| WebGIS is a kind of technology, which employs the Internet as its environment, adopts Web pages asthe user interface of GIS software, combines the GIS technology with the Internet, and provides all kinds ofGIS application with GIS function. It unites Web technology, GIS technology and database technology as awhole, changes the getting, transferring, sharing and applying ways of the spatial data by a new workingand data sharing mode. All of the data resources in the system are stored on the server side. The clientside needs to send service request to the server side in order to get data. After receiving the request, theserver side returns variable data depending on the request. The structure of this kind of system owns theadvantages of cheap developing and applying cost, wide range available to visit, easy to operate andexpand, relatively independent and so on. As a result of it, it's applied in all kinds of GIS field, and showsthe direction of the GIS development in the future.However, there are also some shortcomings in the present WebGIS. In the condition of the upsurge inthe number of clients, it's easy to form "bottleneck" of computing resources and network bandwidth in theserver side, causing the server load and network latency. It can affect the user's normal use in severecondition.The above problem can be resolved in the following two ways:(1) improving the configuration of themachine and the network bandwidth, mainly by improving the hardware conditions;(2) Changing thepresent WebGIS data transferring mode, in order to ensure the balance of the server side load and the highspeed of data transferring, is realized mainly by designing reasonable system structure. It's impossible toimprove the configuration of the machine by improving the hardware conditions in a short time since itneeds a lot of money. Therefore, in order to improving the capability of the GIS application by using thesecond method (changing the data transferring mode on the whole), this paper deals with some workmainly on the following two areas:1, Introducing the data transferring mode of the peer-to-peer into present WebGIS, improving thecurrent system structure, thus changing the data transferring mode. We call the improved structure as theP2PWebGIS structure. It makes every client node devote itself by using the character of every client nodeserving as the server side for each other, thus realizing active collaboration of the device in the edge. The client can get the data from the server and can also obtain data from other nodes within the peer when thereare heavy loads on the server. All clients within the network are able to be aware of each other's presence,forming a virtual group, which, to some extent, can solve "bottlenecks" problem when a large number ofusers visiting the server side.2, Developing the cache policy for the client of WebGIS--L&Nbuffer. The L&Nbuffer can increasethe number of server-side copy of the data within the peer, therefore, reducing the number of Client Accessserver and reducing the degree of pressure of the server by improving the P2PWebGIS mode. This strategydivides the cache space into the local cache (Lbuffer) and neighbor cache (Nbuffer). Lbuffer uses FIFOcache replacement algorithm, while Nbuffer cache adopts replacement algorithm based on data popularityand number of copies of data, making the data resources which are of less number and high popularityalways be retained in the limited buffer space to provide services for other nodes better.On the basis of the above, this paper builds a WebGIS of P2PWebGIS structure and tests the pressurebeard by the server and the speed of data-getting in the client side by simulating a large number of users'accessing to it. A large number of test data show that P2PWebGIS mode can resolve the server-side"bottlenecks" problem. With the rapid increasing in the number of users, the load on the server has been ina relative balanced state, and the speed of the client getting date shows a rising trend with the increase ofthe client user. |
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