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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
case study, we present our ongoing project BP-GServices, i.e.,
BestPeer based GIS Web Services, and the major techniques of
BP-GServices implementation.
The reminder of this paper is organized as follows. An
overview of BestPeer is given in Section 2. Then, the BP-
GServices project details of architecture and major
implementing techniques are exposed in Section 3. Following
that, a simplified prototype is introduced in Section 4. Finally,
the conclusions are given in Section 5.
2. ABOUT BESTPEER
BestPeer' is a generic P2P system, which is designed and
implemented by National University of Singapore and Fudan
University of China, to serve as a platform on which P2P
applications can be developed easily and efficiently. Figure !
illustrates a BestPeer network.
LI GO Servers
Peers
Figure 1. BestPeer network
BestPeer system consists of two types of nodes: a large number
of normal computers (i.e. peers), and a relatively fewer number
of Location-Independent Global names Lookup (LIGLO)
servers. Every peer in the system runs the BestPeer software,
and will be able to communicate and share resources with any
other peers. There are two types of data in each peer: private
data and public (or sharable) data. For a certain peer, only its
public data can be accessed by and shared with other peers. A
LIGLO server is a node that has a fixed IP and running
Location-Independent Global Names Lookup Server software.
It provides two main functions: generate a BestPeer Global
Identity (BPID) for a peer and maintain peer's current status,
such as the current IP address and whether the peer is currently
online or offline (if this information is available). Through
LIGLO servers, a node can exactly who its peer is.
BestPeer overcomes the limitations of existing P2Psystems and
has following four good features (Ling, 2002; Zhou, 2002):
1l) |BestPeer combines the power of mobile agent and P2P
technologies into a single system. On the one hand, P2P
technology provides — resource-sharing capabilities
amongst peers and enables ad-hoc communication and
collaboration of the system. On the other one, since
agents can carry both data and code, they can effectively
perform any kind of functions. With mobile agents,
Bestpeer not only provides files and raw data, it also
provides processed and meaningful information. More
over, the use of agents allows Bestpeer nodes to collect
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information on the entire Bestpeer network, and this can
be done offline. This allows a node to be better equipped
to detérmine who should be directly connected peers or
who can provide it better services.
2) BestPeer not only facilitates a finer granularity of data,
files and services sharing, it also shares computational
power of peer nodes in the system. Since mobile agents
can carry data and code, the requester performs the
filtering task at the provider's end and gets processed
information. This feature has several advantages: (a) it
allows filtering to be performed where the provider's end
does not provide the capability; (b) it allows individual
requester to filter the content according to what (s)he
desires; (c) it facilitates extensibility — new algorithm or
program can be used without affecting other parts of the
system; (d) existing non-distributed objects can be easily
extended for use by a P2P application by leveraging on
the support provided by BestPeer; (e) it optimizes
network bandwidth utilization as only the necessary data
is transmitted to the requester.
3) BestPeer supports mechanisms to dynamically keep
promising(best) peers in some proximity based on some
criterion. Thus, BestPeer will always try to make a direct
connection to these promising(best) nodes that have
bigger possibility to provide information and data. In this
way, promising peers are first traverse before the less
promising ones. BestPeer currently supports two default
reconfiguration strategies: MaxCount and MinHops. The
former maximizes the number of objects a node can
obtain from its directly connected peers; while the latter
implicitly exploits collaboration with peers by minimizing
the number of hops.
4) BestPeer uses location independent global names lookup
(LIGLO) servers to provide each peer node of the system
with a unique global identity (BPID). By the way, each
peer node has a unique global identity that is different
from any other peer in the system even if its IP address
has been changed. A peer node with unique global
identity can communicate with any other peer node and
exchange sharable information. At the same time, LIGLO
servers can maintains peer's current status in the system,
such as the current IP address and whether the peer is
currently online or offline (if this information is available).
3. BP-GSERVICES.BESTPEER BASED GIS WEB
SERVICES FRAMEWORK
3.1 The Architecture of BP-GServices
As aforementioned, BP-GServices is an application system that
is designed and implemented on BestPeer. Similarly, the BP-
GServices also comprises two kinds of entities, i.e. several
LIGLO servers and a large number of normal peer nodes. The
former generates a BestPeer Global Identity (BPID) for a peer
and maintain peer's current status, and the latter takes the roles
of a services provider and a services consumer as well as a
services registrar. Thus, there is no central UDDI registry in
BP-GServices, all services and their services descriptions are
distributed over the peer nodes. Figure 2 illustrates the internals
of a BP-GServices peer node. The system is essentially
composed of seven components that are loosely integrated.
