15.1.1 Discovery Client

The DiscoveryClient class provides methods to discover agents. The active discovery operation sends a discovery request to a multicast group and waits for responses. These messages are proprietary and are not exposed to the user. Discovery clients can only discover agents listening on the same multicast group and port, so your design must coordinate this information between the discovery client and responders.

You can instantiate and perform searches from multiple discovery clients in a single application. Each discovery client can be configured to use different multicast groups or ports, enabling you to discover different groups of agents.

The discovery services enable users to specify a local interface from which to send out multicast messages. This is useful when working on a system that has multihome interfaces connecting to disconnected multinetworks. In addition, the DiscoveryResponder constructor enables you to specify the host address to be used to build the discovery response. The address can be specified either as an IPV4 or IPv6 address, or as a host name.

Example 15–1 Instantiating and Initializing a Discovery Client

public class Client {
    public static void main(String[] args) throws Exception {
	       BufferedReader br
	           = new BufferedReader(new InputStreamReader(System.in));
 
	       discoveryClient = new DiscoveryClient();
	       discoveryClient.start();

	       while(true) {
	          echo("\n>>> Press return to discover connector servers;");
	          echo(">>> Type a host name then return to discover connector 
                servers running on that machine;");
	          echo(">>> Type bye then return to stop the client.");

    	    String s = br.readLine();

	          if (s.equalsIgnoreCase("bye")) {
		           System.exit(0);
	          } else {
		           try {
		                discover(s);
		           } catch (Exception e) {
		                echo("Got exception "+e);
		                e.printStackTrace();
		           }
	          }
	       }
    }

Once you have created the discovery client, before initiating searches, you must call the discovery client's start method, as shown in Example 15–1. This will create its multicast socket and join the multicast group used for broadcasting its discovery request. The default multicast group is 224.224.224.224 and the default port is 9000. These can be set to other values through the multicastGroup and multicastPort attributes, but only when the state of the discovery client is OFFLINE.

The scope of the discovery request depends on the time-to-live used by the multicast socket. Time-to-live is defined by the Java class java.net.MulticastSocket to be a number between 1 and 255. By default, the time-to-live is 1, which corresponds to the host's local area network. You can modify this value at any time by setting the discovery client's TimeToLive attribute.

By default, a discovery client waits for responses for one second after it has sent a discovery request. This period can be customized by setting a value in milliseconds for its TimeOut attribute. When setting this attribute, you should take into account estimated time for a round-trip of a network packet using the given time-to-live.

Once started, the discovery client in the example above awaits user inputs before starting searches.