event_types.idl

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// Event_Types.idl
// Simple demonstration of typed events in a distributed system.
//
// Author:
//   Torsten Kuepper
//

// Event_Types.idl,v 1.3 2000/04/27 22:54:25 nanbor Exp

// Event inheritance hierarchy  ===========================

// Base class  --------------------------------------------

valuetype Event
{
  void do_print ();

  // An operation. In some implementations (e.g. operator terminal)
  // the event should visualize itself. That is of no use at the
  // event producing sensor. So, the declaration of do_print ()
  // could be deferred to the implementation classes, but then you need
  // to downcast from the pointer to the event valuetype to your
  // implementation. Another solution is perhaps to inherit do_print ()
  // through an additional abstract valuetype base only in that
  // IDL that a visualizing implementation sees. But this would change
  // the type and this is a bad thing. The cleanest thing to do may be
  // to apply the visitor pattern. Event::accept (visitor) would be
  // implemented as null-op in the measurement device, if you take this
  // example.


  public long time_;

  // A state member. Don't confuse with attributes, which are
  // ok here too, but they do only map to a pair of local operations,
  // in opposite to (public/private) state members they haven't got no
  // implementation for the state data and finally they are not transmitted
  // over the wire.


  public unsigned long origin_id_;

  // This id should identify the origin (e.g. sensor) in the system.
  // This makes an id-space beside the object references which has to be
  // maintained. It would be useful to implement some consistency check
  // protocol (as CORBA interfaces) to verify that the suppliers and
  // consumers are connected (through some event channel) in the
  // right way.

};


// Derived Events  ----------------------------------------


valuetype Temperature : Event
{
  // do_print () is overridden in the implementation. We can't
  // tell this in IDL, because operations can't be declared again.
  // They are implicit assumed to be polymorph.

  public float temperature_;
  // Extends Event with the state member for the temperature.
};


typedef float Point[3];
// (anonymous arrays are not yet working in this OBV ...%!)
// (( BTW %! <- no emoticon, this is my to do mark))

valuetype Position : Event
{
  attribute float x, y, z;
  // The Position can be accessed both through the coordinates ...

  public Point xyz;
  // ... or as a whole array, which is a state member.
};


valuetype Log_Msg : Event
{
  public short  urgency;
  public string message;
};

// (Valuetypes which hold other types as shown are not yet tested %!)

// You may extend the system with aggregated events, such the status
// message of a boiler, which has temperature and a pressure valuetype
// as state member (recall: unshared valuetypes are well at this time.
// But a shared valuetype splits at the receiving end of an invocation
// in two or more instances, dependend on the number of references on it
// (in the argument list plus in the members of compound types). This
// misbehaviour will go away once valuetype sharing is implemented %!
// But to do this in an efficient and thread safe manner seems a little tricky)


// Passing back the critical events in a list  ----------------------------

// This is the link, that is used internally -----
// (should come after Event_List, but forward decl. is not yet complete %!)

valuetype Event_List_Link
{
  Event get_event ();
  // get the event

  Event_List_Link get_next_link ();
  // get the event

  void attach_next_link (in Event_List_Link chain);
  // Link a chain at the end.

  private Event my_event;
  // event which is held

  private Event_List_Link next;
  // link to the next event container
};

// 'private' state member are mapped to 'protected' in C++, so
// they can be accessed from the implementation class, which should
// be derived from OBV_Event_chain.


// The event list uses links as declared above. But its implementation
// could be changed 'under the hood' to use e.g. a CORBA sequence.
// (This doesn't go yet, because valuetype is only allowed
// as an operation argument for now. Just impl. the visitors in tao_idl %!)

valuetype Event_List
{
  void store_event (in Event e);
  // Attach an event at the lists's end.

  public Event_List_Link first_link;
  // Should better be private, but then the iterator can't access it.
};


// Interface to access the "event server" ------------------

// A client (e.g. sensor) delivers the events via put_event ().
// The server checks against alarm conditions and memorizes
// critical events, which can be passed back
// to a client (e.g. operator terminal) with get_critical_events ().

interface Checkpoint
{
  void  put_event (in Event e);
  // Put event in the server. If it exceeds an alarm criterion
  // it will be stored.

  Event_List  get_critical_events ();
  // Ask for a list of critical events.

  oneway void shutdown ();
  // This operation will shutdown the server.
};


// Checkpoint server side --------------------------------------------
// The Checkpoint should compare the incoming event against a
// criterion for the specific event type. My approach is the following
// (to facilitate separation of application logic and event specific
// code): An abstract valuetype Criterion provides is_critical () to check
// against a boundary. Concrete alarm boundaries for any existing
// event derive from this class and perform the check. Thats's it for
// the event type maintainer --- the customs that use this 'framework'.

// The concrete criterions inherit from Event too. I wanted to reuse
// the list which works on events. The wrapper Criterion_List makes it safe
// that only criterions are accepted to this list. Templates would be fine,
// but currently I have no idea how to apply them to a valuetype. Perhaps
// there is no way to get around custom marshalling [n.y.avail.%!] in the
// area of containers.
// Finally the concrete criterions must have a suitable implementation for
// is_critical ().

// Now the internals of the server which shouldn't need to be touched by
// the final implementer: The above mentioned wrapper Criterion_List
// uses an Event_List to compare an incoming event against the
// boundaries. In this simple example it will just apply the event to
// is_critical () of any criterion, which origin id matches.
// The criterion checks with
// valuetype's _downcast () if the event matches its event type and then
// performs the alarm check. A real world approach with many event types
// and criterions could better use a hash map for the criterions. The
// external map index would be the repository id of the event.

abstract valuetype Criterion
{
  boolean is_critical (in Event e);
  // Check against alarm boundaries.
};


// The specialized criterions. Note: A valuetype can only inherit
// from one non-abstract other valuetype (which then must be the first
// one listed). Further Criterions may only be abstract valuetypes
// without the ability to contain state members. (The support of
// a CORBA interface is not yet supported.)
// P.S. Please don't bother about the class hierarchy
// (Criterion inherits from Event _and_ has some Events as boundary values
// aggregated). I just wanted to reuse the code for the list of events.
// Certainly not an example of good OO design.


valuetype Temperature_Criterion : Event, Criterion
{
  private Temperature meltingpoint;
  // The boundary is stored in a state member.
};


valuetype Position_Criterion : Event, Criterion
{
  private Position leftbottom, topright;
  // Any position should be contained in a box.
};


valuetype Log_Msg_Criterion : Event, Criterion
{
  // No state member. All Log_Msg which have urgency
  // greater zero meet the criterion.
};



// The Criterion_List  ===========================================

valuetype Criterion_List
{
  void store_criterion (in Criterion c);
  // Attach an criterion at the lists's end.

  boolean is_critical (in Event e);
  // Check with the listmembers if e should raise an alarm.

  public Event_List my_list;
  // Used in the implementation. Is public for allowing
  // access to the iterator.
};