Getting started with jSLP

First, download the current 1.0_RC1 release of jSLP and a current release of commons-logging. Similar to RFC 2614, jSLP separates the UA and SA functionalities into two different classes, location of services is provided by ch.ethz.iks.slp.Locator, registration of services by ch.ethz.iks.slp.Advertiser. The starting point of interacting with jSLP is to get an instance of one of the classes (or both) via the static methods of ch.ethz.iks.slp.ServiceLocationManager.

Understanding, what's going on

jSLP can either operate stand-alone (in peer-to-peer mode), or with a dedicated SLP Directory Agent in the network. In the first case, jSLP uses multicast convergence to query the local subnet for peers that offer requested services. In the latter case, the central Directory Agent maintains all registered services and answers requests.

The SLP protocol is self-adaptive in the sense that whenever a Directory Agent is present (and matches the scope), it is exclusively used. Otherwise, multicast convergence is used. jSLP fully complies to this behavior.

By default, the SLP protocol uses the standardized port 427 for communication. This, however, requires to run jSLP as root on Linux/Unix platforms. In some situations, where a closed world can be assumed and interoperability with other SLP peers is not relevant, it might be more convenient to run jSLP on a port > 1024 to allow it to run from an ordinary user account. This is possible from jSLP release 0.7.x by setting net.slp.port to a target port.

jSLP is designed to be accessible from multiple instances of Java running on the same machine. Be aware, that always the first instance started holds the Java part that corresponds to the Deaemon. If this instance is shut down, also the registrations are lost. To prevent this, use a DA in the network of make sure that this first instance runs continously. (In general, this is the behavior of slpd).

Register a service

The following example shows how to register a service with jSLP:

// get Advertiser instance
Advertiser advertiser = ServiceLocationManager.getAdvertiser(new Locale("en"));

// the service has lifetime 60, that means it will only persist for one minute
ServiceURL myService = new ServiceURL("service:test:myService://my.host.ch", 60);

// some attributes for the service
Hashtable attributes = new Hashtable();
attributes.put("persistent", Boolean.TRUE);
attributes.put("cool", "yes");
attributes.put("max-connections", new Integer(5));

advertiser.register(myService, attributes);

Locate a service

The next example shows how to locate a service in the network:

// get Locator instance
Locator locator = ServiceLocationManager.getLocator(new Locale("en"));

// find all services of type "test" that have attribute "cool=yes"
ServiceLocationEnumeration sle = locator.findServices(new ServiceType("service:test")
	, null, "(cool=yes)");

// iterate over the results
while (sle.hasMoreElements()) {
	ServiceURL foundService = (ServiceURL) sle.nextElement();
	System.out.println(foundService);
}

See, what's going on

The following lines added to your VM's command line can help to see and understand what is going on on the level of messages and registrations: -Dorg.apache.commons.logging.log=org.apache.commons.logging.impl.SimpleLog -Dorg.apache.commons.logging.simplelog.defaultlog=trace -Dnet.slp.traceMsg=true -Dnet.slp.traceReg=true

Troubleshooting

There are two common problems that might prevent jSLP from working correctly:

• jSLP cannot open the port 427 : Either run jSLP as root, or change the port, if this is acceptable for your setup.
• jSLP cannot find your OpenSLP DA : First, ensure that your OpenSLP DA is in the same subnet as your jSLP client is and that it is actually running in DA mode. Check /var/log/slpd.log for the agent URL, it should read something like service:directory-agent://URL. If your OpenSLP DA is running as an SA although you configured it to be a DA, it is a hostname lookup problem. OpenSLP will refuse to run as a DA if it cannot find out your host name. Make sure that your /etc/hosts contains an entry that maps your hostname to the actual IP address and that your machine has a hostname set. As long as the result of OpenSLP's effort to find out the machines canonical host name is 127.X, it will silently switch to SA mode (one could argue that this is not a nice behavior but it is implemented this way).
• jSLP cannot detect the local host address on Linux : First, try to set your /etc/hosts, etc. If you cannot do this, you can manually influence the interfaces by setting net.slp.interfaces . The first entry will determine on which network interface jSLP sends outgoing messages. See the properties for details.