Posts on this blog get picked up by Facebook. Alison who's an experienced Web developer spotted my last post over there and couldn't make much sense of it. Hardly surprising, I referred to rather a lot of obscure stuff and used a lot of jargon without much explanation. But given that this affordances thing relates directly to the way everyone uses the Web, a developer should be able to make sense of it. So here I go again, this time trying to stick to the main points, glossing over the detail. [Blimey, but I've ended up rambling on a long while]
So on the Web you've got lots of documents in HTML on servers and lots of people with clients (browsers) that understand HTML. Those documents and various other messages are passed between server and client using the HTTP protocol. Most of HTML is about document structure, which with the aid of CSS can make text look good on the screen. But it has several things built in that allow a client to communicate over HTTP and hence allow the end user to interact with the Web. Most used is almost certainly the <a href="http:/example.org/here">something</a> link. When interpreted by a browser, that bit of markup highlights the word something and enables the link http://example.org/here to be followed by clicking on the something.
One fairly archaic definition of the word afford is to provide or supply (an opportunity or facility). Presumably this is where a 1970's psychologist got the word affordance (Wikipedia) which he defined as an "action possibility" (and some other stuff). This got picked up by human-computer interaction folks and mutated a bit, but "action possibility" is good enough here. So what the browser does with the bit of markup above - enables the link http://example.org/here to be followed by clicking on the something - can be described as an affordance.
The Web can be looked at as an information store with which we interact, and borrowing from database speak we have four basic operations: Create, Read, Update and Delete (CRUD). Through the highlighted, clickable link the browser provides the Read operation. When we want to Create e.g. a new blog entry, Update or Delete it we typically interact through a HTML <form>. So the kind of things a form enables can also be described as affordances. It's not unreasonable to expand the definition to include certain things the browser does that go beyond displaying a document with structure, things like displaying an image file that's linked to by an <img> element. Nowadays we're surrounded by loads of other different potential interactions thanks to Javascript and Ajax, these are also affordances. With the rise of blogging, online photo/video sharing and social platforms like Facebook, Twitter and now Google Plus, there's a new emergent breed of affordances that's been identified that include things like share, like, +1 etc. These are typically powered by Ajax and very often operate across sites and involve some data transfer, e.g. if you post a link on Facebook to a photo on Flickr it'll add it to your wall display a thumbnail of the image and the title. This new breed of affordances has been called Web Intents or Web Actions depending on where you look. (The Web Intents thread is I believe partly derived from a similar thing called Intents on Android phones, but having never used one I can't comment).
Ok, now there's an increasing amount of data on the Web expressed as Linked Data. This is published using the Resource Description Framework, RDF (depending on who you ask, linky non-RDF formats can also be considered linked data, but that's not really relevant here). The question is, how best to interact with this material, in other words what affordances do we need? There's a natural expression of documents on the Web - just show them as documents - but even for a passive display it's not altogether clear how to represent data. Ok, with traditional databases we usually have a table of some kind. But in that context we have a good idea in advance what can go in the rows and columns. On the Web, where the data can potentially be any shape it's a much trickier creature to pin down. With documents there is the familiar constraint of the individual document or page, whereas data doesn't chunk so neatly - the data we're interested in might be spread wide across the Web, between files containing only a handful of statements and stores containing millions. Links are part of the expression of the data, and links are the fabric of the Web, twisty eh? And this is just considering the Read aspect, there's also (at bare minimum) Create, Update and Delete to throw into the mix. We also need to not only interface with simple file-like linked data representations, there are also triplestores with SPARQL interfaces to consider (although the linked data API should help there, it can make a triplestore+SPARQL setup look more like normal Web representations).
However, to put these kind of problems into context - we don't need every possible operation for all data in all environments, far from it. One thing the work around Web Intents shows is that a handful of little facilities (share, like etc) are making a big difference in the benefit people get out of the Web. One thing that should really be avoided is making things as special cases - if you can share from A to B then you should be able to use the same mechanism to share from C to D and so on (this isn't that different from the centralized system setup, things on the Web should be distributed and ideally federated).
Ok, seems that affordances are going to be pretty important for working with the Web of Data. Some fairly good analysis has been done of HTML-in-browser affordances, and taking a leaf from the HTML book the simple hypermedia click-following of links seems a reasonable place to start in assembling suitable tools (in fact there are quite a few tools out there that support this in one form or another). It's fairly certain that some of the affordances will be a vastly different than those we're familiar with - data supports things like merging (trivial in RDF), query and inference, completely different kinds of transformation and analysis than text and so on. At the moment it's not even really clear that a general-purpose tool like the HTML browser is for documents makes sense for Web data (my guess is most likely a variety of different tools will be built inside the Javascript-capable browser, with different tasks being spread between clients and services).
But again to put these problems into context, there's no reason why any individual applications should be much different than they are today. Passing an image and its title between Flickr and Facebook requires the same basic machinery whatever kind of markup is used to describe the material. One of the aims for the Web as a whole, augmented by the Web of Data, has to be a reduction in complexity for common tasks. The fact that a whole new world of potential applications becomes feasible is just, well, interesting.
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