Blog use cases
Currently, the main use of TrackBack is as a remote commenting system: if I post on my weblog about a post on your weblog, my weblogging tool will notify yours to inform you of that. Your weblog will then display the excerpt of the post that I made, with a link back to the post on my site. This allows visitors to your site (and you) to know what others are saying about your post--like comments, in other words, but the post is on my site instead of yours, as it would be if I had just left a comment. This gives me control over my post. If I want to fix a typo, or change some wording, then I can do that; whereas if I had left a comment on your site, I would not have access to change the text. In other words, TrackBack provides more control over your content.
The power of this method is that the TrackBack ping has created an explicit reference between my site and yours. These references can be utilized to build a diagram of the distributed conversation. Say, for example, that another weblogger posted her thoughts on what I wrote, and sent me a TrackBack ping. The conversation could then be traced from your original post, to my post, then to her post. This threaded conversation can be automatically mapped out using the TrackBack metadata. For example, this thread: http://www.sixapart.com/safari-thread.html. This is a diagram of the conversation started by this weblog post: http://www.sixapart.com/log/2003/01/initial_reactio.shtml.
Other use cases
Transaction or synchronization protocol
- responding to wantads
- bidding on self-hosted auctions or requests for quotations
Comment aggregation (collecting the comments of a single person across the Blogosphere into one place) could be implemented using TrackBack fairly easily.
Although TrackBack's most prevalent use thus far has been as a form of remote commenting, a more exciting use has been emerging: using TrackBack to aggregate content into topic-based repositories. This was actually the original intended use of TrackBack--the remote commenting grew out of a special case of a topic-based repository, the "topic" being a single weblog post.
Content aggregation sites collect content about a particular topic. If you've ever tried to look for weblog posts about a particular subject, it's pretty much impossible to do, unless the subject is a news story or something timely. If your subject is something like 80's Music, you'd have a much more difficult time finding all weblog posts about that subject. This is where TrackBack comes in: by establishing a repository for posts about 80's Music, other content authors can use TrackBack to automatically ping this central category. Anyone looking for weblog posts about 80's Music can come to this page to find pointers to such posts.
These content repositories can be either centralized, like the Internet Topic Exchange (http://www.topicexchange.com/), or distributed. In Movable Type, for example, you can set any of your weblog categories to receive TrackBack pings--this enables you to transparently become a source of information on a particular topic of interest to you:"
As time goes on and you invest more time and content into your weblog, you will likely want control over all of the content that you post on other weblogs and systems. For example, if you post your thoughts about a post on someone else's weblog, you'll want to post those thoughts on your own weblog, so that you are in control of them. Or, if you post a review on amazon.com, you may want this review to be syndicated on your own site.
TrackBack can be used to help with this. As an example, Matt Haughey's Posted Elsewhere sidebar (http://a.wholelottanothing.org/) aggregates content that he has written elsewhere. The reverse would also work: instead of posting the content on someone else's site, an author could post the content in his own weblog, then send a TrackBack ping to the other site. For example, if amazon.com reviews accepted TrackBack pings, you could control the content on your own site, and let amazon.com link back to you.
How does this use case differ from Remote Commenting? The quoted text was written before the Atom Publishing Protocol effort begun. This may overlap with APP now.
I think, eventually, the TrackBack notification format should supplant the weblogs.com ping format. Why? Although weblogs.com sports a plain vanilla HTTP POST interface, the implementations in weblog software are overwhelmingly RPC-based. This is not as good as it could be, because it means that endpoints other than weblogs.com need to use XML-RPC libraries to parse incoming pings.
And for what? Two lousy pieces of data. Compare this to the POST-based TrackBack format and the far richer data it contains. But a TrackBack isn't the same as an update notification. TrackBack creates a relationship from one post to another post on another weblog, or posts across categories. To my knowledge, no weblog software emits a non-contextual, TrackBack-compatible update notification.
The specific goal of replacing the weblogs.com API along is not a good goal. This API is in use by many public ping sites such as weblogs.com or blo.gs now and does its job. Having a simple, standardized and general purpose means for any system to communication a notifications of a new resource is a fine one though.
TrackBack could possibly be used as a way of standardizing the posting of bookmarks to social bookmarking sites. Alternatively this may be better suited to Atom Pub, it's not really clear to me which would be better.