Client-Server Architecture

Common Examples

Common examples of client-server architectures include e-mail exchange, database access, and Web access. The most obvious example is Web access. In this case, the Web browser (e.g., Microsoft Internet Explorer or Mozilla Firefox) is the client. When one types a Web address into the Web browser, the browser (the client) initiates a connection to the Web address (the server). The server responds based on the request sending back information to be displayed in the Web browser. In this example, the response is typically a Web page.

Geospatial Examples

Geospatial examples include Google Maps and Microsoft Virtual Earth. Both of these examples use a Web browser as the client to connect to servers hosting large amounts of geospatial data that are returned, in the form of maps, and displayed by the server. Another common geospatial example is virtual globes, such as NASA World Wind and Google Earth. In this case, the client no longer uses a Web browser; rather, a custom application is installed. Both NASA World Wind and Google Earth have highly customized servers that are designed to provide spatial information to a specific client; this creates the base data for the virtual globe. These clients also have the ability to connect to any number of other servers that adhere to certain Web standards. Additionally, desktop geographic information system (GIS) software, such as ArcGIS Desktop (from the Environmental Systems Research Institute), ENVI (from ITT), and Quantum GIS (open source), can be considered clients in client-server architecture. All these products have the ability to connect to both local and remote sources of data. A user can access data in a remote database or via geospatial Web standards, and in doing so, these software products are acting as clients contacting a server. In these cases, the data being returned are often very different from what is returned to a Web browser as these clients can deal with many complex remote sensing and GIS data formats. However, the underlying architecture remains the same—the client makes a request to a server that is awaiting such requests, and the server processes the request and returns a response.

Open Geospatial Consortium Standards

The client-server architecture relies on standards for communication for clients and servers to easily communicate with one another. In the case of general Web communications, this standard is the Hypertext Transfer Protocol (HTTP). Additional standards can be used in addition to HTTP for specific types of communications. The Open Geospatial Consortium (OGC) has developed and maintains a number of standards for requesting and receiving geospatial data via client-server architecture. While these standards are too numerous and complex to cover them all in detail here, a brief overview of some of the more common standards is warranted. The Web Map Service (WMS) is used for returning maps as images. It is important to note that the returned map is not data; it is just a picture of data. The Web Feature Service (WFS) is used to request/return feature data such as points, lines, and polygons. The Web Coverage Service (WCS) is used to request/return raster data. Unlike WMS, both WFS and WCS return actual data, not just images. These services among others are commonly used to allow clients to request data from servers via the Web.

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