At the Forge - Thinking about APIs
The idea that a Web site could provide a regularly updated, machine-parseable version of its content whetted the appetite of many developers for more. Many developers wanted a method to add and modify data, as well as retrieve it.
This came in several different forms, all of which still are used today. The first was XML-RPC, a simple RPC protocol that used HTTP to send an XML-encoded function invocation on a remote server. The server turned the XML-RPC request into a local function call and sent the result of that call (or an error message) in an XML-encoded response. The good news was (and is) that XML-RPC is simple to understand and use, that there are implementations in many different languages, and that they are generally compatible and interoperable.
At the same time, XML-RPC was unable to handle some of the more complex data types that people wanted to use. Plus, it didn't have the official seal of approval or complete standard that would have been necessary for it to enter the corporate arena. So, some of the original XML-RPC developers created SOAP (originally known as the Simple Object Access Protocol, but now an acronym that doesn't expand). SOAP is more sophisticated and complete than XML-RPC, but it had a great many issues with compatibility, implementation and complexity. Today, there are SOAP implementations for many languages, and it continues to be used in a variety of ways, despite some compatibility issues.
But, at the same time that XML-RPC and SOAP were being touted as the foundations for a new type of interactive, machine-parseable Web, along came Roy Fielding, who described the current state of affairs as unnecessarily complex. He proposed that instead of using XML in both the request and the response, we instead use Representational State Transfer (REST). In other words, the URL should contain everything needed to describe the request, without any additional XML markup or payload. The response, of course, could be in XML or any other format.
The idea of published Web services, procedures invoked via HTTP and URLs that transferred data in XML and other formats, soon became widespread. Creating and using Web services became the biggest thing, with every company talking about how it would take advantage of such Web services. Many standards were proposed for describing and finding Web services; for all I know, these standards still exist, but for the average programmer, they don't, and I'm not sure if and when they ever will.
Given two read-only protocols and three read-write protocols, it was a matter of time before people started to create applications that would take advantage of these. Amazon was one of the first companies to do so, opening up its catalog in a set of Web services now known as Amazon E-Commerce Services, or ECS. Amazon made its entire catalog available via ECS, and it allowed programmers to choose between SOAP and REST. Over the years, ECS has become an increasingly sophisticated and capable system, making it possible to retrieve particular slices of Amazon's catalog and pricing information.
But, retrieving information from Amazon is only half the story: Amazon also makes it possible to manage a shopping cart via ECS and even has some facilities for managing third-party products for sale. Amazon has made a huge commitment to ECS, and a large community of developers and third-party software vendors now exist around this facility. By turning Amazon into a platform for software development, rather than a simple on-line store, Amazon simultaneously has made a community of people dependent on ECS and has created opportunities for the creation of software applications that otherwise never would have been built.
eBay, Google and Yahoo! (among others) also have provided a number of APIs via Web services, which developers can use and access using various protocols. I've read reports, which I can't confirm but that I'm willing to believe, claiming the majority of requests submitted to eBay's servers are through its Web services. Given that most eBay users are not technically sophisticated enough to create their own HTTP clients, we may assume there are a number of software development shops that see eBay as a distribution platform, much as others might see Windows or Linux as their platform.
Google also has exposed a number of its applications to read-write APIs. Rather than use one of the existing protocols, Google uses a version of Atom for both requests and responses, along with a data format it calls GData. There are read-write APIs for a number of Google's applications, including the calendar, Blogger and the spreadsheet program. Programmers no longer are limited by the interface that Google provides to their spreadsheet data; they may create their own programs that use the spreadsheet for storage and retrieval. (One slightly far-fetched example would be the creation of a distributed database server that depended on Google's spreadsheet for locking and version control.)
Although new APIs of this sort constantly are being rolled out, the trend has seemed clear. Make the data easily available and downloadable by the users, in a variety of formats. And, make it possible for them to interact with your Web-based application either using your Web site or (alternatively) the command line or their own home-grown application.