I am not arguing that the documentation calls the module "experimental", but if you follow the discussion by the developers about this module you will find out that the fact that it is still called this is due only to a combination of 1) the major version of Apache not having been bumped in a very long time, 2) a number of Apache modules in the wild that are poorly coded (not that I've ever managed to actually find one), and most importantly: 3) it does not work on all platforms (Linux is great). Things that have previously been considered "the reason" mpm_event is marked experimental are all now obsolete; for a specific example, SSL now works 100% correctly with mpm_event.

Also, nginx being a "darling of many high traffic production sites" does not mean it actually works well for this purpose: if you do a Google search for "nginx" and "ephemeral" you get lots of evidence to the contrary, and you can also prove my statements from first principals of TCP if you really don't believe me: this need not be based on silly anecdotes, you would simply expect nginx to have issues with ephemeral ports due to the way it is designed and implemented (a reverse proxy making new outgoing connections for each incoming one), and if it didn't you would be surprised and probably want to publish a paper on it. ;P

""Compared to putting tornado processes behind nginx, this approach is simpler (since fdserver is much simpler than configuring nginx/haproxy) and avoids issues with ephemeral port limits that can be a problem in high-traffic proxied services.""" -- http://tornadogists.org/1073945/

"""This makes load testing complicated since the nginx machine quickly runs out of ephemeral ports.""" -- http://mailman.nginx.org/pipermail/nginx/2008-February/00352...

My service has tens of millions of users distributed worldwide, making many billions of requests per month to my hostnames. My setup is mostly coded for mod_python (generally considered to be an "older module", especially considering it is no longer even maintained by the upstream developers). I make complex usage of requests making recurrent subrequests through different languages. A good amount of my traffic is SSL.

Of course, most requests are cached at the CDN, so they don't have to go through to my backends, but I still handle way more than a billion requests all the way through to my dynamic webapp every month. These are all handled, eventually, by two boxes running Apache, and I only need two boxes because I want to handle one of them randomly failing (I can easily handle the load on one box: each box can handle, and actually has under previous concepts for my architecture, 3200 concurrent clients).

As for mpm_event in this environment? It works, is stable, is why I could handle 3200 concurrent clients, and you should not be avoiding it because you feel it is "experimental" (yes, even with mod_python). I did run across one or two Linux kernel builds that had regressions that affected Apache+mpm_event (horrible concurrent performance), but you are better off noticing that and steering away from them than avoiding mpm_event.

That said, I want to make it clear that I am not arguing against reverse proxies: I am only making the point that your CDN /is/ a reverse proxy, so there's little point in additionally adding nginx to the setup unless you can't handle enough concurrent connections from the master CDN nodes around the world, in which case what you really want is "just" a load balancer, and you really still want one that is smart enough to use HTTP/1.1 to connect to its backends, and that simply isn't nginx. (Humorously, DNS round-robin, if you think of it as a load balancer, actually works great for this HTTP/1.1 problem, but there are other reasons to avoid it, of course. ;P)

(Now, this said, I heard a few days ago that the just released nginx 1.1 branch now supports persistent backend connections, but I haven't been able to find it in the release notes.)

(Also, as your comment about "now and then you need an actual web server to do actual work" implies to me, but this might totally be incorrect, that you didn't yet notice that a CDN actually provides insanely high latency benefits even if all of your content is dynamic and all of it has to go through to the backend. If you did not know this, you should read my commentary here: http://news.ycombinator.com/item?id=2823268 .)