How Do Web Services Work?

Many people fear new technology because they don't understand how it works, and many of those who do know how it works enjoy the mystique of knowledge too much to share it with anyone else. Web services are actually quite easy to understand if you look at them in a way that relates the task to everyday occurrences. For example, you might compare the operation of a Web service to making a withdrawal at the bank—the process really is the same. The one thing to remember is that the process a Web service uses to perform a task is always the same. No matter what technology you use to make a request or receive a response, the steps are still the same. Here are the steps that most Web services, including Google Web Services, use to complete a transaction.

The client discovers the Web service. During the act of discovery, the client might do things like download a file that tells how to interact with the Web service. This step is the same as someone walking into the bank. The person knows the bank exists and the bank teller might have noticed the person. The bank posts the rules for making a withdrawal or the teller might help a first-time customer understand the rules.

The client makes a request based on the rules delivered during the discovery phase. The rules might specify that the request has to appear in a certain form, and the client must provide specific data. This step is the same as the person walking up to the teller's window with a withdrawal request. The request must contain the person's account number, the amount they wish to withdraw, and other identifying information. The bank specifies the format of the request and the information it must contain.

The server might ask the client for credentials depending on the openness of the Web service. Google Web Services is public but still requires that you supply a developer license (account) number as identification. This step is the same as the bank teller asking you for a driver's license or other form of identification before honoring your withdrawal request.

The Web service performs the work required to honor your request. In most cases, the Web service accesses a database for information, it could enter an order, and it might even provide some level of formatting information about the original information (such as the typeface used for a word-processed document). Google Web Services performs a number of tasks depending on the request you make. The easiest request is a general search, but you can also perform checks such as making a spelling check. This step equates to the bank teller getting the money from the drawer and counting it.

The Web service sends the data to the client. The content of the information depends on the Web service. Google Web Services provides data in a very specific format based on the content of the associated database and the nature of the request. This step equates to the teller handing the person their money. In general, the teller orders the money in a specific way and counts it out to the person, rather than simply handing the money over.

The client logs out of the Web service or the Web service disconnects the client after some period of inactivity. This step equates to the person leaving the bank, money in hand. If the person doesn't leave the bank (they just hang out in the lobby), you can be sure that someone will ask them to leave.

The client does something with the data it receives. In many cases, it formats the data and presents it on screen for the user. This step equates to the person spending the money they receive from the bank.

You can add any amount of complexity needed to the individual steps, but these seven steps define the process every Web server follows. When you break a Web service down into these seven steps, the process that used to appear as magic suddenly becomes quite doable. Chapters 5 through 9 are essentially options you can use to perform these seven steps using different technologies. This book explores the seven steps using various languages and platforms—Google Web Services makes information available to just about anyone who needs it. However, it's important to remember that everything comes down to a client making a request and the Web server returning data.