Both peers and peer groups can offer services. A service offered by a peer individually, at a personal level, is called a peer service, a concept equivalent to centralization. No other peer needs to offer that service; if the peer is not active, the service might become unavailable.
Peer groups offer services called peer group services. Unlike peer services, these services are not specific to a single peer but available from multiple peers in the group. Peer group services are more readily available, because even if one peer is unavailable, other peers offer the same services. The key to Jxta lies in a set of common protocols defined by the Jxta community.
These protocols can be used as a foundation to build applications. Designed with a low overhead, the protocols assume nothing about the underlying network topology over which an application that uses them is built.
Performance scalability of the JXTA P2P framework
Figure 1. Figure 2. The peer-to-peer model Though peers all have equal status in the network, they don't all necessarily have equal physical capabilities. The Jxta solution Different protocols, different architectures, different implementations. From the Jxta vision statement: Project Jxta is building core network computing technology to provide a set of simple, small, and flexible mechanisms that can support P2P computing on any platform, anywhere, and at any time.
The project is first generalizing P2P functionality and then building core technology that addresses today's limitations on P2P computing. The focus is on creating basic mechanisms and leaving policy choices to application developers. XML in Jxta Undoubtedly, the first step towards providing a universal base protocol layer is to adopt a suitable representation that a majority of the platforms currently available can understand.
Understand the Jxta jargon Before proceeding any further, let's quickly look at the various concepts in Jxta.
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Peers Any entity on the network implementing one or more Jxta protocols. A peer could be anything from a mainframe to a mobile phone or even just a motion sensor. A peer exists independently and communicates with other peers asynchronously. Peer groups Peers with common interests can aggregate and form peer groups. Peer groups can span multiple physical network domains.
Messages All communication in the Jxta network is achieved by sending and receiving messages. These messages, called Jxta messages, adhere to a standard format, which is key to interoperability. Pipes Pipes establish virtual communication channels in the Jxta environment. Peers use them for sending and receiving Jxta messages. Pipes are deemed virtual because peers don't need to know their actual network addresses to use them.
That is an important abstraction. Services Both peers and peer groups can offer services. Codats can be published, discovered, and replicated if necessary. Advertisements An advertisement publishes and discovers any Jxta resource such as a peer, a peer group, a pipe, or a codat.
Advertisements are represented as XML documents. Identifiers Identifiers play a key role in the Jxta environment. Identifiers specify resources, not physical network addresses. World peer group Every Jxta peer is, by default, a member of the world peer group. Each Jxta peer intrinsically knows the world peer group and can join this peer group, even if it can't find any other peers on the network.
Even disconnected peers are members. Net peer group On a local network, the network administrators can usually configure a peer group that every peer on the network can join: the net peer group. This group resembles a DHCP dynamic host configuration protocol service. The net peer group provides peers with a global connectivity according to restrictions imposed by the administrators. Rendezvous peers A rendezvous peer is a special peer that stores information about other peers it knows about by caching these known peers' advertisements.
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Thus, a rendezvous peer can help peers discover other peers in the network. Rendezvous peers can also forward discovery requests to other rendezvous peers. Endpoints Endpoints are destinations on the network and can be represented by a network address. Peers don't generally use endpoints directly; they use them indirectly through pipes, which are built on top of endpoints. Routers Anything that moves packets around the Jxta network is called a Jxta router.
Not all peers need to be routers.
Peers that are not routers must find a router to route their messages. The Jxta protocols The key to Jxta lies in a set of common protocols defined by the Jxta community. Related: Enterprise Java P2P. Java tip: When to use composition vs inheritance. JDK The new features coming to Java Loop, switch, or take a break? Deciding and iterating with Java statements. The documents are displayed with the most recent first. A closer look at the information provided about each document reveals that the software provides basic bibliographic information, a extract or summary, and a visual cue about the type of file.
Shown in the image below, Scopeware's stream view displays documents using a notecard metaphor. Documents appear with the newest at the top of the stack. Each notecard provides information about the author, title, and date of the document.
Summaries of each document are generated for each retrieved document. You can click on the image to see a larger version. The Scopeware software offers the licensing organization several interesting options. First, Scopeware supports a range of wireless devices, including mobile telephones and such hand held devices as Palm, Handspring, and iPaq among others. No special programming is required to enable a user's access to documents or other types of content when away from a wireline connection.
Second, Scopeware has a legacy document module. Organizations can scan documents, index them, and then display the page image on the screen of a suitable viewing device.
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Although Adobe provides a version of the Acrobat Reader for PDAs, complex documents are best reserved for large-screen monitors. Third, Scopeware is a "data aware" network application. A user interested in a particular subject has access to the most recent information available. Scopeware recognizes new documents, launches the indexing modules, and performs the updating of queries as a native task. The system is almost automatic, and it delivers solid performance. Large documents do require bandwidth, so Scopeware performs best when there is sufficient network capacity to move documents from their location on client machines or servers to the user who requests them.
Finally, Scopeware can be integrated into existing network applications via an Applications Programming Interface. Note that Scopeware is a Java-centric program. It makes use of Java spaces to perform a wide range of alerting, indexing, and administrative services. Nevertheless, Scopeware works effectively within an Internet Explorer browser.
Scopeware uses the Sun ONE technologies. Scopeware is evidence that Java provides more or equivalent technical horsepower than Microsoft's Dot Net offerings. What does this mean to a organization planning to reduce its costs for computing and information services?
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In the short term, most organizations may not need to embrace a particular approach. Many factors will go into making a particular architectural decision. One certainty exists in the application world. Intelligent, Web-centric, network messaging applications are beginning to define the next "big thing" in software and system services.
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Peer-to-peer applications appear to be the easiest, cheapest, and fastest way to provide applications that integrate legacy data, support real-time messaging, and mesh with the types of work flows that are emerging in our post-Internet world. Groove Technologies Groove Networks, Inc.
Mirror World Technologies, Inc. Sun Microsystems www.