A message shared between the content provider and an OPDS client. They're meant to do all the hard-work for you – download your content, provide extra metadata, add your shelf and provide a catalog – all with a single click.
Callbacks are an ideal way to link the physical world to the digital world.
Imagine walking into your favorite indie book retailer, but to buy digital content.
You can browse around the store or chat with a sales clerk to get recommendations.
Once your selection is done, you can pay with cash or any other payment method.
No need to set up any kind of account, just give your name and email and you instantly receive a link (to an OPDS callback).
When you open this link, the reading application receives metadata about your purchase and immediately downloads the content.
Opening the link also sets up your bookshelf and the retailer's catalog for you.
All your future acquisitions (purchases from a retailer, books borrowed from a library) will be automatically downloaded on your device from now on.
You can also continue browsing for additional content to acquire since you can now access the full catalog, anywhere you want, right from your device.