International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
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Figure 2: The architecture of BP-GServices Peer
The first component is a GServices Manager and also the most
important component of BP-GServices. It can facilitate GIS
services discovery, GIS services composition and GIS services
Deploying. Corresponding to its functionalities, GServices
Manager consists of three sub-components, namely the
GServices Discovery Engine, the GServices Composer and the
GServices Deployer. The GServices Discovery Engine is
responsible for the publication and location of GIS services.
The GServices Composer provides facilities for defining new
composite GIS services from existing GIS services, and editing
existing GIS services (local), which is finished in a visual
interface (as a part of the user interface). The GServices
Deployer facilitates the binding and invocation of requested
GIS services as well as coordination of composite GIS services.
The second component is the GIS Web Services Agent System,
or simply GWSAgent. GWSAgent mainly provides the
environment on which mobile agents operate. Each BP-
GServices node in the system has a master agent that manages
the GIS services discovery and GIS services description
retrieval. In particular, it will clone and dispatch worker agents
to neighbouring nodes, receive GIS services or processed
results and present them to the user. It also monitors the
statistics and manages the network reconfiguration policies.
The third component is a Cache Manager, which is used for
caching the results of GIS services discovery and retrieval in
order to reduce the response time of subsequent answers. A
Cache Manager has the following functions: 1) cache remote
GIS services in secondary storage; 2) determine the
caching/replacement policy; 3) by collaboration among the
cache managers, form a P2P cache subsystem under the BP-
services framework so that all peers can share the caching
results among themselves as in Buddy Web (Wang, 2002).
The fourth component is the User Interface. Here the user
interface consists of several interface modules, corresponding to
GIS services discovery and retrieval, GIS services composition
and deploying. This provides a user-friendly environment for a
user to submit their GIS services query, to maintain their
sharable GIS services, and insert/delete their GIS services.
The system also includes the other three components:
GServices Key Indexes, Local UDDI Registry and Local
GServices Repository. Here, GServices repository can provide
GIS services provided locally. The description (or publication)
information of local GIS services is kept in the Local UDDI
registry. GServices Key indexes holds lists of services
keywords extracted from the description information of local
services, mainly business names and service types, etc, in order
to speed up GIS services discovery.
3.2 Nodes Initiating in BP-GServices
Each peer node in the system installs and runs the BP-
GServices software, which is firstly used by the new peer to
process the files it maintains. By now, the peer is only an
autonomous information system and is not a participant of BP-
GServices system. If the peer node wants to become a
participant of BP-GServices system, the following process is
taken.
1) The user uses an application table reflecting his/her
favourites to register with a LIGLO server, which is
similar to registering a mail server in Internet
environment.
2) Then, the LIGLO server will provide the node with a
global and unique identifier, i.e., BPID (BestPeerID),
which includes two parts: LOGLOID and NodelD. The
former is the IP address of LIGLO server, and the latter is
the unique ID for the peer assigned by the LIGLO server.
By the way, a user with the same BPID will be recognized
as the same user even if its IP address has been changed.
3) At the same time, the LIGLO server will also send the
node a list of peer nodes that have already registered in
the network, i.e., the initial direct peers of the node.
4) After the above steps were over, the node's initiating
process has been finished.
If a Peer P; who has been participant wants to rejoin the BP-
GServices system after disconnection of failure, taking the
following process (Ling, 2002):
1) Firstly, Peer P; sends its current IP address to its LIGLO
server to allow its LIGLO server to update its IP address
if it has changed.
2) Secondly, it sends an active message to each of peers in
its ConfidantCircle (say P;) to restore its connection with
them. If P; receives an. Active OK message from Pj, then
it restores their connection successfully. Otherwise, with
the help of LIGLO server, P; should first determine the
status of the confidant, i.e., InActive, Active but IP
changed, and then it takes corresponding actions.
3) By now, it has finished its rejoining process.
3.3 Neighbor Nodes finding in BP-GServices
In BP-GServices, neighbor nodes of a peer node are these nodes
that can provide more services similar to that in the given node
than other nodes. Here we use information retrieval method to
find neighbor nodes.
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