Contents

24.1 The Default Locator

24.2 Writing a New Locator

24.1 The Default Locator

1. The locator first attempts to contact an OrbixWeb daemon process at the local (client) host. If no such process exists, then the location attempt fails.


2. The locator invokes the method lookUp(), which contacts the local OrbixWeb daemon and requests a list of host names for the specified server name.


3. The host names returned by the daemon are arranged in a random order. OrbixWeb then iterates through this list, attempting to verify the registration of the server at each host in turn. The locator returns the first host at which the desired server is registered.


4. If the server is registered at none of the configured hosts, the locator attempts to locate the server at the local host. If the server is registered at the local host, the location attempt succeeds. Otherwise, the location attempt fails.

For successful operation of the default locator, it is necessary to specify server names and corresponding target hosts in advance. The configuration of the default locator must be carried out with respect to the local OrbixWeb daemon process, which manipulates the locator configuration files. This configuration and some additional details of the default locator functionality are described in Chapter 3, "OrbixWeb Configuration" of the OrbixWeb Reference Guide.

The lookUp() method

 // Java
 public _sequence_String lookUp(String ServiceName, 
 			int MaxHops, Context ctx);

Unlike the location mechanism in some versions of Orbix, the OrbixWeb default locator does not make use of a publicly accessible default locator object in order to call the method lookUp(). Consequently, an OrbixWeb client cannot call the default locator lookUp() method directly. However, it is possible to implement this functionality and this is discussed in the next subsection.

Note that the object _CORBA.locator (defined in package IE.Iona.Orbix2) is only used to allow programmers to override the default lookUp() implementation and is assigned to null unless explicitly replaced, as described in section 24.2.

Default lookUp() functionality

The default lookUp() implementation uses the IT_daemon IDL interface, which is implemented by the OrbixWeb daemon. In order to mimic the behaviour of lookUp(), it is simply necessary to bind to the local OrbixWeb daemon process and invoke the IT_daemon::lookUp() operation. The IDL signature of this operation is:

 // IDL
 boolean lookUp (in string service,
 			out stringSeq hostList,
 			in octet hops, in string tag);

 // Java
 // in file Client.java
 // in class Client
 
 IE.Iona.Orbix2.IT_daemonRef dRef;
 String service;
 String tag = null;
 byte hops;
 _sequence_String hostList = null;
 
 // initialise server name
 service = "MyServer";
 
 // initialise number of locator hops
 hops = 5;
 
 try {
 	// bind to Orbix daemon
 	dRef = IE.Iona.Orbix2.IT_daemon._bind ();
 
 	// invoke lookUp() operation
 	dRef.lookUp (service, hostList, hops, tag);
 }
 catch (SystemException se) {
 	...
 }
 
 if (hostList.length > 0) {
 	int i;
 
 	for (i=0; i<hostList.length; i++)
 		System.out.println (hostList.buffer[i]);
 }
 else {
 	// server not found in configuration file ...
 }

The hops parameter to lookUp() specifies the maximum number of hops which can be used to fulfil a request, thereby limiting the number of hosts involved in a search. The _bind() method uses the value _CORBA._LOCATOR_HOPS (which can be changed by a programmer who wishes to modify how _bind() uses lookUp()). Explicit calls to lookUp() can specify any byte value, except that the constant value _CORBA._MAX_LOCATOR_HOPS is used if a greater value is specified.

The tag parameter is simply a string used in daemon diagnostic messages and can generally be assigned the value null.

24.2 Writing a New Locator

• Defining a derived class of locatorClass.
  
• Creating a single instance of the new class.
  
• Assigning the pointer _CORBA.locator to point to that instance.
  

The locator lookUp() method is passed the name of the server being sought and it should return a list of names of hosts on which that server is registered in the Implementation Repository.

Class locatorClass is defined as follows:

 // Java
 package IE.Iona.Orbix2.CORBA;
 
 public class locatorClass {
 	public _sequence_String lookUp(
 		String ServiceName, int MaxHops, 
 		Context ctx);
 }

It is often advisable that a locator randomises the sequence before returning it.