The Term Tagging

The popular feature of ‘tagging’ content is nothing new. The average netizen should have encountered it by now. Tagging was made popular by sites like del.icio.us and flickr, where users can attach free-form strings, so-called ‘tags’ to their bookmarks and images. The viewer can then use these tags to navigate through one or more user’s contents and locate related content.

Scientific Background

The most-cited work on tagging is this research paper from HP, which starts categorizing tagging as ‘folksonomy’ (folk taxonomy) in contrast to the conventional term taxonomy. A taxonomy is usually a categorization of content according to a hierarchical and exclusive tree of attributes, while the folksonomy is neither. Also, usually a taxonomy is created by an individual or a small group, whereas the folksonomy can be created and expanded by any number of users.

Use of tagging

The most obvious use of tags attached to a set of items is to be able to quickly search for a subset without having to care about spelling or being limited because the items are only indexed by title. The most common user interface for searching according to tags is the ‘tag cloud’, a cluster of tags sorted alphabetically. For ease of use the font size in which each individual tag is shown corellates to the number of items it is attached to.

It is quite convenient to search in a tag cloud with a couple of hundred items. A drawback of the cloud, though is that one can only search for one tag at a time. When the search hits are shown, a list of related tags is shown that share at least one item with the chosen tag. The user can then proceed to view the items attached to one of these tags, but the first tag is subsequently forgotten.

To understand why tagging was invented it is neccessary to look at the prior status quo of hierarchical ordering like it is done in directories like dmoz.org, ISP portals or most probably in your browser’s bookmarks feature. Items are sorted into a kind of taxonomy. This leads to difficulties since the universe of topics is multidimensional.
One might have a folder ‘Tutorials’ for sites that contain tutorials, and a folder ‘Software’ for sites on software. Consider further, that he might want to divide the bookmarks between ‘Windows’ and ‘Linux’. The hierarchical model now makes it necessary to duplicate one set of the tree’s nodes to attach them to both branches, i.e. you either have a first-level dichotomy between Windows and Linux and two second level ones between Tutorials and Software or the other way round. Since there’s more aspects to expect, this is not a feasible solution, not even if you flatten out the hierarchy by combining the two aspects and create four folders.

The Alternative: Instant Hierarchy

A method that combines the flexibility of tagging with the search-narrowing power of a deep hierarchy is to combine the tags to an ‘instant hierarchy’: The user chooses from a pool of keywords. Like in tag clouds, he gets to see some items then and a list of subsequient keywords. He can then choose a second keyword and get the items that are tagged with both chosen keywords, and so on. This instant tree is at it’s deepest in the path of the item with the most tags.

These four images show an example of navigation through the instant hierarchy. The choice of keywords in the selectboxes narrows downn, in each step the items that match the exact set of keywords are shown. The best results are achieved when the items are tagged diversely and with a diverse number of keywords, because every step then show some information. If all items have, for instance, three keywords, the first two steps show nothing new.

Database-wise, this approach is a lot more complicated than the single-tag navigation of del.icio.us or technorati. Depending on the database available, n-fold joining or subquerying can be used. In both cases we start with a set of three tables, ‘bookmarks’, ‘keywords’ and ‘relations’ since we want to map an n:m relationship:

+--------------+  +--------------+ +--------------+
| bookmarks    |  | keywords     | | relations    |
+--------------+  +--------------+ +--------------+
| ID           |  | ID           | | ID           |
| url          |  | keyword      | | bookmark     |
| title        |  +--------------+ | keyword      |
+--------------+                   +--------------+

The ID columns are the primary keys (although the relations table might not need an extra key), the keywords should be unique anyway as long as no ownership is implemented.

Implementation

I assembled a small Addon for our OpenSource software PHProjekt and plan to release it on PHProjekt.com as soon as it’s presentable (it’s a hobby project so it proceeds a little slower than I would wish it to). If you are interested in a snapshot, drop me a mail (brotzeller at mayflower dot de). Currently it lets the user add and delete keywords and bookmarks and assign keywords to the bookmarks. The above screenshots show the navigation, which can also return xml and can therefore be used as sidebar in gecko browser. I also added a feature to get RSS feeds for keyword choices, so firefox users can add dynamic bookmarks. Since it’s about bookmarks, usage is observed via redirection, so the RSS feeds can also show new or most used or last used bookmark lists.