Defining Web Services

Web services are a collection of protocols that are used to exchange data between disparate applications or systems. The essence of web services is the open standards on which they are built, by leveraging public and common protocols like HTTP, along with the XML document model. Web services are easy to implement with existing technologies. Not only are you (as a PHP developer) already familiar with many aspects of web services and the tools required, but you already have the facilities required to deploy them.

Why They Are Used

Simply put, web services allow information to easily pass from those who have it to those who desire it. Both feeds and APIs allow the requestor to obtain information from the service, regardless of the information type. Web services allow customers to access the information they desire, without the extraneous information generally presented on web pages. For example, running a search for a book on Amazon.com will yield several relevant results, generally (under the current layout) in the center column of the page. The right and left columns will contain other information, not directly related to your search (recommended titles, recently viewed items, and so on). Performing the same search via the API will yield the same results, but without that extraneous information.

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This points to a key factor in web services — they are used to obtain specific information, or complete specific tasks. Unlike people, the automated processes that utilize web services don't make impulse decisions (like choosing a recommended title from Amazon), so there is no point in presenting them with those types of options.

How They Are Used

Web services generally present information already available via another method (that is, a website). The advantage in the services is the consistent presentation of the information (in stark contrast to methods like screen scraping) in an easy-to-parse format. Here are some examples:

• Federal Express — Allows customers to do rate lookups and schedule shipments after logging into its online system. These same tasks can be accomplished via its API; as such, customers with larger shipping needs can integrate the API with their own systems to quickly automate shipping, and allow customers to determine how much it will cost to ship a particular product.

• Amazon — Allows customers to search for products via its website, and similar tasks can be accomplished via its API (often with more granularity than the traditional web interface provides). Using the API allows small booksellers to transparently integrate with Amazon to offer additional books and apply their own pricing.

• My Personal Blog — Allows people to read my thoughts on various professional matters. The feeds provided by my blog allow those people to read the posts on their own terms, within their own client, without ads or cute pictures of my cat.

It's important to recognize both types of interaction presented here. In some cases the web service is accessed directly by the end user (as is often the case with blogs and feeds). In other cases the service is consumed by an intermediate service, then presented (through various means) to its users.

Who Uses Them

If you accept my broad definition of web services, including both feeds and APIs, the answer is almost everyone. Most news sites offer at least some of their information via an XML feed. Blog sites almost without exception include some form of feed, and APIs are becoming more prevalent offerings from businesses of all sizes.

Also keep in mind that the scope of web services varies widely depending on who offers them. Many are merely informational in nature (such as the National Weather Service API), providing read-only access to information. Others, however, allow you to present information to the server; this could be purely digital in nature (such as adding a bookmark to your del.icio.us account), or it could set into motion a series of physical events (like scheduling a package pickup through FedEx).