Jini: New
technology for a networked world - JavaWorld June
1999
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Jini: New technology for a networked world
Jini: New technology for a networked world
By: By Bill Venners
How Jini raises the level of abstraction for distributed systems programming
elcome to the first installment of Jiniology , JavaWorld 's new column dedicated to distributed systems programming with Java, RMI (Remote Method Invocation), and Jini. In this first article, I'll present an introduction to Jini technology and show how Jini's architecture helps to raise the level of abstraction in distributed systems programming, effectively turning network programming into object-oriented programming.
Jini in context
Traditionally, operating systems have been designed with the assumption that a computer will have a processor, some memory, and a disk. When you boot a computer, the first thing it does is look for a disk. If it doesn't find a disk, it can't function as a computer. Increasingly, however, computers are appearing in a different guise: as embedded devices that have a processor, some memory, and a network connection -- but no disk. The first thing a cellphone does when you boot it up, for example, is look for the telephone network. If it doesn't find the network, it can't function as a cellphone. This trend in the hardware environment, from disk-centric to network-centric, will affect how we organize our software -- and that's where Jini comes in.
Jini is an attempt to rethink computer architecture, given the rising importance of the network and the proliferation of processors in devices that have no disk drive. These devices, which will come from many different vendors, will need to interact over a network. The network itself will be very dynamic -- devices and services will be added and removed regularly. Jini provides mechanisms to enable smooth adding, removal, and finding of devices and services on the network. In addition, Jini provides a programming model that makes it easier for programmers to get their devices talking to each other.
Building on top of Java, object serialization, and RMI, which enable objects to move around the network from virtual machine to virtual machine, Jini attempts to extend the benefits of object-oriented programming to the network. Instead of requiring device vendors to agree on the network protocols through which their devices can interact, Jini enables the devices to talk to each other through interfaces to objects.
What is Jini?
Jini is a set of APIs and network protocols that can help you build and deploy distributed systems that are organized as federations of services. A service can be anything that sits on the network and is ready to perform a useful function. Hardware devices, software, communications channels -- even human users themselves -- can be services. A Jini-enabled disk drive, for example, could offer a "storage" service. A Jini-enabled printer could offer a "printing" service. A federation of services, then, is a set of services, currently available on the network, that a client (meaning a program, service, or user) can bring together to help it accomplish some goal.
To perform a task, a client enlists the help of services. For example, a client program might upload pictures from the image storage service in a digital camera, download the pictures to a persistent storage service offered by a disk drive, and send a page of thumbnail-sized versions of the images to the printing service of a color printer. In this example, the client program builds a distributed system consisting of itself, the image storage service, the persistent storage service, and the color-printing service. The client and services of this distributed system work together to perform the task: to offload and store images from a digital camera and print out a page of thumbnails.
The idea behind the word federation is based on the Jini view of the network -- there isn't a central controlling authority. Because no one service is in charge, the set of all services available on the network form a federation -- a group composed of equal peers. Instead of a central authority, Jini's runtime infrastructure merely provides a way for clients and services to find each other (via a lookup service, which stores a directory of currently available services). After services locate each other, they are on their own. The client and its enlisted services perform their task independently of the Jini runtime infrastructure. If the Jini lookup service crashes, any distributed systems brought together via the lookup service before it crashed can continue their work. Jini even includes a network protocol that clients can use to find services in the absence of a lookup service.
How Jini works
Jini defines a runtime infrastructure that resides on the network and provides mechanisms that enable you to add, remove, locate, and access services. The runtime infrastructure resides on the network in three places: in lookup services that sit on the network; in the service providers (such as Jini-enabled devices); and in clients. Lookup services are the central organizing mechanism for Jini-based systems. When new services become available on the network, they register themselves with a lookup service. When clients wish to locate a service to assist with some task, they consult a lookup service.
The runtime infrastructure uses one network-level protocol, called discovery , and two object-level protocols, called join and lookup. Discovery enables clients and services to locate lookup services. Join enables a service to register itself in a lookup service. Lookup enables a client to query a lookup service for services that can help the client accomplish its goals.
The discovery process
Discovery works like this: Imagine you have a Jini-enabled disk drive that offers a persistent storage service. As soon as you connect the drive to the network, it broadcasts a presence announcement by dropping a multicast packet onto a well-known port. Included in the presence announcement is an IP address and port number where the disk drive can be contacted by a lookup service.
Lookup services monitor the well-known port for presence announcement packets. When a lookup service receives a presence announcement, it opens and inspects the packet. The packet contains information that enables the lookup service to determine whether or not it should contact the sender of the packet. If so, it contacts the sender directly by making a TCP connection to the IP address and port number extracted from the packet. Using RMI, the lookup service sends an object, called a service registrar, across the network to the originator of the packet. The purpose of the service registrar object is to facilitate further communication with the lookup service. By invoking methods on this object, the sender of the announcement packet can perform join and lookup on the lookup service. In the case of the disk drive, the lookup service would make a TCP connection to the disk drive and would send it a service registrar object, through which the disk drive would then register its persistent storage service via the join process.
The join process
Once a service provider has a service registrar object, the end product of discovery, it is ready to do a join -- to become part of the federation of services that are registered in the lookup service. To do a join, the service provider invokes the register() method on the service registrar object, passing as a parameter an object called a service item, a bundle of objects that describe the service. The register() method sends a copy of the service item up to the lookup service, where the service item is stored. Once this has completed, the service provider has finished the join process: its service has become registered in the lookup service.
The service item is a container for several objects, including an object called a service object, which clients can use to interact with the service. The service item can also include any number of attributes, which can be any object. Some potential attributes are icons, classes that provide GUIs for the service, and objects that give more information about the service.
Service objects usually implement one or more interfaces through which clients interact with the service. For example, a lookup service is a Jini service, and its service object is the service registrar. The register() method invoked by service providers during join is declared in the ServiceRegistrar interface, which all service registrar objects implement. Clients and service providers talk to the lookup service through the service registrar object by invoking methods declared in the ServiceRegistrar interface. Likewise, a disk drive would provide a service object that implemented some well-known storage service interface. Clients would look up and interact with the disk drive by this storage service interface.
The lookup process
Once a service has registered with a lookup service via the join process, that service is available for use by clients who query that lookup service. To build a distributed system of services that will work together to perform some task, a client must locate and enlist the help of the individual services. To find a service, clients query lookup services via a process called lookup.
To perform a lookup, a client invokes the lookup() method on a service registrar object. (A client, like a service provider, gets a service registrar through the process of discovery, as described earlier in this article.) The client passes as an argument to lookup() a service template, an object that serves as search criteria for the query. The service template can include a reference to an array of Class objects. These Class objects indicate to the lookup service the Java type (or types) of the service object desired by the client. The service template can also include a service ID , which uniquely identifies a service, and attributes, which must exactly match the attributes uploaded by the
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1999
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1999
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