TAO Trading Service Documentation

The TAO transient Trading Service implements the COS TradingObject Service specification, and  conforms to the Linked Trader conformance criteria. This document details how to use the TAO Trading Service from the following perspectives:

• as an importer bootstrapping to the Trading Service;
• as a service offer exporter;
• as an administrator;
• as an out-of-the-box server process;
• as a collocated object.

In addition, it covers running the Trading Service tests and discusses known bugs and workarounds.

This document assumes you are familiar with Trading Service concepts, such as "importer," "exporter", "service type", "service offer," and "dynamic property", as well as the roles of each of the Trading Service's interfaces --- Lookup, Register, Admin, and Link (the TAO implementation doesn't currently support Proxy). I recommend reading the first two sections of the Trading Service specification. This document has the following layout:

1. The Client Role
   • Bootstrapping to the Trading Service
   • The Importer Role --- Performing a Query
     • Constraints
     • Preferences
     • Policies
     • Filtering Properties
     • Offer Iterators
     • Property Evaluation
   • The Exporter Role --- Registering a Service Type and Offer
     • The Service Type Repository
     • Exporting, Withdrawing, and Modifying Service Offers
     • Implementing Dynamic Properties
   • The Administrator Role --- Tweaking Policies and Adjusting Links
2. The Server Role
   • The TAO Trading_Service Application
   • Colocating the Trading Service in a TAO Application
3. Running the Trading Service Tests
4. Known Bugs and Workarounds
5. Future Work

The Client Role

There are three categories of operations that a client can perform on a Trading Service instance: exporting a service offer to the Trading Service, importing a list of Service Offers whose properties satisfy a constraint expression, and attending to administrative duties --- tweaking policies or adjusting links. The first order of business, of course, is obtaining a reference to a Trading Service instance, assuming that instance is not colocated with the client.

Bootstrapping to the Trading Service

Like with the Naming Service, the ORB will obtain a reference to a Trading Service instance's Lookup interface when a client invokes the CORBA::ORB::resolve_initial_references method and passes to it the ObjectID "TradingService". The following TAO code bootstraps to the Trading Service:

{
  TAO_ORB_Manager orb_manager;
  orb_manager.init (argc, argv, );
  CORBA::ORB_var orb = orb_manager.orb ();
  CORBA::Object_var trading_obj =
    orb->resolve_initial_references ("TradingService");
  CosTrading::Lookup_var lookup_if =
    CosTrading::Lookup::_narrow (trading_obj.in ());
}
catch (const CORBA::Exception& ex)
{
  ex.print_exception ("Failed to bootstrap to a trader");
}

The first time resolve_initial_references is called, the ORB uses a multicast protocol to locate an existing trader. The ORB emits a multicast packet containing a field identifying the desired service --- Naming or Trading --- and the port number that the client is listening on for the response (the IP address can be inferred from the packet). When the trader receives the packet and finds that the id contained within matches its own, it opens a socket to the client on the designated port, and sends its IOR, which the ORB converts to an object reference that it caches.

If the trader IOR is known ahead of time, the string can be passed to the client in the environment variable TradingService, or by the command line option -ORBtradingserviceior <IOR>. Likewise, if the multicast port is known ahead of time and differs from the default port, the port number can be passed to the client in the environment variable TradingServicePort, or by the command line option -ORBtradingserviceport <PORTNUM>.  

The Importer Role --- Performing a Query

Once the importer has obtained a reference to a trader's Lookup interface, it next needs to fire up a query. The query method takes nine parameters (aside from the CORBA::Environment):

Constraints

A constraint is a string in the OMG standard constraint language (the BNF can be found at the end of the specification). The trader iterates over applicable offers, and for each offer determines if its properties meet the constraints, replacing property names in the string with their values and computing the result. If the constraint evaluates to true, the offer is placed in the pool of matched offers. If the constraint string is syntactically invalid, contains property names not found in the service type description for the listed service type, or has operators with mismatched operand types, the query method will throw an InvalidConstraint exception.

Operands can be of two types: property names or literals. A property name is an unquoted string of alphanumeric characters and underscores that begins with a letter. The service type describes the type of a property. A literal is an signed or unsigned integer, floating point number --- scientific notation acceptable ---, single-quoted string, or boolean --- written TRUE or FALSE.

The constraint language supports the following operations:

Preferences

A preference is a constraint language string that determines the order of offers in the returned offer sequence. There are five types of preferences:

Policies

The following import policies are descibed in the specification and supported by the TAO Trading Service:

The TAO Trading Service comes with a handy utility --- TAO_Policy_Manager --- for creating a policy sequence to pass to the query method that won't incur any exceptions.   Use the TAO_Policy_Manager in the following way:

TAO_Policy_Manager policies;
policies.exact_type_match (CORBA::B_FALSE);
policies.search_card (16*NUM_OFFERS);
policies.match_card (16*NUM_OFFERS);
policies.return_card (16*NUM_OFFERS);
policies.link_follow_rule (CosTrading::local_only);
const CosTrading::PolicySeq& policy_seq = policies.policy_seq ();

Filtering Properties

If the client wants only a subset of the properties defined for a service type returned in matching offers, it can specify those property names in the desired_properties parameter of the query method. Pass the prop_names method of CosTrading::Lookup::SpecifiedProperties a CosTrading::PropNameSeq:

char* props[] = {"Name", "Description", "Location", "Host_Name" };
CosTrading::Lookup::SpecifiedProps desired_props;
CosTrading::PropertyNameSeq prop_name_seq (4, 4, props, CORBA::B_FALSE);
desired_props.prop_names (prop_name_seq);

Offer Iterators

Those offers returned from the query in excess of how_many are placed in an offer iterator for deferred retrieval. The CosTrading::OfferIterator::next_n method will allocate a sequence and fill it with either n offers, or if it has fewer than n offers, the remaining offers. The next_n method returns true if the iterator contains more offers, and false if it's been depleted. After finishing with the iterator, invoke its destroy method to release any server-side resources.

The following code is an example of obtaining offers from a CosTrading::OfferIterator:

CORBA::Boolean any_left = CORBA::B_FALSE;
CORBA::Environment _env;

do
  {
    CosTrading::OfferSeq_ptr iter_offers_ptr;
    CosTrading::OfferSeq_out iter_offers_out (iter_offers_ptr);

    any_left = offer_iterator->next_n (length,
                        	       iter_offers_out,
                                       _env);
    ACE_CHECK_ENV_RETURN (_env, 0);

    CosTrading::OfferSeq_var iter_offers (iter_offers_ptr);
    // Process offers...

  } while (any_left);

Property Evaluation

After the client completes a query that used dynamic properties, to review the property values of the returned offers, it has to distinguish between Anys containing static properties and Anys containing dynamic property structures. The TAO_Property_Evaluator class is a handy utility to obtain property values that hides how it evalutes properties for the client --- by simple Any value extraction for static properties, or by calling back to a dynamic property interface. The TAO_Property_Evaluator caches the value of a dynamic property, and frees the allocated Anys during its destruction.

The following code demonstrates how to use the TAO_Property_Evaluator to dump the properties of an offer to the screen.

TAO_Property_Evaluator prop_eval (prop_seq);
for (int length = prop_seq.length (), k = 0; k < length; k++)
  {
    ACE_DEBUG ((LM_DEBUG, "%-15s: ", prop_seq[k].name.in ()));
    ACE_TRY
      {
        CORBA::Boolean is_dynamic = prop_eval.is_dynamic_property (k);
        ACE_CHECK_ENV;

	value = prop_eval.property_value(k, env);
    	ACE_CHECK_ENV;

	if (value != 0)
	  CORBA::Any::dump (*value);
      }
    ACE_CATCHANY
      {
        ACE_DEBUG ((LM_DEBUG, "Error retrieving property value.\n"));
      }
    ACE_ENDTRY;
  }

The Exporter Role --- Registering a Service Type and Offer

Before an exporting client can register a new service offer with the Trading Service, it needs to ensure first that its service type is present in the service type repository of the target trader. The most efficient way to do this is to first invoke the export method on the Register interface, and if it raises an UnknownServiceType exception, obtain a reference to the Repository, add the Service Type, and attempt the export a second time. Here's the boilerplate code:

CORBA::Object_var trading_obj =
  orb_ptr->resolve_initial_references ("TradingService");
CosTrading::Lookup_var lookup_if =
  CosTrading::Lookup::_narrow (trading_obj.in ());
CosTrading::Register_var register_if = lookup_if->register_if ();
CosTrading::TypeRepository_ptr obj = this->trader_->type_repos ();
CosTradingRepos::ServiceTypeRepository_var str =
  CosTradingRepos::ServiceTypeRepository::_narrow (obj);

try
  {
    // Attempt to export the offer.
    offer_id = register_id->export (object_ref, type, props);
  }
catch (const CosTrading::UnknownServiceType&, excp)
  {
    // If the ServiceTypeName wasn't found, we'll have to add the
    // type to the Service Type repository ourselves.
    str->add_type (type,
		   object_ref->_interface_repository_id (),
		   prop_struct_seq,
		   super_type_name_seq);

    // Now we'll try again to register the offer.
    offer_id = reg->export (object_ref, type, this->tprops_);
  }
catch (...)
  {
    // Sigh, all our efforts were for naught.
    throw ();
  }
    

The Service Type Repository

Creating a service type description is simply a matter of filling in two sequences: a CosTradingRepos::ServiceTypeRepository::PropStructSeq and a CosTradingRepos::ServiceTypeRepository::ServiceTypeNameSeq. When filling in the value_type field, remember to up the reference count of the TypeCode, since otherwise the TypeCode_var will sieze control of the memory and free it. Here's a code excerpt taken from export_test showing how to build the first couple elements of such sequences:

this->type_structs_[TT_Info::PLOTTER].props.length (2);
this->type_structs_[TT_Info::PLOTTER].super_types.length (1);
this->type_structs_[TT_Info::PLOTTER].super_types[0] =
TT_Info::INTERFACE_NAMES[TT_Info::REMOTE_IO];
this->type_structs_[TT_Info::PLOTTER].props[0].name =
TT_Info::PLOTTER_PROPERTY_NAMES[TT_Info::PLOTTER_NUM_COLORS];
this->type_structs_[TT_Info::PLOTTER].props[0].value_type =
CORBA::TypeCode::_duplicate (CORBA::_tc_long);
this->type_structs_[TT_Info::PLOTTER].props[0].mode =
CosTradingRepos::ServiceTypeRepository::PROP_NORMAL;
this->type_structs_[TT_Info::PLOTTER].props[1].name =
TT_Info::PLOTTER_PROPERTY_NAMES[TT_Info::PLOTTER_AUTO_LOADING];
this->type_structs_[TT_Info::PLOTTER].props[1].value_type =
CORBA::TypeCode::_duplicate (CORBA::_tc_boolean);
this->type_structs_[TT_Info::PLOTTER].props[1].mode =
CosTradingRepos::ServiceTypeRepository::PROP_READONLY;

Exporting, Withdrawing, and Modifying Service Offers

Like with adding a Service Type, exporting an offer is just filling in the sequences. For offers, of course, property values are passed, so this involves employing the Any insertion operators. Here's a code exerpt from export_test:

CosTrading::PropertySeq prop_seq (2);
prop_seq[0].name =
  TT_Info::PLOTTER_PROPERTY_NAMES[TT_Info::PLOTTER_NUM_COLORS];
prop_seq[0].value <<= ACE_static_cast (CORBA::Long, 256);
prop_seq[1].name =
  TT_Info::PLOTTER_PROPERTY_NAMES[TT_Info::PLOTTER_AUTO_LOADING];
prop_seq[1].value <<= CORBA::Any::from_boolean (CORBA::B_TRUE);

The export_test returns a CosTrading::OfferId string, which is required to perform the withdraw and modify operations on the exported offer. withdraw requires that you simply pass the OfferId of the offer to be withdrawn, while modify takes two additional sequences: a CosTrading::PropertyNameSeq of property names to be removed from the offer, and a CosTrading::PropertySeq of offers to be added or changed in the offer.

Implementing Dynamic Properties

To export an offer with a dynamic property:

• inherit from the TAO_Dynamic_Property class and implement its DP_Eval method;
• create a CosTradingDynamic::DynamicProperty structure using the TAO_Dynamic_Property::construct_dynamic_prop method;
• insert the CosTradingDynamic::DynamicProperty in the value field of the property.

The following code, taken from the export_test example, illustrates this:

// Step 1: Write the Dynamic Property callback handler.
class Simple_DP : public TAO_Dynamic_Property
{
public:

  virtual CORBA::Any* evalDP (const char* name,
                              CORBA::TypeCode_ptr returned_type,
                              const CORBA::Any& extra_info,
			      CORBA::Environment& _env)
    ACE_THROW_SPEC ((CosTradingDynamic::DPEvalFailure));
};

CORBA::Any*
Simple_DP::evalDP (const char* name,
                   CORBA::TypeCode_ptr returned_type,
                   const CORBA::Any& extra_info,
                   CORBA::Environment& _env)
  ACE_THROW_SPEC ((CosTradingDynamic::DPEvalFailure))
{
  CORBA::Any* return_value = 0;
  ACE_NEW_RETURN (return_value, CORBA::Any, 0);

  (*return_value) <<= ACE_static_cast (CORBA::ULong, ACE_OS::rand ());
  return return_value;
}

// Step 2: Create the Dynamic Property
Simple_DP dp;
CORBA::Any extra_info;
CosTrading::PropertySeq prop_seq (1);
CosTrading::DynamicProp* dp_struct =
  dp.construct_dynamic_prop ("prop_name",
                             CORBA::_tc_ulong,
			     extra_info);

// Step 3: Turn over the dynamic property to the propery value Any.
CORBA::Environment env;
prop_seq[0].name = "prop_name";
prop_seq[0].value.replace (CosTrading::_tc_DynamicProp,
                           dp_struct,
                           CORBA::B_TRUE,
                           env);
ACE_CHECK_ENV_RETURN (env, -1);

The Administrator Role --- Tweaking Policies and Adjusting Links

The trader can be configured remotely through two interfaces: the Admin interface, for tweaking global policies, enabling and disabling interfaces, and dumping the trader contents; and the Link interface, for attaching to and detaching from other traders.

Adjusting policies is straightforward. Here's an example of setting the max_search_card policy:

// lookup_if returned from resolve_initial_references.
CosTrading::Admin_var admin_if =
  lookup_if->admin_if ();

admin_if->set_max_match_card (200);

Here's an example of using the list_offers method on the Admin interface to remove all offers from the Trader:

{
CosTrading::OfferIdIterator_ptr offer_id_iter;
CosTrading::OfferIdSeq_ptr offer_id_seq;

// lookup_if returned from resolve_initial_references.
CosTrading::Admin_var admin_if =
  lookup_if->admin_if ();

CosTrading::Register_var register_if =
  lookup_if->register_if ();

admin_if->list_offers (10,
                       CosTrading::OfferIdSeq_out (offer_id_seq),
                       CosTrading::OfferIdIterator_out (offer_id_iter));

if (offer_id_seq != 0)
 {
   CosTrading::OfferIdSeq_var offer_id_seq_var (offer_id_seq);
   for (CORBA::ULong i = 0; i < offer_id_seq_var.length (); i++)
     {
       register_if->withdraw (offer_id_seq_var[i]);
     }
 }

if (offer_id_iter != CosTrading::OfferIdIterator::_nil ())
 {
   CORBA::Boolean any_left = CORBA::B_FALSE;
   CosTrading::OfferIdSeq_ptr id_seq = 0;
   CosTrading::OfferIdIterator_var offer_id_iter_var (offer_id_iter);

   do
     {
       any_left =
         offer_id_iter->next_n (length,
                                CosTrading::OfferIdSeq_out (id_seq));

       CORBA::ULong offers = id_seq->length ();
       for (CORBA::ULong i = 0; i < offers; i++)
        {
          register_if->withdraw (id_seq[i]);
        }

       delete id_seq;
     }
   while (any_left);

   offer_id_iter->destroy ();
 }
}
catch (...)
{
  // Handle Errors.
}

Here's an example a trader linking itself to another trader (this->trader_ is a colocated trader --- see the next section for more information):

 {
   CosTrading::Link_var link_if =  lookup_if->link_if ();

   TAO_Trading_Components_Impl& trd_comp =
     this->trader_->trading_components ();
   CosTrading::Lookup_ptr our_lookup = trd_comp.lookup_if ();
   CosTrading::Link_ptr our_link = trd_comp.link_if ();

   link_if->add_link (this->name_.in (),
                      our_lookup,
                      CosTrading::always,
                      CosTrading::always);

   our_link->add_link ("Bootstrap_Trader",
                       lookup_if.in (),
                       CosTrading::always,
                       CosTrading::always);
 }
catch (...)
{
  // Handle Errors.
}

The Server Role

The TAO Trading Service comes with an out-of-the-box executable suitable for common use. However, it can also easily be colocated with any other TAO server to add Trading Service functionality to that server.

The TAO Trading Service Application

This out-of-the-box server takes a number of command line arguments:

By default the trader will listen for multicast resolve_initial_references requests, and respond with the IOR of its Lookup inteface. For the purposes of testing federated queries, when passed the -TSfederate method, instead of becoming a bootstrappable server, the Trading_Service application will bootstrap itself to a multicast trader, link itself to that trader and every other trader accessible through that trader. This protocol will have all traders on the multicast network form a complete graph.

Colocating the Trading Service in a TAO Application

Colocating the Trading Service in a TAO application amounts to constructing a TAO_TRADER object using the TAO_Trader_Factory::construct_trader call. The argc and argv parameters to construct_trader contain the configuration parameters described in the previous section. The trader is also configurable programatically through its attribute classes. The follow code exerpt demonstrates this.

In addition the application will need to create a service type repository implementation --- TAO's being the TAO_Service_Type_Repository --- and configure the trader with it. The service type repository is separate from the trader in this way to allow, for example, multiple traders to share the same service type repository. The following code exerpt also demontrates configuring the repository:

TAO_TRADER* trader = TAO_Trader_Factory::create_trader (argc, argv);
TAO_Support_Attributes_Impl& sup_attr = trader->support_attributes ();
TAO_Import_Attributes_Impl& imp_attr = trader->trading_components ();

// Configure the trader with a service type repository.
CORBA::Environment _env;
TAO_Service_Type_Repository type_repos* type_repos = 0;
ACE_NEW (type_repos, TAO_Service_Type_Repository);
sup_attr.type_repos (type_repos->_this (_env));
ACE_CHECK_ENV_RETURN (_env, -1);

// Configure other policies, overriding the command line arguments.
imp_attr.search_card (20);
sup_attr.supports_dynamic_properties (CORBA::B_FALSE);

The trader interfaces register themselves with the default POA during the Trading Service's construction. All that remains is to activate the POA and enter the ORB event loop.

Running the Trading Service Tests

There are two executables that test the Trading Service functionality --- one to test the exporter role, and the other, the importer role. To run the tests simply launch the Trading_Service application, then run the export_test executable found in the orbsvcs/tests/Trading directory. When the export_test ceases to output data and enters the event loop, run the import_test found in the same directory.

Also of importance: the -TSdumpior filename argument to the trader dumps its IOR to the file. You can then paste the contents on the command line to the tests with -ORBtradingserviceior IOR, or into the environment variable TradingServiceIOR.

The expected output of the tests can be found in the README file in the tests directory.

To test federated queries, run at least three copies of the Trading_Service application, each using the -TSfederate flag. The traders will form a complete graph, with each link follow policy set to CosTrading::always. When run with the -f flag, the export_test will add the service types and export offers to each of the traders in the federation. When run with the -f flag, the import_test will perform a directed query to a trader two links distant from the trader boostrapped to, in addition to performing federated queries.  

By default the tests dump the contents of service types and offers to the screen so the tester can validate the results. To run the tests in quiet mode, where the results of the describe and query methods are concealed, use the -q flag.

Known Bugs and Workarounds

At this point there are no known problems with TAO that affect the Trading service.

Future Work

• Persistence --- Have the Trading Service offer database and service type repository survive the lifetime of a single Trading Service process. This would be accomplished by either taking advantage of the ability to serialize IDL types --- using CDR streams --- or through memory-mapped files, a la the ACE Naming Service.

• The Proxy Interface --- Should we ever feel so motivated, we might implement the Proxy interface.