<tt>bgchannel://</tt> Considered Harmful?

Recently Bryan posted about background channels — a system for automatic updating desktop wallpaper. One of the features of the design is a new URI scheme based on the same ideas as webcal://, which I think is a bad idea (as dobey has also pointed out).

The usual reasoning for creating a URI scheme like this go something like this:

  1. You want to be able to perform some action when a link in a web page is clicked.
  2. The action requires that you know the URI of the link (usually to allow contacting the original server again).
  3. When the web browser activates a helper application bound to a MIME type, you just get the path to a saved copy of the resource, which doesn’t satisfy (2).
  4. Helper applications for URI types get passed the full URI.

So the solution taken with Apple’s iCal and Bryan’s background channels is to strip the http: off the start of resource’s URI, and replace it with a custom scheme name. This works pretty well for the general case, but causes problems for a few simple use cases that’ll probably turn out to be more common than you think:

  • Serving a background channel (or calendar, or whatever) via a protocol other than http. The first alternative protocol you’ll probably run into is https, but there may be other protocols you want to support in the future.
  • Any links to a background channel will need to be fully qualified since they use a different scheme. If you move your site, you’ll need to update every page that links to the background channel. If you could use relative URIs in the links, this wouldn’t be the case.

One alternative to the “new URI scheme” solution, that doesn’t suffer from the above problems is to serve a “locator file” from the web server that contains the information needed to request the real information. Even though the helper application will only get the path of a temporary file, the content of the file lets the app connect to the server. This is the approach taken by BitTorrent, and various media players like RealPlayer.

The separate “locator file” can even be omitted by placing the background channel location inside the background channel itself. This is the approach taken for Atom, via a <link rel="self"/> link.