At the Forge: Bloglines Web Services

More and more Web sites are offering machine-friendly versions of their services. Here's an example of a simple but useful service—updates on new Web site content.

Last month, we looked at ways in which we can gather, or aggregate, content from a number of different Web sites and put together a single summary of the day's news. Although it was amazing to see how much we could accomplish with a little bit of code, the application I presented is merely a toy when compared with actual aggregators. My example application supports only one user, is controlled by a primitive configuration file, doesn't categorize Weblogs into groups, checks for Weblog updates only when we explicitly ask it to do so and doesn't check for or handle errors.

Creating a robust, user-friendly aggregator is beyond the scope of this column, given the attention to technical and design details that would be necessary. But several days before I sat down to write this column, something amazing happened. The free, Web-based Bloglines.com aggregation service, which many people use to keep track of their favorite Weblogs, announced the availability of a Web service API that allows independent developers to create and deploy applications that use the data and applications developed by Bloglines. The publication and availability of the Bloglines API marks the growing popularity of Web services among well-known sites and opens the door to new applications built on the underlying Bloglines infrastructure.

This month, we take a look at the Bloglines API, including the creation of a simple application based on it. The API is brand new as of this writing (early October 2004) and undoubtedly will evolve as more people use it. If Weblogs interest you, and if you still are waiting to see practical uses for Web services, this combination of events might have come just in time.

What Is a Web Service, Anyway?

The basic idea behind Web services is quite simple: the Web's success is due in no small part to the fact that the client and server operating systems are irrelevant. So long as the client and server adhere to the HTTP and HTML specifications, they can communicate seamlessly. Linux has made inroads into the server space precisely for this reason.

Web services take this one step further, saying that computers and not people should be the biggest users of the Web. Although computers exchange information over HTTP, they send and receive data in XML, the markup language or meta-language, that has caught on like wildfire in recent years. If my computer can send XML in the HTTP request it sends to your computer, and your computer then returns XML in its HTTP response, we can exchange information regardless of what languages and operating systems we're using.

The original form of this service, known as XML-RPC, still exists and is great for fast, easy communication. But this idea was extended further, and a variety of data types, error-checking mechanisms and object serialization techniques were introduced that XML-RPC lacked. This extension became known as SOAP (Simple Object Access Protocol). SOAP theoretically can run on top of a variety of protocols, but it most often is sent on top of HTTP.

SOAP is a great solution to many problems, except that it is terribly complex, can be slow and is difficult to implement. And, both XML-RPC and SOAP require that the HTTP request include a well-formed XML request containing the query. One response to this growing complexity is REST (representational state transfer), in which all transactions are initiated by a simple HTTP GET request and all parameters are specified in the URL itself. The response then is an XML document containing the records and fields appropriate to the request. All of the Bloglines API calls are done with REST, although it's hard to say if this reflects the relatively simple queries now provided or if it's a design preference of the developers.

Although Web services probably are taking off behind corporate doors, only a few of the larger Web sites have made their plans and APIs public. The best-known examples are some of the largest and most profitable sites on the Web, including Amazon, eBay and Google. eBay charges for access to its Web services, with annual fees as well as per-transaction costs. By contrast, Amazon and Google have made their APIs freely available to the public, subject to usage restrictions and without making any promises regarding future availability.

In making its API public, Bloglines is indicating its interest in creating the same sort of developer community that Amazon, Google and eBay have created. This move also demonstrates its interest in remaining a leader in the world of Weblog aggregation and applications. Given Google's purchase of Blogger several years ago and the extensive search features that Bloglines is making available with its API, we might be witnessing the beginning of a new type of application or platform battle, with the Google and Bloglines APIs competing for attention.

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