The Location Service Example

This example illustrates VisiBroker for Java's location service. It enables developer code to discover, or be notified of, object instances that have particular attributes, such as satisfying an interface. The location service can be used for any function that requires knowledge of what objects are presently accessible and where they are. This information could be used for load balancing or debugging. This example subdirectory also contains a program illustrating triggers which are a part of the Location Service. Triggers are an asynchronous alternative to polling the OSAgent to learn when an instance becomes available or becomes unavailable. Note that usage of the LocationService (the OSAgent interface) with VisiBroker for Java currently requires the execution of the standalone server locserv which is provided with the VisiBroker for Java release. The program is shipped as part of the archive of platform-specific executables and scripts which should have been installed in the directory bin. Assuming the installation's bin directory has been added to your path, this executable can be run with the command:

prompt> locserv

• Use the Location Service API provided by VisiBroker for Java to detect instances becoming available.
• Implement triggers for notification when an instance becomes available or unavailable.

Directory Contents

• locate.idl


IDL interface for the Location Service provided by VisiBroker for Java.

• AccountFinder.java


This program gathers information using the Location Service about all instances satisfying the account interface. To do this, the Repository ID IDL:Account:1.0 of the account interface is passed to the Agent's all_instances().

• Find.java


This example is more generic than the AccountFinder.java example above. It shows an example of acquiring an Agent (The Agent is the CORBA interface to access the Location service) to obtain all Globally scoped instances. The Agent is acquired by passing "Location Service" to orb.resolve_initial_references() and then narrowing the object returned to ObjLocation.Agent. ObjLocation.all_repository_ids() is then used to retrieve the repository IDs of all interfaces that are known to the OSAgent. Following this, information about all the interfaces that have been retrieved is obtained using all_instances_descs().

• AccountTrigger.java


This program implements a TriggerHandler. The main class creates and registers a Handler instance. The trigger is set to notify when an instance with the repository ID IDL:Account:1.0 registers itself with the OSAgent and becomes accessible. The trigger handler is sticky in that it will continue to fire when new instances of this interface become available, until explicitly unregistered by the user. The program gives an option to the user to unregister the trigger handler each time an instance of IDL:Account:1.0 becomes available or unavailable.

Running this example

To run the examples, first make sure that the VisiBroker Smart Agent (osagent executable) is running on your network. You should also have the Location Service (locserv executable) running on your network. Then start the Bank Server from the Bank example using the command:

prompt> vbj Server  
(start vbj Server on Windows)
// Make Server run in the background

Next, start the AccountFinder, Find and AccountTrigger programs:

prompt> vbj -DORBservices="com.visigenic.vbroker.ObjLocation" AccountFinder  

Note that the Location Service detects the Bank Server. Kill the bank server and again run the AccountFinder program as indicated above. Notice the change in output.

prompt> vbj -DORBservices="com.visigenic.vbroker.ObjLocation" Find

To test the find program, start one or more bank servers (or other servers). The Find program provides with a command line user interface which allows the user to obtain information about specific objects using their repository id. To obtain a list of the options the Find program provides, start the Find program and type at the shell prompt it provides.

prompt> vbj -DORBservices="com.visigenic.vbroker.ObjLocation" AccountTrigger   

The AccountTrigger program will notify the user when a provider goes down or when a provider comes back up. Try running the AccountTrigger program after either starting the bank server or stopping the bank server.