Jini for DistJava

Jan Newmarch
jan@newmarch.name
Monash University

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Background

Communication

• In socket-level programming all communication between client and server is coded by the programmer
• In RPC, the code for communication is generated from a specification. The client includes this generated code and makes calls on it. The server code is generated and calls implementation code
• In CORBA, a specification is used to generate a server and client half-objects. The client calls methods on the half-object which does communication to the server which calls a server-side object
• Java RMI does the same, but also allows copies of objects to be made across the network, so that the programmer can deal with local objects, remote references, or copies of remote objects
• Jini adds the possibility of making the remote references into remote copies, so that there are more choices of remote object: "dumb" references or "smart" proxies

Lookup

• In socket-level programming, you need to know the location and port of a service in order to connect to it
• In RPC, you need to know the host. The portmapper service is a well-known service that handles RPC calls on a standard port
• CORBA has three methods to locate services
  • A string format, which can be passed by e.g. email
  • A naming service running anywhere in the network that stores name/reference pairs. A service is told the host name and port of the naming service and registers with it. A server is also told the host name and port of the naming service and looks up services on it
  • A trader running anywhere in the network, that clients and services know about
• RMI has a naming service which runs on the same machine as the service. Clients must know this hostname
• Jini has a lookup service which is either located by multicast on a local network or is on a host known to clients and services

OVERVIEW

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What is Jini?

• Jini brings ``network plug and play'' to hardware devices and software services
• Jini is in pure Java
• It makes hardware and software available as Java networked services
• ``Legacy'' systems can be wrapped in a Java proxy
• Jini has competitors including CORBA Trader, DCOM, HP's Chai, Enterprise Java Beans, JNDI, etc

Jini Federation

• A Jini federation or ``djinn'' consists of a number of clients, services and service locators
• Typically connected by TCP/IP
• Objects are moved around from service to locator to client using MarshalledObject's

Figure 1: Components of a Jini federation

Service Registration

Figure 2: Querying for a service locator

Figure 3: Registrar returned

Figure 4: Service uploaded

Client Lookup

Figure 5: Querying for a service locator

Figure 6: Registrar returned

Figure 7: Asking for a service

Figure 8: Service returned

Support Services

• Jini objects are moved in ``marshalled'' form
• Class files may need to follow these for proper activation
• The class files may need to be available from HTTP servers
• Security permissions will need to be set using JDK 1.2 security model

Proxies

• A toaster will be controlled by some service software
• The service software will be moved across the network to the client
• The mobile code must be a proxy for the real service code
• The proxy may be explicitly written or be an RMI proxy stub
• Remote calls from the proxy may be RMI Remote calls, or by some other mechanism such as private TCP connection

Attribute Registration

Service Location

Leasing

• Because services can come and go, network connections can be lost, etc, a distributed garbage collection mechanism is needed
• Services which register are granted a lease
• Registration lasts for as long as the lease
• When the lease expires, the service is removed
• Leases can be renewed

Non-distributed Application

Distributed Version

Client Structure

Internally a client will look like

Server Structure

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

SIMPLE SYSTEM

Jini Converter Service

• This shows a Jini version of the Converter service
• It is as minimal as possible!
• Some poor practises are used, like throwing exceptions out to the main process

Jini Service Specification

Jini Service Implementation

Jini Server

Jini Client

Jini Environment

• There must be a Jini lookup service running to act as the "billboard"
• The Jini distribution contains one, called reggie
• An HTTP server should be running, to deliver class files
• The converter server runs with one set of class files
  • Converter
  • ConverterImpl
  • Server
• The converter client runs with another set of class files
  • Converter
  • Client
• These files should be on the HTTP server so that clients can find the class definitions
  • Converter
  • ConverterImpl

SECURITY

JDK 1.2 Security

• Jini security uses the security model of JDK 1.2, with no changes or extensions
• Security is based on granting permissions (access rights), in security files
• Debug the application with a slack policy

  
        grant {
            permission java.security.AllPermission "", "";
        };
        

• Leave it like this of production only at your peril

Service requirements

grant {
    permission net.jini.discovery.DiscoveryPermission "*";
    // multicast request address
    permission java.net.SocketPermission "224.0.1.85", "connect,accept";
    // multicast announcement address
    permission java.net.SocketPermission "224.0.1.84", "connect,accept";

    // RMI connections
    permission java.net.SocketPermission "*.dstc.edu.au:1024-", "connect,accept";
    permission java.net.SocketPermission "130.102.176.249:1024-", "connect,accept";
    permission java.net.SocketPermission "127.0.0.1:1024-", "connect,accept";

    // reading parameters
    // like net.jini.discovery.debug!
    permission java.util.PropertyPermission "net.jini.discovery.*", "read";
};

Client requirements

Client policy

grant {
    permission net.jini.discovery.DiscoveryPermission "*";

    // multicast request address
    permission java.net.SocketPermission "224.0.1.85", "connect,accept";
    // multicast announcement address
    permission java.net.SocketPermission "224.0.1.84", "connect,accept";

    // RMI connections
    permission java.net.SocketPermission "127.0.0.1:1024-", "connect,accept";
    permission java.net.SocketPermission "*.dstc.edu.au:1024-", "connect,accept";
    permission java.net.SocketPermission "130.102.176.249:1024-", "connect,accept";

    // DANGER
    // HTTP connections - this is where external code may come in - careful!!!
    permission java.net.SocketPermission "127.0.0.1:80", "connect,accept";
    permission java.net.SocketPermission "*.dstc.edu.au:80", "connect,accept";

    // reading parameters
    // like net.jini.discovery.debug!
    permission java.util.PropertyPermission "net.jini.discovery.*", "read";

};

Running things

Jini classes

The common classes needed by all of reggie, the clients and servers are

1. jsk-lib.jar
2. jsk-platform.jar

Jini Lookup Service

The command Launch-All in the Jini installverify directory will start the needed support services.

Client

The client needs to have the following classes in its CLASSPATH, in addition to the Jini classes

1. The Converter interface
2. The class Client

The client can then be run by

java -Djava.security.policy=policy.all Client

Server

The server needs to have the following classes in its CLASSPATH, in addition to the Jini classes

1. The Converter interface
2. The class Server
3. The class ConverterImpl

The server can then be run by

java -Djava.rmi.server.codebase=http://hostname/classes \
     -Djava.security.policy=policy.all \
     Server

Variations

• If the implementation implements Serializable and the implementation itself is advertised, then a copy of the service is run on the client
• Same as above, but the copy communicates to a third party service by e.g. SOAP

URLs