references (section 11.2.4), activating objects (section 11.2.5), and processing requests (section 11.2.6) in more detail than is done in this document. Please refer to the CORBA 2.3.1 Specification for more information.

Step 6: Create the object reference

Object references are created in servers. Once created, they may be exported to clients. Object references encapsulate object identity information and information required by the ORB to identify and locate the server and the POA with which the object is associated. References are created in the following ways:

Explicitly activate a servant and associate it with an object reference.

The following example is from Hello World: Persistent Server. This example uses the servant_to_reference operation to map an activated servant to its corresponding object reference.

// Resolve Root Naming context and bind a name for the
// servant.
org.omg.CORBA.Object obj = orb.resolve_initial_references(
  "NameService" );
NamingContextExt rootContext = NamingContextExtHelper.narrow( obj );

NameComponent[] nc = rootContext.to_name( 
  "PersistentServerTutorial" );
rootContext.rebind( nc, persistentPOA.servant_to_reference( 
  servant ) );

Server application directly creates a reference.
The following example is from the RMI-IIOP with POA example. In this example, the following code directly creates a reference. In doing so, they bring the abstract object into existence, but do not associate it with an active servant.
```
// Publish the object reference using the same object id
// used to activate the Tie object.
Context initialNamingContext = new InitialContext();
initialNamingContext.rebind("HelloService", 
tPOA.create_reference_with_id(id, 
  tie._all_interfaces(tPOA,id)[0]) );
```
Server application causes a servant to implicitly activate itself.
The behavior can occur only if the POA has been created with the IMPLICIT_ACTIVATION policy, which is the default behavior.

Once an reference is created in the server, it can be made available to clients. For more information on creating object references and exporting to clients, refer to section 11.2.4 of the CORBA 2.3.1 Specification for more information.

Adapter Activators

For an example application that uses Adapter Activators, see Using Adapter Activators.

An adapter activator is optional. You would use an adapter activator if POAs need to be created during request processing. If all needed POAs are created when the application is executed, an adapter activator is not required.

An adapter activator supplies a POA with the ability to create child POAs on demand, as a side-effect of receiving a request that names the child POA (or one of its children), or when the find_POA method is called with an activate parameter value of TRUE. The ORB will invoke an operation on an adapter activator when a request is received for a child POA that does not currently exist. The adapter activator can then create the required POA on demand.

A request must be capable of conveying the Object Id of the target object as well as the identification of the POA that created the target object reference. When a client issues a request, the ORB first locates an appropriate server (perhaps starting one if needed) and then it locates the appropriate POA within that server.

If the POA does not exist in the server process, the application has the opportunity to re-create the required POA by using an adapter activator. An adapter activator is a user-implemented object that can be associated with a POA. It is invoked by the ORB when a request is received for a non-existent child POA. The adapter activator has the opportunity to create the required POA. If it does not, the client receives the ADAPTER_NONEXISTENT exception.

Once the ORB has located the appropriate POA, it delivers the request to that POA. The further processing of that request depends both upon the policies associated with that POA as well as the object's current state of activation.

For more information on Adapter Activators, refer to section 11.3.3 of the CORBA 2.3.1 Specification or the AdapterActivatorOperations API documentation.

Servant Managers

Servant Managers are optional. You would use a servant manager to allow the POA to activate servants on demand when a request for an inactive object is received. If your server loads all objects when it starts up, you do not need a servant manager.

A servant manager is a callback object that the application developer can associate with a POA. The ORB will invoke operations on servant managers to activate servants on demand, and to deactivate servants. Servant managers are responsible for managing the association of an object reference (as characterized by its Object Id value) with a particular servant, and for determining whether an object reference exists or not. Each servant manager type contains two operations, the first called to find and return a servant and the second to deactivate a servant. The operations differ according to the amount of information usable for their situation.

To use servant managers, the USE_SERVANT_MANAGER policy must be set. Once set, the type of servant manager used in a particular situation depends on other policies in the POA. The two types of servant managers are:

ServantActivator
When the POA has the RETAIN policy, it uses servant managers that are ServantActivators.

This type is typically used to activate persistent objects.
- For an example that uses Servant Activators, see Using Servant Activators.
ServantLocator
When the POA has the NON_RETAIN policy, it uses servant managers that are ServantLocators. Because the POA knows that the servant returned by this servant manager will be used only for a single request, it can supply extra information to the servant manager's operations and the servant manager's pair of operations may be able to cooperate to do something different than a ServantActivator. When the POA uses the ServantLocator interface, immediately after performing the operation invocation on the servant returned by preinvoke, the POA will invoke postinvoke on the servant manager, passing the ObjectId value and the Servant value as parameters (among others). This feature may be used to force every request for objects associated with a POA to be mediated by the servant manager.

This type is typically used to activate transient objects.
- For an example that uses Servant Locators see Using Servant Locators.

For more information on Servant Managers, refer to section 11.3.4 of the CORBA 2.3.1 Specification.

POA Q&A

Is `POAManager.activate()` required for a newly created POA?

POAManager.activate() is required for a newly created POA if a null is passed for the POAManager parameter to POA::createPOA . If null is passed, a new POAManager is created and associated with the created POA. In this case, POAManager.activate() is needed.

To control several POAs with the same POAManager, you would:

Create a POA or use the rootPOA
Obtain the POA's manager via POA::the_POAManager
Pass the POAManager to subsequent createPOA calls

There is no implicit relationship between the Root POA's POAManager and other POAs unless explicitly programmed by the programmer as shown above.

For more information, read section 11.3.2 of the CORBA specification, formal/99-10-07.

For more information

For more information about the Portable Object Adapter, read Chapter 11 of the CORBA/IIOP v.2.3.1 Specification from the Object Management Group's website.