is Yahoo's resident MySQL Geek and he explains that the developer community requested Yahoo! to expose their search services. They saw it as a way to encourage 3rd party innovation. The initial goals were to keep it simple, collect feedback and create a low barrier to entry.

The Developer Community contains the SDK, a wiki, a blog, and general documentation of the API—which offers search on Web, News, Image, Video, Local plus a new feature of contextual search.

The architecture is based on backend clusters (for web, image, video, etc.) that are accessed by the application via a XML proxy developed in PHP. The proxy cleans up the XML results coming from the backend already existing internally to Yahoo! PHP is the de facto language of choice at Yahoo!

There's a restriction of 5,000 queries/day/service/IP - there's no developer token. So if you distribute your app each install wiill have its own limit.

The usual debate to decide for REST or SOAP also happened at Yahoo! "REST is 80% of the usage and SOAP is 80% of the support burden"—so REST was the choice. If there's community demand for SOAP they might consider implement it too.

Some cool apps were already created by 3rd parties and can be found here.