# Servant Managers

In our previous example, we extended our simple client using synchronous method invocation to handle asynchronous requests: Asynchronous Method Invocation by using the reply handlers.

In applications which have many objects, activating all of them all the time would be unnecessary and also might require too much memory or too many database lookups. For such applications, the POA provides an option for the application to supply servant managers that can dynamically supply servants on a per-request basis.

A servant manager is a call-back object that the application registers with a POA. When the POA attempts to determine the servant associated with a particular request, it calls back the application's servant manager to obtain the servant. To be able to register a servant manager with the POA, the `RequestProcessingPolicyValue`, which controls the matching of requests to servants, is to be set to `USE_SERVANT_MANAGER`.

There are two types of servant managers depending on whether the POA retains the associations of objects to servants in its Active Object Map or not. This is determined by the value of the `ServantRetentionPolicy` set when the POA is created. If the value of this policy is set as RETAIN, the POA retains the associations, and if the policy value is set to be NON_RETAIN, the POA doesn't retain any associations between the object and the servant.

For a POA with the RETAIN value, the servant manager must activate the servant associated with the object. This would need the servant manager object to support the ServantActivator interface. In the case of a POA with the NON_RETAIN value set, our servant manager object should be able to locate the servant for the requested object and then invoke it.

In this example, let's use a servant locator to locate the servant associated with our Stock_Factory object when a request is invoked on this object.

### The Stock Factory Locator Implementation

Our implementation of the Stock_Factory_Locator_i will help us find the Quoter_Stock_Factory servant.

A servant Locator interface provides two operations: preinvoke and postinvoke. The preinvoke operation is invoked to obtain the servant to dispatch the request to. The servant returned by the preinvoke is used only for a single request. The postinvoke operation is later invoked to destroy the servant created by the preinvoke operation.

          #include "tao/corba.h"

          class Quoter_Stock_Factory_Locator_i : public POA_PortableServer::ServantLocator
          {
           public:
            Quoter_Stock_Factory_Locator_i (CORBA::ORB_ptr orb);

            // Preinvoke function
            virtual PortableServer::Servant preinvoke (const PortableServer::ObjectId &oid,
                                                       PortableServer::POA_ptr poa,
                                                       const char * operation,
                                                       void * & cookie)
              throw (CORBA::SystemException, PortableServer::ForwardRequest);

            // Postinvoke function
            virtual void postinvoke (const PortableServer::ObjectId & oid,
                                     PortableServer::POA_ptr poa,
                                     const char * operation,
                                     void * cookie,
                                     PortableServer::Servant servant)
              throw (CORBA::SystemException);

           private:
            CORBA::ORB_var orb_;
          };

In the implementation of the `preinvoke` operation, we check if the object ID is valid and then check for the servant we want to create and create and return the requested servant.

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         PortableServer::Servant
         Quoter_Stock_Factory_Locator_i::preinvoke (const PortableServer::ObjectId &oid,
                                                     PortableServer::POA_ptr poa,
                                                     const char * operation,
                                                     void * & cookie)
           throw (CORBA::SystemException, PortableServer::ForwardRequest)
         {

           try {

            // Get the ObjectID in string format
            CORBA::String_var oid_str =
              PortableServer::ObjectId_to_string (oid);

            // Check if the ObjectId is valid
            if (strcmp (oid_str.in (), "Quoter/Stock_Factory") != 0) {
              // Create the requested servant.
              PortableServer::Servant servant =
                new Quoter_Stock_Factory_i ();

             cookie = servant;

             return servant;
          }catch (const CORBA::BAD_PARAM &) {
            throw CORBA::OBJECT_NOT_EXIST ();
          }

        The implementation of the postinvoke operation is
          simple. We just destroy the servant which we created by the
          preinvoke operation. The Cookie IDL
          type which is a parameter in both these operations
          helps associate the invocation of preinvoke with
          its postinvoke operation.

          void
          Quoter_Stock_Factory_Locator_i::postinvoke (const PortableServer::ObjectId &oid,
                                                      PortableServer::POA_ptr poa,
                                                      const char * operation,
                                                      void * cookie,
                                                      PortableServer::Servant servant)
            throw (CORBA::SystemException)
          {

            // Delete the servant as it is no longer needed.
            PortableServer::Servant my_servant = (PortableServer::Servant) cookie;
            if (servant == my_servant)
              delete servant;
          }

         Server Implementation
        Our first steps would be to create a new POA from the RootPOA
          with the USE_SERVANT_MANAGER value for the
          RequestProcessingPolicy and NON_RETAIN
          for the ServantRetentionPolicy.

            CORBA::PolicyList policies (3);
            policies.length (3);

            // Assign the polices
            policies [0] =
              poa->create_id_assignment_policy (PortableServer::USER_ID);

            policies [1] =
              poa->create_request_processing_policy
                                    (PortableServer::USE_SERVANT_MANAGER);

            policies [2] =
              poa->create_servant_retention_policy (PortableServer::NON_RETAIN);

            // Create the POA with these policies
            PortableServer::POA_var child_poa =
              poa->create_POA ("childPOA",
                               poa_manager.in (),
                               policies);

            // Destroy the policy objects
            for (CORBA::ULong i = 0; i != policies.length (); ++i) {
              policies[i]->destroy ();
            }

         The policy values are assigned, the childPOA is
          created with these policies, and later these policy objects can be
          deleted, as a copy of these objects is made by the
          create_POA and we would not need these objects any more.

         Now that we have the POA which can support servant managers,
          the next step would be to create a servant for the servant
          locator object, activate it to obtain its reference, and set it as
          the servant manager with the childPOA.

           // Create a Stock_Factory_Locator servant
           Quoter_Stock_Factory_Locator_i servant_locator_i(orb.in ());

           // Need to activate a servant_manager object in the Root POA
           PortableServer::ServantLocator_var servant_locator =
             servant_locator_i._this ();

           // Set the SM with the childPOA
           child_poa->set_servant_manager (servant_locator.in ());

        Now that we have set the servant manager with the childPOA, the
          next step would be to create a reference with the user-created
          ID in the childPOA which uses the Quoter_Stock_Factory_Locator_i.
          The create_reference_with_id operation lets us
          create the required object without actually creating its servant.
          The application supplies the ObjectId which signifies the
          identity of the object in the application domain.

           // Get the Object Id
           PortableServer::ObjectId_var child_oid =
             PortableServer::string_to_ObjectId ("childFoo");

           //Create the Object without creating the servants
           CORBA::Object_var stock_factory =
             child_poa->create_reference_with_id (child_oid.in (),
                                                  "IDL:Quoter/Stock_Factory:1.0");

        After this, as before, let's put this object reference as an IOR
          string and print it out.

           // Put the object reference as an IOR string
           CORBA::String_var ior = orb->object_to_string (stock_factory.in ());

           // Print it out!
           std::cout << ior.in () << std::endl;

        Excercise
        Modify the server.cpp in
          the simple server to use servant managers and locators. Use
          these files to help complete the implementation.
          Stock_Factory_locator_i.h
          Stock_Factory_locator_i.cpp
          Makefile.

        Solution
        Look at the server.cpp file. It should not
    	be much different from yours.

        Testing
        A client which uses request handlers is provided:
          client.cpp. As before the
          following files are provided.
          Quoter.idl
          Stock_i.h
          Stock_i.cpp
          Stock_Factory_i.h
          Stock_Factory_i.cpp
          Handler_i.h and
          Handler_i.cpp.

        More Reading
        The
        The Henning and
          Vinoski
          CORBA book
          discusses POA policies in detail.  Likewise, the Schmidt and Vinoski
          columns
          in C++ Report also include several articles about the POA.  Finally,
          the TAO
          distribution includes
          examples that illustrate how to use the POA policies.

    Last modified: Tue Apr 24 17:50:17 CDT 2001