Chapter 12 The Dynamic Skeleton Interface
The Dynamic Skeleton Interface (or DSI) allows applications to provide
implementations of the operations on CORBA objects without static
knowledge of the object's interface. It is the server-side equivalent
of the Dynamic Invocation Interface.
This chapter presents the Dynamic Skeleton Interface and explains how
to use it. A toy example use of the DSI can be found in the omniORB
distribution in the src/examples/dsi directory. For further
information refer to the Dynamic Skeleton Interface and C++ Mapping
sections of the CORBA 2.3 specification.
The DSI interface has changed in CORBA 2.3. omniORB 3 uses the new
mapping, but since the mapping depends on
PortableServer::Current, which is not yet implemented, not all
facilities are available. This chapter describes an approach to
building DSI servers which works with omniORB 3.
12.1 Overview
When an ORB receives an invocation request, the information includes
the object reference and the name of the operation. Typically this
information is used by the ORB to select a servant object and call
into the implementation of the operation (which knows how to unmarshal
the parameters etc.). The Dynamic Skeleton Interface however makes
this information directly available to the application---so that it
can implement the operation (or pass it on to another server) without
static knowledge of the interface. In fact it is not even necessary
for the server to always implement the same interface on any
particular object!
To provide an implementation for one or more objects an application
must sub-class PortableServer::DynamicImplementation and
override the method invoke(). An instance of this class is
registered with a POA and is assigned an object reference (see below).
When the ORB receives a request for that object the invoke() method
is called and will be passed a CORBA::ServerRequest object
which provides:
-
the operation name
- context strings
- access to the parameters
- a way to set the returned values
- a way to throw user-defined exceptions.
12.2 DSI Types
12.2.1 PortableServer::DynamicImplementation
This class must be sub-classed by the application to provide an
implementation for DSI objects. The method invoke() will be called
for each operation invocation.
namespace PortableServer {
...
class DynamicImplementation : public virtual ServantBase {
public:
virtual ~DynamicImplementation();
CORBA::Object_ptr _this();
// Must only be called from within invoke(). Caller must release
// the reference returned.
virtual void invoke(CORBA::ServerRequest_ptr request) = 0;
virtual char* _primary_interface(const ObjectId& oid, POA_ptr poa) = 0;
virtual CORBA::Boolean _is_a(const char* logical_type_id);
// The default implementation uses _primary_interface(),
// but may be overridden by subclasses.
};
...
};
12.2.2 ServerRequest
A ServerRequest object provides the interface between a dynamic
implementation and the ORB.
namespace CORBA {
...
class ServerRequest {
public:
virtual const char* operation() = 0;
virtual void arguments(NVList_ptr& parameters) = 0;
virtual Context_ptr ctx() = 0;
virtual void set_result(const Any& value) = 0;
virtual void set_exception(const Any& value) = 0;
protected:
inline ServerRequest() {}
virtual ~ServerRequest();
};
...
};
12.3 Creating Dynamic Implementations
The application must override the invoke() method of
DynamicImplementation to provide an implementation for DSI
objects. This method must behave as follows:
-
It may be called concurrently by multiple threads of execution,
and so must be thread-safe.
- It may not throw any exceptions. Both user-defined and system
exceptions are passed in a value of type Any via a call to
ServerRequest::set_exception().
- The operations on the ServerRequest object must be
carried out in the correct order, as described below.
12.3.1 Operations on the ServerRequest
operation() will return the name of the operation, and may be
called at any time. For attribute access the operation name is the IDL
name of the attribute, prefixed by _get_ or
_set_. If the operation name is not recognised a
CORBA::BAD_OPERATION exception should be passed back
through set_exception(). This will allow the ORB to then see if it
is one of the standard object operations.
Firstly arguments() must be called passing a
CORBA::NVList1 which must be initialised to contain
the type and mode of the parameters. The ORB consumes this value and
will release it when the operation is complete. At this point any
in/inout arguments will be unmarshalled, and when this
operation returns, their values will be in the NVList. The
application may set the value of inout/out arguments by
modifying this parameter list.
If the operation has user-context information, then ctx() must be
called after arguments() to retrieve it.
set_result() must then be called exactly once if the operation has
a non-void return value (unless an exception is thrown). The value
passed should be an Any allocated with new, and will be freed
by the ORB.
At any point in the above sequence set_exception() may be called
to set a user-defined exception or a system exception. If this happens
then no further operations should be invoked on the
ServerRequest object, and the invoke() method should return.
Within the invoke() method _this() may be called to obtain the
object reference. This method may not be used at any other time.
12.4 Registering Dynamic Objects
To use a DynamicImplementation servant, a CORBA object must be
created and associated with it, just as for any other servant. Dynamic
servants can also be created on demand by Servant Managers, just like
static servants.
This implementation of DynamicImplementation::invoke() is taken
from an example which can be found in the omniORB distribution. The
echoString() operation is declared in IDL as:
string echoString(in string mesg);
Here is the Dynamic Implementation Routine:
void
MyDynImpl::invoke(CORBA::ServerRequest_ptr request)
{
try {
if( strcmp(request->operation(), "echoString") )
throw CORBA::BAD_OPERATION(0, CORBA::COMPLETED_NO);
CORBA::NVList_ptr args;
orb->create_list(0, args);
CORBA::Any a;
a.replace(CORBA::_tc_string, 0);
args->add_value("", a, CORBA::ARG_IN);
request->arguments(args);
const char* mesg;
*(args->item(0)->value()) >>= mesg;
CORBA::Any* result = new CORBA::Any();
*result <<= CORBA::Any::from_string(mesg, 0);
request->set_result(*result);
}
catch(CORBA::SystemException& ex){
CORBA::Any a;
a <<= ex;
request->set_exception(a);
}
catch(...){
cout << "echo_dsiimpl: MyDynImpl::invoke - caught an unknown exception."
<< endl;
CORBA::Any a;
a <<= CORBA::UNKNOWN(0, CORBA::COMPLETED_NO);
request->set_exception(a);
}
}
- 1
- obtained by calling
CORBA::ORB::create_list()