Network systems based on service discovery can provide a consistent view of their distributed components even during changing network conditions. The ability of a system to heal itself during a network catastrophe, including architectural change and system breakdown, will help the system to realign its content traversal route intelligently and swiftly. This ability can be obtained from various healing strategies like failure detection, consistency maintenance, and distributed service activation techniques.

A complete understanding of the interactions among self-healing strategies would provide architects of distributed systems with the knowledge necessary to build the most effective catastrophe-free network system with minimum overhead. A self-healing, manageable distributed system can be developed using a Jini federation. Since Jini is focused on service-oriented programming and supports the discovery of services, active or dormant, identifying service failure and recovering from a major disaster is possible.

As a rule of thumb, a network system is not complete if it's not manageable remotely. Hence there should exist a framework for exposing the application or Jini service for remote configuration. MBeans do just that.

Accessing MBeans from a different VM or a remote location can be done using a protocol adapter or connector. Connectors are similar to protocol adapters but they need the presence of a wrapper at both the server and the client sides; adapters are software, listening on the server side, built on a common protocol that the client is expected to understand and access. The JMX agent that starts the MBean server should let remote clients invoke methods on the MBeans registered in the MBean server.

The conventional way of accessing the MBean server from a different location is by using an RMI connector. RMI connectors are inherently MBeans registered in the MBean server. The remote client can access the MBean server using the RMI connector client. While this seems to be a neat solution for remotely accessing MBeans, the client needs to know the physical location of the MBean server. Even if the the MBean proxy is bound to a lookup, the client should know the lookup location. Even when using protocol adapters like the HTML and SNMP adapters, the client is expected to know the server location.

Consider a typical setup, such as a distributed content management system running on an agent framework, where the publishing server doesn't need to know where the content updating modules are running since it can be run by an editor, writer, or a designer in different locations. Implementing a simple connector does not solve this problem. This article explores the possibilities of using a discovery mechanism to find out where the JMX agent is running. This can be achieved by using a Jini connector, which is registered as an MBean with the MBean server.

However, we don't actually discover a running JMX agent but we discover the Jini lookup service that holds the proxy of the Jini service, which is also registered as an MBean with the MBean server. Figure 1 shows the MBean invocation process between two different VMs.

Management Through MBeans
An agent application is a piece of software written in Java that contains an MBean server and interfaces to access its functionality. This would be a minimal agent and anything less couldn't be managed. In our example of a minimal agent, we'll access the MBean server through the HTML protocol adapter from a Web browser and also through a Jini client running in a different VM. Jini is a Java-based network federation where the services that want to expose themselves to the clients register themselves with a lookup registry, and the client that needs to access the service discovers the service through the lookup registry and invokes its methods. Since MBean is not serializable, a Jini connector wrapper registers the proxy with the lookup service.

Discovering MBean Agents
A JMX client can access and manage MBeans exposed by the MBean agent running in a different VM through various known techniques. The JMX Remote API (JSR 160) proposes a viable solution to remotely access an MBean agent. Hence it's possible for a remote client to get a reference for the JMX Remote API Connector. But a JMX Remote API can be used only when you know where the MBean server is running. The standard does not provide any solution for discovering MBean agents. Instead, you can try traditional service discovery processes like Jini lookup and Service Location Protocol (SLP). SLP is an IETF standard that provides a framework for allowing networking applications to discover the existence, location, and configuration of networked services in the network. But the Java SLP Implementation (JSR 140) is still being tested.

Reggie for Discovering Agents
The Jini framework provides a reference implementation of a lookup ser- vice called "reggie" that holds service references and enables remote clients to discover it and get the remote service reference. It's very simple to discover a running Jini lookup service using API calls. The service, which intends to be a part of the Jini federation, registers a serializable object with the lookup service, enabling remote clients to use this object as a proxy. Another advantage of using Jini is that the classes required for instantiating the proxy objects can be downloaded dynamically from an HTTP server, and the Jini framework provides the necessary security for code download based on the RMI security manager. Figure 2 shows the classes being downloaded dynamically from the client in our example application.

Registering Services
In Jini the services can be registered through a serializable Java object or a stub. The stub provides a direct reference to the underlying Jini service. Since the Jini lookup service, which is based on RMI, automatically downloads from the server all the classes needed to deserialize the service object on the client, the server can register any class and the client can use the same class without having prior information of the class implementation. An easier way of registering services is by calling the JMXConnectorFactory .newConnector of the JMX Remote API.

Accessing Remote Agents
By using Jini, a JMX agent can be distributed the same way as an RMI connector, but without mandating that the clients know the location of the running agent. Hence, to access a JMX agent, the clients can just create a Jini connector and locate the nearest running agent. If multiple agents are running on the network, the clients can select which agent they want to access based on the lookup entry provided by the agent with the lookup service. The Jini connector can also advertise itself by using the default domain name of the MBean server in which it's registered.

In this article we'll build a simple Jini connector that registers itself with the MBean server and exposes the MBean agent for remote administration. Figure 3 shows the MBean list through an HTTP adapter showing the Jini connector and a configurable test string. The Jini connector is comprised of the service, which we want to register with the lookup service, the MBean, and a Jini client. Figure 4 shows the Jini lookup browser showing the Jini service class and the JiniWrapper registered with it. The MBean enables the agent to control the Jini service and, as you refer to the source code (which you can download from www.sys-con.com/java/sourcec.cfm), the MBean has a reference to the MBean server, which allows the Jini service to perform callbacks to the MBean server methods. The Jini client forwards its method invocation to the MBean server via the Jini service. The Jini client-side application discovers and uses the Jini connector service and therefore can use the agent.

An Example
The example source code provides a basic way of enabling the Jini client application to discover the lookup ser- vice running on the network and obtain the proxy of the Jini Connector service that enables the client to invoke the MBean server methods through callbacks. An excellent book to read, especially if you're new to Jini, is Jini in a Nutshell by Scott Oaks and Henry Wong; to learn the basics of JMX, read JMX in Action by Benjamin G. Sullins, Mark Whipple, and Ben G. Sullins.

The process of setting up a Jini environment can be frustrating at times. For this reason, I've provided some instructions (see sidebar). Though our service does not register with the RMI activation daemon, enabling auto-restart during crashes, the Jini lookup service, an activateable service, requires RMID running on the same VM. So make sure the lookup service and RMID are running in the same VM. The Jini connector service can be run in any VM as long as the correct server codebase is specified for dynamic classloading. When the client gets started, it finds out the lookup service and hence the agent, and tries to get the MBean registration information, MBean count, and the test string value. It also dynamically changes the value of the string that can be viewed in a browser (see Figure 2). The output of the client is shown in Figure 5.

Conclusion
MBean provides a powerful interface for managing services. Jini extends the functionality of MBeans by letting the client discover the JMX agent in a network on the fly. And since the agent is pulled into the Jini federation by the Jini connector service, other advantages like agent failure notification, event mailbox, and dynamic service reconfiguration are possible for attaining a true catastrophe free, self-healing, manageable, and intelligent network.