JINI Technology
JINI Overview
What is JINI?
A Jini system is a distributed system based on the idea of federating groups
of users and the resources required by those users. The overall goal is
to turn the network into a flexible [3], easily administered tool with
which resources can be found by human and computational clients. Resources
can be implemented as either hardware devices, software programs, or a
combination of the two. The focus of the system is to make the network
a more dynamic entity that better reflects the dynamic nature of the workgroup
by enabling the ability to add and delete services flexibly.
A Jini system consists of the following parts:
- A set of components that provides an infrastructure for federating services in a distributed system
- A programming model that supports and encourages the production of reliable distributed services
- Services that can be made part of a federated Jini system and that offer functionality to any other member of the federation
The end goals of the system span a number of different audiences; these goals include the following:
- Enabling users to share services and resources over a network
- Providing users easy access to resources anywhere on the network while allowing the network location of the user to change
- Simplifying the task of building, maintaining, and altering a network of devices, software, and users
Figure 1 :Jini Overview
The Jini technology infrastructure provides mechanisms for devices,
services, and users to join and detach from a network. Joining and leaving
a Jini system are easy and natural, often automatic, occurrences. Jini
systems are far more dynamic than is currently possible in networked groups
where configuring a network is a centralized function done by hand.
Key Concept 
The purpose of the Jini architecture is to federate groups of devices and
software components into a single, dynamic distributed system. The resulting
federation provides the simplicity of access, ease of administration, and
support for sharing that are provided by a large monolithic system while
retaining the flexibility, uniform response, and control provided by a
personal computer or workstation .
The architecture of a single Jini system is targeted to the workgroup. Members of the federation are assumed to agree on basic notions of trust, administration, identification, and policy. It is possible to federate Jini systems themselves for larger organizations.
Services
The most important concept within the Jini architecture is that
of a service. A service is an entity that can be used by a person, a program,
or another service. A service may be a computation, storage, a communication
channel to another user, a software filter, a hardware device, or another
user. Two examples of services are printing a document and translating
from one word-processor format to some other.
Members of a Jini system federate to share access to services. A Jini
system should not be thought of as sets of clients and servers, users and
programs, or even programs and files. Instead, a Jini system consists of
services that can be collected together for the performance of a particular
task. Services may make use of other services, and a client of one service
may itself be a service with clients of its own. The dynamic nature of
a Jini system enables services to be added or withdrawn from a federation
at any time according to demand, need, or the changing requirements of
the workgroup using the system.
Jini systems provide mechanisms for service construction, lookup, communication,
and use in a distributed system . Examples of services include: devices
such as printers, displays, or disks; software such as applications or
utilities; information such as databases and files; and users of the system.
Services in a Jini system communicate with each other by using a service
protocol, which is a set of interfaces written in the Java programming
language. The set of such protocols is open ended. The base Jini system
defines a small number of such protocols that define critical service interactions.
Lookup
Service
Services are found and resolved by a lookup service. The lookup service
is the central bootstrapping mechanism for the system and provides the
major point of contact between the system and users of the system. In precise
terms, a lookup service maps interfaces indicating the functionality provided
by a service to sets of objects that implement the service. In addition,
descriptive entries associated with a service allow more fine-grained selection
of services based on properties understandable to people .
Objects in a lookup service may include other lookup services; this provides hierarchical lookup [3, 20]. Further, a lookup service may contain objects that encapsulate other naming or directory services, providing a way for bridges to be built between a Jini Lookup service and other forms of lookup service. Of course, references to a Jini Lookup service may be placed in these other naming and directory services, providing a means for clients of those services to gain access to a Jini system.
A service is added to a lookup service by a pair of protocols called discovery and join--first the service locates an appropriate lookup service (by using the discovery protocol), and then it joins it (by using the join protocol).
Java Remote
Method Invocation (RMI)
Communication between services can be accomplished using Java Remote Method
Invocation (RMI) [2, 24]. The infrastructure to support communication between
services is not itself a service that is discovered and used but is, rather,
a part of the Jini technology infrastructure. RMI provides mechanisms to
find, activate, and garbage collect object groups.
Fundamentally, RMI is a Java programming language-enabled extension to traditional remote procedure call mechanisms. RMI allows not only data to be passed from object to object around the network but full objects, including code. Much of the simplicity of the Jini system is enabled by this ability to move code around the network in a form that is encapsulated as an object.
2.2.4 Security
The design of the security model for Jini technology is built on the twin notions of a principal and an access control list. Jini services are accessed on behalf of some entity--the principal-- which generally traces back to a particular user of the system. Services themselves may request access to other services based on the identity of the object that implements the service[3]. Whether access to a service is allowed depends on the contents of an access control list that is associated with the object.
Leasing
Access to many of the services in the Jini system environment is lease
based. A lease is a grant of guaranteed access over a time period. Each
lease is negotiated between the user of the service and the provider of
the service as part of the service protocol: A service is requested for
some period; access is granted for some period, presumably taking the request
period into account. If a lease is not renewed before it is freed--either
because the resource is no longer needed, the client or network fails,
or the lease is not permitted to be renewed--then both the user and the
provider of the resource may conclude that the resource can be freed [3].
Leases are either exclusive or non-exclusive. Exclusive leases ensure that no one else may take a lease on the resource during the period of the lease; non-exclusive leases allow multiple users to share a resource.
Transactions
A series of operations, either within a single service or spanning multiple
services, can be wrapped in a transaction [3]. The Jini Transaction interfaces
supply a service protocol needed to coordinate a two-phase commit. How
transactions are implemented--and indeed, the very semantics of the notion
of a transaction--is left up to the service using those interfaces.
Events
The Jini architecture supports distributed events. An object may allow
other objects to register interest in events in the object and receive
a notification of the occurrence of such an event [3]. This enables distributed
event-based programs to be written with a variety of reliability and scalability
guarantees.
Component
Overview
The components of the Jini system can be segmented into three categories:
infrastructure, programming model, and services[22, 24]. The infrastructure
is the set of components that enables building a federated Jini system,
while the services are the entities within the federation. The programming
model is a set of interfaces that enables the construction of reliable
services, including those that are part of the infrastructure and those
that join into the federation.
These three categories, though distinct and separable, are entangled
to such an extent that the distinction between them can seem blurred. Moreover,
it is possible to build systems that have some of the functionality of
the Jini system with variants on the categories or without all three of
them. But a Jini system gains its full power because it is a system built
with the particular infrastructure and programming models described, based
on the notion of a service. Decoupling the segments within the architecture
allows legacy code to be changed minimally to take part in a Jini system.
Nevertheless, the full power of a Jini system will be available only to
new services that are constructed using the integrated model.
A Jini system can be seen as a network extension of the infrastructure,
programming model, and services that made Java technology successful in
the single-machine case.
Infrastructure
The Jini technology infrastructure defines the minimal Jini technology
core [3]. The infrastructure includes the following:
- A distributed security system, integrated into RMI, that extends the Java platform's security model to the world of distributed systems.
- The discovery and join protocols, service protocols that allow services (both hardware and software) to discover, become part of, and advertise supplied services to the other members of the federation.
- The lookup service, which serves as a repository of services. Entries in the lookup service are objects written in the Java programming language; these objects can be downloaded as part of a lookup operation and act as local proxies to the service that placed the code into the lookup service.
Programming
Model
The infrastructure both enables the programming model and makes use of
it. Entries in the lookup service are leased, allowing the lookup service
to reflect accurately the set of currently available services [3]. When
services join or leave a lookup service, events are signaled, and objects
that have registered interest in such events get notifications when new
services become available or old services cease to be active. The programming
model rests on the ability to move code, which is supported by the base
infrastructure.
Both the infrastructure and the services that use that infrastructure are computational entities that exist in the physical environment of the Jini system. However, services also constitute a set of interfaces, which define communication protocols that can be used by the services and the infrastructure to communicate between themselves.
These interfaces, taken together, make up the distributed extension of the standard Java programming language model that constitutes the Jini programming model. Among the interfaces that make up the Jini programming model are the following:
- The leasing interface, which defines a way of allocating and freeing resources using a renewable, duration-based model.
- The event and notification interfaces, which are an extension of the event model used by JavaBeans™ components to the distributed environment, enable event-based communication between Jini services.
- The transaction interfaces, which enable entities to cooperate in such a way that either all of the changes made to the group occur atomically or none of them occur.
- The lease interface extends the Java programming language model by adding time to the notion of holding a reference to a resource, enabling references to be reclaimed safely in the face of network failures.
The transaction interfaces introduce a lightweight, object-oriented protocol enabling Jini applications to coordinate state changes. The transaction protocol provides two steps to coordinate the actions of a group of distributed objects. The first step is called the voting phase, in which each object "votes" whether it has completed its portion of the task and is ready to commit any changes it made. In the second step, a coordinator issues a "commit" request to each object.
The Jini Transaction protocol differs from most transaction interfaces in that it does not assume that the transactions occur in a transaction processing system. Such systems define mechanisms and programming requirements that guarantee the correct implementation of a particular transaction semantics. The Jini Transaction protocol takes a more traditional object-oriented view, leaving the correct implementation of the desired transaction semantics up to the implementor of the particular objects that are involved in the transaction. The goal of the transaction protocol is to define the interactions that such objects must have to coordinate such groups of operations.
The interfaces that define the Jini programming model are used by the infrastructure components where appropriate and by the initial Jini services. For example, the lookup service makes use of the leasing and event interfaces. Leasing ensures that services registered continue to be available, and events help administrators discover problems and devices that need configuration. The JavaSpaces™ service, one example of a Jini service, utilizes leasing and events, and also supports the Jini Transaction protocol. The transaction manager can be used to coordinate the voting phase of a transaction for those objects that support transaction protocol.
The implementation of a service is not required to use the Jini programming model, but such services need to use that model for their interaction with the Jini technology infrastructure. For example, every service interacts with the Jini Lookup service by using the programming model; and whether a service offers resources on a leased basis or not, the service's registration with the lookup service will be leased and will need to be periodically renewed.
The binding of the programming model to the services and the infrastructure is what makes such a federation a Jini system not just a collection of services and protocols. The combination of infrastructure, service, and programming model, all designed to work together and constructed by using each other, simplifies the overall system and unifies it in a way that makes it easier to understand.
Services
The Jini technology infrastructure and programming model are built to enable
services to be offered and found in the network federation. These services
make use of the infrastructure to make calls to each other, to discover
each other, and to announce their presence to other services and users.
Services appear programmatically as objects written in the Java programming language, perhaps made up of other objects. A service has an interface that defines the operations that can be requested of that service. Some of these interfaces are intended to be used by programs, while others are intended to be run by the receiver so that the service can interact with a user. The type of the service determines the interfaces that make up that service and also define the set of methods that can be used to access the service. A single service may be implemented by using other services.
Example Jini services include the following:
- A printing service, which can print from Java applications and legacy applications
- A JavaSpaces service, which can be used for simple communication and for storage of related groups of objects written in the Java programming language
- A transaction manager, which enables groups of objects to participate in the Jini Transaction protocol defined by the programming model
Service
Architecture
Services form the interactive basis for a Jini system [1], both at the
programming and user interface levels. The details of the service architecture
are best understood once the Jini Discovery and Jini Lookup protocols are
presented.
Discovery
and Lookup Protocols
The heart of the Jini system is a trio of protocols called discovery, join,
and lookup [1]. A pair of these protocols--discovery and join--occur when
a device is plugged in. Discovery occurs when a service is looking for
a lookup service with which to register. Join occurs when a service has
located a lookup service and wishes to join it. Lookup occurs when a client
or user needs to locate and invoke a service described by its interface
type (written in the Java programming language)
and possibly other attributes.
The heart of the Jini system is a trio of protocols called discovery, join, and lookup [2]. A pair of these protocols--discovery and join--occur when a device is plugged in. Discovery occurs when a service is looking for a lookup service with which to register. Join occurs when a service has located a lookup service and wishes to join it. Lookup occurs when a client or user needs to locate and invoke a service described by its interface type (written in the Java programming language) and possibly other attributes. Figure 2 outlines the discovery process.
Figure 2: Discovery
Jini Discovery/Join is the process of adding a service to a Jini system. A service provider is the originator of the service--a device or software, for example. First, the service provider locates a lookup service by multicasting a request on the local network for any lookup services to identify themselves (discovery, see Figure 2). Then, a service object for the service is loaded into the lookup service (join, see Figure 2.4). This service object contains the Java programming language interface for the service, including the methods that users and applications will invoke to execute the service along with any other descriptive attributes.
Figure 3: Join
Services must be able to find a lookup service; however, a service may delegate the task of finding a lookup service to a third party. The service is now ready to be looked up and used, as shown in the following diagram (Figure 4).
Figure 4: Lookup
A client locates an appropriate service by its type--that is, by its
interface written in the Java programming language--along with descriptive
attributes that are used in a user interface for the lookup service. The
service object is loaded into the client.
The final stage is to invoke the service, as shown in the following
diagram (Figure 5).
The service object's methods may implement a private protocol between
itself and the original service provider. Different implementations of
the same service interface can use completely different interaction protocols.
The ability to move objects and code from the service provider to the
lookup service and from there to the client of the service gives the service
provider great freedom in the communication patterns between the service
and its clients. This code movement also ensures that the service object
held by the client and the service for which it is a proxy are always synchronized
because the service object is supplied by the service itself. The client
knows only that it is dealing with an implementation of an interface written
in the Java programming language, so the code that implements the interface
can do whatever is needed to provide the service. Because this code came
originally from the service itself, the code can take advantage of implementation
details of the service that are known only to the code.
The client interacts with a service via a set of interfaces written
in the Java programming language. These interfaces define the set of methods
that can be used to interact with the service. Programmatic interfaces
are identified by the type system of the Java programming language, and
services can be found in a lookup service by asking for those that support
a particular interface. Finding a service this way ensures that the program
looking for the service will know how to use that service, because that
use is defined by the set of methods that are defined by the type.
Programmatic interfaces may be implemented either as RMI references
to the remote object that implements the service, as a local computation
that provides all of the service locally, or as some combination. Such
combinations, called smart proxies, implement some of the functions of
a service locally and the remainder through remote calls to a centralized
implementation of the service.
A user interface can also be stored in the lookup service as an attribute
of a registered service. A user interface stored in the lookup service
by a Jini service is an implementation that allows the service to be directly
manipulated by a user of the system.
In effect, a user interface for a service is a specialized form of
the service interface that enables a program, such as a browser, to step
out of the way and let the human user interact directly with a service.
In situations in which no lookup service can be found, a client could
use a technique called peer lookup instead. In such situations, the client
can send out the same identification packet that is used by a lookup service
to request service providers to register. Service providers will then attempt
to register with the client as though it were a lookup service. The client
can select the services it needs from the registration requests it receives
in response and drop or refuse the rest.
Service
Implementation
Objects that implement a service may be designed to run in a single address
space with other, helper, objects especially when there are certain location
or security-based requirements. Such objects make up an object group. An
object group is guaranteed to always reside in a single address space or
virtual machine when those objects are running. Objects that are not in
the same object group are isolated from each other, typically by running
them in a different virtual machine or address space.
A service may be implemented directly or indirectly by specialized hardware. Such devices can be contacted by the code associated with the interface for the service.
From the service client's point of view, there is no distinction between services that are implemented by objects on a different machine, services that are downloaded into the local address space, and services that are implemented in hardware. All of these services will appear to be available on the network, will appear to be objects written in the Java programming language, and, only as far as correct functioning is concerned, one kind of implementation could be replaced by another kind of implementation without change or knowledge by the client. (Note that security permissions must be properly granted.)
Conclusion
Bibliography
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| Jini Technology Home page |
http://www.jini.org
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| Jini From Sun |
http://www.sun.com/jini
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