What Is a Web Service?

You can look at a Web service from a number of perspectives. The easiest way to view a Web service is as a means of obtaining access to information. Essentially, you ask the server for information and the server returns that information in some form. The request and the returned information normally appear in eXtensible Markup Language (XML) form. Using XML preserves the meaning behind the information, regardless of the diversity of the platforms involved, so that you receive not only the information, but understand the context in which the information is used. The "Understanding XML Basics" section of Chapter 3 tells you more about XML. All you need to know now is that you receive information in XML format.

From a Google Web Services perspective, you request information based on any of a number of search criteria. Google supports a number of search techniques and not every technique works well for every kind of search. Chapter 2 discusses search techniques in detail. For now, just think of the search criteria as a form of request. The request defines the kind of information you want to know and how detailed that information will be. Google Web Services returns the information you request (when available) in a standardized format.

Note Google's database schema specifies the format of the information. A schema defines the organization of information in a database. Fortunately, the format of the data returned by Google is relatively simple. You only have to consider a few return types. However, the content of the return data is a different story. Learn more about the Google database schema in the "Understanding the Google Data Output" section of Chapter 4.


A Web service also performs some type of useful work. The useful work might be something as simple as interpreting your request, calculating the answer, and sending the result back. In the case of Google Web Services, the Web service accepts your request in the form of a search request, interacts with the database through a search engine to obtain the information you requested, and sends the information back to you. The search can take various forms. For example, you don't have to search all Web sites—you can concentrate on just one. You might want to look for pictures, rather than text, and might only have an interest in newsgroups. The rest of the book shows how to perform all of these tasks. The main idea is that you can submit a variety of search request types—the request type affects the information you receive back from Google.

The final consideration for a Web service (at least from the Web service user perspective) is that it executes on the remote machine, not on your machine. In short, this means you're using resources on that other machine with the permission of the machine's owner. The remote machine can set requirements for using the Web service, as well as require you to perform specific setup and security checks as part of your request. In the case of Google Web Services, you need to obtain this permission by requesting a license. You also need to download the Google Web Services Kit to ensure you follow the terms of the licensing agreement. The "Downloading and Installing the Kit" section of this chapter tells how to obtain the required permission and what this permission means to you.

Tip You may find that Google Web Services is so indispensable that you'll want to work with Web services from other vendors. For example, Microsoft supports the MapPoint Web Service (http://www.microsoft.com/mappoint/net/). In time, standards organizations will set up directories of these Web services that you can access with ease. In the meantime, you can search for companies that offer Web services using the Web Services Finder page at http://www.15seconds.com/WebService/. Some people have problems using the Web Services Finder; it might produce an error instead of presenting a list of Web services. In some cases, you'll need to use a specialty Web service list such as the one at http://www.flash-db.com/services/. The Web services on this site are special because many of them perform one task well, such as providing you with a location based on a domain name.