This is not your father's Internet. When the Web was first emerging onto the scene, it was simple. Individual web pages were self-contained static blobs of text, with, if you were lucky maybe an image or two. The HTTP protocol was designed to be "dumb". It knew nothing of the relationship between an HTML page and the images it contained. There was no need to. Every request for a URI (web page, image, download, etc.) was a completely separate request. That kept everything simple, and made it very fault tolerant. A server never sat around waiting for a browser to tell it "OK, I'm done!"
Much e-ink has been spilled (can you even do that?) already discussing the myriad of ways in which the web is different today, mostly in the context of either HTML5 or web applications (or both). Most of it is completely true, although there's plenty of hyperbole to go around. One area that has not gotten much attention at all, though, is HTTP.
Well, that's not entirely true. HTTP is actually a fairly large spec, with a lot of exciting moving parts that few people think about because browsers offer no way to use them from HTML or just implement them very very badly. (Did you know that there is a PATCH command defined in HTTP? Really.) A good web services implementation (like we're trying to bake into Drupal 8 as part of the Web Services and Context Core Initiative </shamelessplug>) should leverage those lesser-known parts, certainly, but the modern web has more challenges than just using all of a decades-old spec.
Most significantly, HTTP still treats all URIs as separate, only coincidentally-related resources.
Which brings us to an extremely important challenge of the modern web that is deceptively simple: Caching.
I've spoken a great deal recently about architectural priorities. In short, we as software developers cannot eat our cake and have it too. Improving flexibility in one area may hurt performance, while improving usability one another area may hinder flexibility. These trade-offs are not necessarily right or wrong, except in the context of the goals and purpose of the project with respect to its target audience.
But what is Drupal's target audience, and how does that impact our architectural decisions?
DrupalCon Copenhagen was a watershed event in terms of understanding how to conceptualize that question, in my view, based on conversations I had with the likes of Mark Boulton, Jen Simmons, and Sam Boyer. In his (excellent) keynote, Jeremy Keith noted that the HTML5 Working Group had a specific, defined set of priorities:
In case of conflict, consider users over authors over implementers over specifiers over theoretical purity.
That may be a good priority order; it may be bad. That depends on your point of view and your goals. It lays out the order in which different stakeholders should be listened to, and if you come to a decision where you have to screw one group over to benefit another how that decision should be made. Having such a clear understanding of your constituent priority is critical to making good, consistent architectural decisions.
So what are Drupal's priorities?