The response format may be either image/gif, image/jpeg or image/png


To make efficient use of browser-side image caching, the Cover Image Resolver uses HTTP redirects. If an image is requested using its well-known URL that contains multiple ISBNs and a valid image is found for one of the ISBNs, the response will be an HTTP redirect to the well-known URL of the single valid ISBN. This ensures that individual images will be cached only once and that the cached copy will be used if possible.

Example – An image URL redirect in action

The following requests an image for a document that has multiple ISBNs:

The Cover Image Resolver determines that there is a valid image for ISBN 9781920898625.

The Cover Image Resolver returns a redirect as follows:

HTTP/1.x 301 Moved Permanently
Date: Tue, 10 Nov 2009 02:24:46 GMT
Content-Length: 0

The browser can then request the permanently redirected URL, which returns the following:

HTTP/1.x 200 OK
Content-Type: image/gif
Date: Tue, 10 Nov 2009 02:24:46 GMT
Content-Length: 7073

The redirect and the image are cached, so that when the next time the full three-ISBN url or the short single-ISBN URL is requested, the image will be pulled the from local browser’s cache.