Contents

• Service attribute entries
• Clients getting and using services
• Service implementation choices
• Example: proxy is the service
• JoinManager - simplifying service code
• Remote events
• User interfaces for distributed services

ServiceRegistrar

• Both lookup methods return a ServiceRegistrar. This is a proxy for the lookup service

        public interface ServiceRegistrar {
            public ServiceRegistration register(ServiceItem i,
                                                long leaseDuration);
  
            public Object lookup(ServiceTemplate tmpl);
            public ServiceMatches lookup(ServiceTemplate tmpl, int maxMatches);
  
            LookupLocator getLocator();
        }
        

• The register() method is used by a service
• The lookup() methods are used by a client. The first returns a single service, the second a set of services
• The getLocator() method can say where the lookup service is

ENTRIES

Entry class

• Entries are additional information about a service, such as location, name, cost
• This information is not part of the service object itself
• The additional information can be used to select between otherwise identical services e.g. a colour printer, a four-slice toaster
• Entries implement Entry

        public interface Entry {}
        

• There are a bunch of convenience implementations such as
  • Address
  • Comment
  • Name
  • ServiceInfo

Creating entries

• You can create your own classes, or use the given ones
• Typical code

        Entry[] entries = new Entry[] {new Comment("best toaster made"),
                                       new Name("Classy Toaster")};
        

Interfaces, implementations and entries

• A client looks for an instance of an interface
• It may qualify the search with entries
• A service exports an implementation of this interface
• A service will also export additional entry information, as much as it can

CLIENT

Problem domain

• Files have a type, that can be described by a MIME type

        public class MIMEType implements java.io.Serializable {
            protected String contentType;
            protected String subType;
  
            public MIMEType(String contentTpe, String subType) {...}
  
            public String toString() {...}
        }
        

  (It may need to be serialized, to move it from one machine to another) The full class is at http://jan.newmarch.name/java/jini/tutorial/SimpleExample.xml
• A file classifier can take a filename and use this to work out the MIME type, using an internal table

        public interface FileClassifier {
            MIMEType getMIMEType(String fileName)
                throws java.rmi.RemoteException;
        }
        

  (The method may be executing remotely, so can throw a remote exception)

Unicast client

Client and service locator JVM's

Multicast client

PROXY IMPLEMENTATION CHOICES

Proxy is the Service

• The complete service may be uploaded, so there is no separate proxy
  
• The client uses FileClassifier
• The server creates a FileClassifierImpl
• The server exports the FileClassifierImpl
• This is a "fat" proxy model, with no processing being done on the server side
• This is suitable for software services that do not need to maintain state on the server side

RMI Proxies

• If RMI stubs are used to implement the proxy, it looks like
  
• The client uses FileClassifier
• The server creates a FileClassifierImpl
• The RMI runtime ensures that a FileClassifierimpl_Stub is exported to the service locator
• This is a "thin" proxy, with no processing being done on the client side
• This is closest to the standard RPC model between client and service

Non-RMI Proxy

• If RMI is not used, the proxy must be explicitly created
  
• The client uses FileClassifier (like before - it doesn't care what the proxy is)
• The server creates a FileClassifierImpl and a FileClassifierProxy
• The server exports a FileClassifierProxy
• The proxy and service communicate using some protocol
• This can have processing done on both sides
• This is good for client-server problems communicating on their own ports with their own protocol. The proxy is the heart of the client

RMI and Non-RMI Proxies

• You can use both RMI and a non-RMI proxy
• The client uses FileClassifier
• The server creates a FileClassifierImpl and a FileClassifierProxy
• The server exports a FileClassifierProxy and a FileClassifierImpl_Stub
• The proxy uses the stub locally (on the client), which uses RMI to talk back to the service
• This can have processing done on both sides, using an RPC-like model for communication

PROXY IS THE SERVICE

Service implementation

File classifier server

All the JVM's

References

Jan Newmarch A Programmers Guide to Jini