Jini-like
discovery for RMI
Tutorial Details:
Jini-like discovery for RMI
Jini-like discovery for RMI
By: By Philip Bishop and Nigel Warren
Take advantage of Jini's discovery mechanism for RMI development
f you follow Jini developments, you know that Jini clients don't need to know where a service is located; they simply use the discovery mechanism to obtain a proxy to the service they want to use. Conversely in RMI (Remote Method Invocation), you must know the URL of the server you want to use. In this article, we show how you can implement a Jini-like discovery mechanism for RMI, which frees some clients from having to know an RMI server's lookup URL.
Your first thought may be, Why bother to do this in the first place; why not just use Jini? We would agree with this logic, especially for new systems. However, many RMI-based systems still exist, and until Jini is accepted into mainstream Java development, we need to provide more elegant RMI solutions. In fact, the work described here is the result of such a requirement: to develop a Jini service that can also run as a standalone RMI server, but uses Jini-like discovery.
This article is primarily targeted at RMI developers who haven't adopted Jini. By giving insight into what occurs under the Jini hood, we hope you begin to understand how powerful Jini mechanisms are. We certainly don't encourage you to reimplement Jini, but this article may help you understand how these mechanisms work. It may even help you convince your managers or department heads that they should consider Jini as a viable technology for distributed systems.
We won't go into depth about the Jini discovery mechanism, so if you are not familiar with it, we recommend you quickly review Bill Venners's " Locate Services with the Jini Lookup Service ."
Basic RMI and Jini lookups
In RMI, a client must know the location of the server to which it wants to connect. An RMI server's address is in the URL form rmi://:/ , where the port number is the port on which the rmiregistry listens for requests. For example:
Translator service
=(Translator)Naming.lookup("rmi://theHost/SpanishTranslator");
In Jini, a client finds a service using a Jini utility class, such as ServiceDiscoveryManager . In the example below, we create a ServiceTemplate instance with a list of classes; or in our case, the class we want to match -- the Translator.class :
Class [] classes=new Class[]{Translator.class};
ServiceTemplate tmpl=new ServiceTemplate(null,classes,null);
ServiceDiscoveryManager lmgr=new ServiceDiscoveryManager(null,null);
ServiceItem serviceItem =lmgr.lookup(tmpl,null);
Translator service=serviceItem.service;
As you can see from the example, the ServiceDiscoveryManager uses the lookup() method to find any available services that match the ServiceTemplate . You may also associate any number of attributes with a service lookup; we didn't here in order to keep things simple and essential.
Comparing both lookup mechanisms, you will notice that the service location isn't specified in the Jini version. It's worth pointing out that you can specify a lookup service location if required, but not the location of the actual service you want. The Jini model's power is that we don't have to know or care where a service is located.
Having compared both RMI and Jini lookup mechanisms, we can now think about how to access an RMI server in a Jini-like fashion.
A location-neutral RMI lookup
Ideally, we would like to find the first matching instance of a discovered Translator :
Translator service
=(Translator)RMIDiscovery.lookup(clazz,id);
Here, clazz is the RMI service's interface, and id is some unique string to differentiate between server instances implementing the clazz interface. For example, to find a Spanish translator, we use the following:
Class clazz=Translator.class;
String id="Spanish";
Now that we have a high-level idea of how to use RMI discovery, we can start investigating how to implement it. As we attempt to implement a "poor man's" discovery for RMI, we can look at how Jini does it and then adapt those principles/concepts in a way that suits an RMI server and client.
The discovery mechanism
Jini's primary discovery mechanism uses a combination of multicast UDP (User Datagram Protocol) and unicast TCP/IP. In simple terms, this means that a client sends a multicast request packet, which gets picked up by lookup services listening for it. The lookup services make unicast connections back to the client and serialize their proxy down the stream available over the connection. The client then interacts with the lookup service (proxy) to locate the service it wants. Figure 1 illustrates this process.
Figure 1. Jini multicast discovery
There is considerably more to discovery than this, but we're only interested in the key concepts of multicast UDP and unicast TCP/IP.
We won't attempt to implement a standalone RMI lookup service equivalent. Instead we will implement a simple multicast listener/unicast dispatcher that an RMI server can use, effectively making each RMI server act as its own lookup service. On the client side, we write the counterparts for the server-side sockets -- a multicast dispatcher/unicast listener. This means that the participants in Figure 1 become the RMI client and RMI server, shown below in Figure 2.
Figure 2. RMI multicast discovery
The table below describes in more detail the interactions between the RMI client and RMI server.
Interactions between RMI client and RMI server
Server
Client
Start listening on multicast address.
Start ServerSocket to listen for unicast responses from the server.
Begin sending UDP packets to multicast address.
Parse received UDP packet. If valid, connect back to the client via unicast TCP/IP.
Send remote stub to client.
Read remote object from stream.
Close ServerSocket . Stop sending UDP multicast packets.
Start using service.
The discovery protocol
Earlier we outlined how we want an RMI client to discover a server: it would specify an interface class and a unique name to identify a server instance. This is because multiple servers that implement the same interface may be running concurrently.
Before we implement our RMI discovery mechanism, we must define the protocol for message passing between participants. For simplicity, we'll use a delimited string containing all the information a RMI server needs to respond to a matching request. First, we define a protocol header. This prevents the server classes from attempting to fully parse packets that arrive from other sources. The remainder of the message packet will contain the unicast response port, the server's interface class name, and the server instance's unique identifier.
Below is the format of the discovery request message we will use:
,,,
Now let's look at a sample message packet a client might send to discover a Spanish instance of the Translator server. RMI-DISCOVERY is the protocol header, and 5000 is the port number on which the client is listening for responses:
RMI-DISCOVERY,5000,Translator,Spanish
We don't include the client's hostname in the request because that information can be obtained from the UDP packet received on the server. Having defined our message format, we can start implementing the discovery classes.
Implement the server-side classes
Our cunning plan is to write a utility class that RMI servers can use to create their own personal lookup service:
//instantiate RMI server
Remote server=new SpanishTranslator();
//initiates discovery listener
RMILookup.bind(server,"Spanish")
The Remote parameter checks if the server implements the interface the client is trying to discover and whose RMI stub is ultimately serialized back to the client. The String parameter compares the server name with the name in the request packet.
Before going forward, let's quickly recap the server-side classes' responsibilities:
Set up a multicast UDP socket to listen for requests
Check the protocol header when a packet arrives
Parse the message packet
Match the unique server name parameter
Match the interface parameter
If 4 and 5 match, serialize the server's remote stub to the client via unicast TCP/IP socket
Set up a multicast UDP listener
To set up a multicast listener, you must use a known multicast address and port; those in the range 224.0.0.1 to 239.255.255.255, inclusive. Some vendors reserve some of these address/port combinations; for example, Sun reserves Jini the combination 224.0.1.85:4160. (A list of reserved addresses can be found at http://www.iana.org/assignments/multicast-addresses .) Operating on the same frequencies as other vendors is not recommended, so we chose to use the same combination used in the MulticastSocket Javadoc example:
int port=6789;
String multicastAddress="228.5.6.7";
MulticastSocket socket=new MulticastSocket(port);
InetAddress address=InetAddress.getByName(multicastAddress);
socket.joinGroup(address);
byte[] buf = new byte[512];
DatagramPacket packet=new DatagramPacket(buf, buf.length);
socket.receive(packet);
//parse packet etc
socket.leaveGroup(address);
The above example shows how simply you can set up a multicast listener and receive packets on that address/port combination. In the above example, only a single packet can be processed, so we must create a loop around the DatagramPacket creation and socket.receive() ; otherwise only one client will be able to discover the server:
while(active){
byte[] buf=new byte[512];
DatagramPacket packet=new DatagramPacket(buf,buf.length);
socket.receive(packet);
//process packet
}
We could use several strategies to process packets received:
Thread per request : Create a new thread to handle each request
Thread from a thread pool : Use a preinstantiated thread from a (possibly fixed) resource thread pool (see " Java Tip 78: Recycle Broken Objects in Resource Pools ")
Blocking: Only process one request at a time, other r
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