Sunday, February 21, 2010

HATEOAS - Hypermedia As The Engine Of Application State

Roy Fielding's PhD dissertation on Architectural Styles and the Design of Network-based Software Architectures is the seminal paper on the RESTful style of web architecture. Before discussing the paper, let's first discuss what it is, because it seems quite unusual in its approach as a PhD dissertation. Reading this paper in the 21st century, one might conclude that the paper is an "experience paper" (as defined here). However, as noted, "An experience paper that merely confirms that which is well known as opposed to that which is widely believed is of little value." However, in the early 1990's, the topic of the dissertation was probably not widely known. The irony now, is that as the term REST becomes more and more commonplace, and is applied to non-RESTful systems, the level of confusion over REST might be rising, not falling.

I was certainly confused for a long time about what constitutes a RESTful system. Isn't anything that has a URL RESTful? Definitely not, as the presence of resource identifier is just one of several properties that make a system RESTful. Fielding has recently written this article listing the properties he sees as essential to a RESTful system. After working quite a bit on the RESTful system for, it seems that me that the principle of Hypermedia As The Engine Of Application State (HATEOAS), identified on the original dissertation section 5.1.5, might be the most important principle of all. Incidentally, of all the properties outlines in the recent article, Fielding chose to title the article "REST APIs Must be Hypertext Driven". To my mind, that confirms my hunch that HATEOAS is perhaps the least understood principle of REST, and the principle in need of the most discussion and explanation.

The essence of HATEOAS is that the client needs to understand as little as possible. In essence, this is quite similar to the experience a human requires when he browses a website. In order to move from one page to the next, we must be given links that we can navigate. This is our common experience on web pages. Consider an alternative, and less user friendly,  scenario in which the home page simply consisted of a set of instructions (an API, if you will). One instruction might read "To view articles about sports, visit a URL consisting of "http://sitename/topics?name=topic-name" and replace topic-name with 'SPORTS'". You would be forced to manually construct the URL and enter it into the browser. The notion of such a website seems absurd, and yet it describes the scenario common to RPC or "fake REST" APIs in which the client is responsible for constructing the state-transition links rather than being handed a hypermedia document that provides the available state transitions. 

Quoting Fielding: "...all application state transitions must be driven by client selection of server-provided choices that are present in the received representations or implied by the user’s manipulation of those representations".

This is an interesting article that shows how non-HTML hypermedia can be used in keeping with HATEOAS. However, I think there are excellent reasons why Plain Old HTML (POH) is the best hypermedia choice for use with RESTful systems:

  1. HTML was made for hypermedia (it has well known structures for encoding links)
  2.  Your mom knows what HTML is
  3. By using POH for HATEOAS, your RESTful system's content can be indexed by search engines
  4. You can easily view and traverse your system states using a web browser
POH for HATEOAS is the approach taken by NextDB's REST system. In the documentation, we do describe the explicit structure of several of the URL's, but we stick strongly to the principle outlined by Fielding that, beyond the entry point URL, or "bookmark" you shouldn't need to know the structure of any of the URLs. Using POH for HATEOAS, the proof of NextDBs' adherence to this principle is quite straightforward: I simply give you an entry point URL (a bookmark) to a table and allow you to navigate the links served in the POH. For example, here is the bookmark to an account named 'geoff' with a database named 'testchars' and a table named 'lines':

From the URL above you can click on links to visit various pages of the content and sort the content. No knowledge of an API is required. Taken to its logical conclusion, I see no reason why the RESTful service should not be allowed to directly present styled content, when the consumer of the content is human. So that is exactly what we do in NextDB, by allowing the POH to include CSS. Here is an example:

We're techically violating HATEOAS because we don't provide links to the styled content, rather the developer has to know about the available styles by reading our documentation. However, we'll soon correct this, as well as allowing the developer to pass in links to his own CSS for inclusion in the returned POH.

Finally, not only do we allow you to style the POH, but we also allow you to apply XSLT transformations to the POH in order to alter the structure (as opposed to style) of the POH. Fielding discusses "Processing Elements" or "Transformers of Data" in his thesis. I believe XSLT to be the POH analog for processing elements, in that they are well understood, easily encapsulated in a markup, and supported by a wide array of processors (including browser-side processing, although this tends to be less portable, which is why NextDB opted to perform the transformation on the server).

In summary, NextDB is a truly RESTful database. This is important not because it conforms to a buzzword, but because it has made NextDB so easy to use that even non-programmers are able to embed the POH hypermedia in their sites. Project Have Hope is a great example of one such site. The "catalogs" of sponsorable children and women in Africa is POH served out of  The HATEOAS architecture is the key to opening the database content to CMS engine-driven sites, as well as accessibility to web search engines.

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