Flickr has achieved prominence as a socially oriented photo site. Not only can members post their photos (as well as tag, categorize, annotate, and share them), but they can also comment on each others' work, join forums, and integrate Flickr services with other third-party applications. You can get a free Flickr account with limitations on how much bandwidth you can use to upload photos every month, or you can buy virtually unlimited bandwidth and storage with a "pro" account.
Flickr was bought by Yahoo! and can be accessed at http://flickr.com. The Flickr API, which we'll be looking at in this section, can be found at http://www.flickr.com/services/api, which also includes a link to the page to sign up for a Flickr key. Using the API requires a unique key, but there's no charge for its use.
Flickr's web services are some of the most complete, offering support for REST, XML-RPC, SOAP, a PHP library (since PHP is so ubiquitous), and recently, JSON. The latter is particularly important because it allows us to make direct service calls using Ajax without running up against the browser sandbox.
The Flickr API is split into classes of objects, such as photos, people, groups, and photosets. Within each class is a set of services that can be requested. The Flickr API site provides excellent documentation about what each service provides and what has to be passed to the service call. All service requests require the API key.
In the examples in this section, we'll use ...