Because PaaS is a generic technology for running and scaling applications with a multitude of different language/framework/stacks, and many/most of those apps do not share the specific properties of single-threaded Rails (including many Ruby/Rails apps!)

And Rails 4 is going to bake-in "live streaming", making single-threaded app servers even more of an edge case.

That sounds a lot more like IaaS to me. PaaS should be providing the entire platform, hence the name.

The entire promise of the space is that the customer only has to worry about his own code and perhaps tweaking a few knobs.

That's like saying Craigslist did it correctly and AirBnB didn't because AirBnB is only tailoring to a specific segment of the world's supply and demand market.

Rails is very widely used. How can you consider that an edge case?

It's like saying EC2 should tailor its virtualization to Fedora 16, or Mac OS X should tailor its windowing system to Photoshop CS4, or Apache should tailor mod_proxy to Joomla. There may be specific attributes of popular applications that need to be adapted to, but those adaptations need to be built in a generic way and exposed through a standard API.

Since even many Rails apps now do not follow a single threaded request-response model, that model of running a web application needs to be considered as one case of many, and building a platform that supports many/all use-cases as well as possible is more complicated than building a platform that fits one use case like a leather glove.

> Mac OS X should tailor its windowing system to Photoshop CS4

I think statements like these obscure away the very tight coupling Heroku has historically had with Rails. While certainly Heroku now perhaps envisions itself as a do-it-all PaaS, there's no denying Rails at one point (and, numerically, perhaps still) was their bread and butter.

While I don't have numbers to support or refute the assertion that "most Rails apps are primarily single threaded", my suspicion is that this is in fact still the case.

> or Mac OS X should tailor its windowing system to Photoshop CS4

I'm taking this example out specifically, because it illustrates my point quite nicely.

If there were a sizeable community of people who only wanted to use a computer for Photoshop, and tailoring the windowing system to them made it a significant usability improvement for those people, then it would be a completely imaginable situation that upon first opening your brand new Mac, it'd ask you whether you're one of those Photoshop people and want the special windowing system setup.

Well, ok, haha Apple and customizing anything for anyone, ever. But many other vendors might make such a choice.

The apparent stubborn refusal of many PaaS services, including Heroku and yours, to particularly tailor to a very common configuration of Rails sounds like a hole in the market, to me. As a customer, I don't care whether this is "incorrect" because Rails does not conform to some yet-to-be-defined standard or simply because the PaaS doesn't have their shit together. The customer experience is the same: I'm running a blog tutorial app, and the performance sucks.

Microsoft have definitely done exactly this. Windows versions have famously been "rebroken" in development to keep bug-for-bug compatibility so they didn't break big third party applications.

The 'P' stands for platform. Providing the platform as a service absolutely means catering to the specific needs of the platform. There is no generic platform. If you want to support multiple platforms, then you support multiple platforms. You don't stop supporting any platform at all.