When things aren't scalable


I know this site is for scalable web site design. But as there aren't any sites I can find for graceful failure under "slashdotted" like pressure I'll ask here.

Does anyone have a sensible way, once you have a "web application" that either won't scale, or can't scale, that you can give some users a good consistent experience and bounce other users to a busy site page. I have seen sites do this to varying degrees, some of which work better than others, but no explanations beyond simply bouncing requests to a "we're busy page server" when you have more than a given number of connections. This is obviously useless as a web page likely requires multiple connection (ignoring keep-alive, pipelining etc) multiple connection to completely render properly.

The normal problem is users getting a page and not the "furniture" for that page like images or css. Other problems are having to wait ages to get the busy page or the site being slow even if you do "get in". And some site let a user "in" and then as they browse around they get bounced out suddenly to the busy page.

Obviously not being the developer for sites I deal with (I am an infrastructure bod) I can't solve the problem where it should have been pre-emptively solved. That is to say I can't write the code to be scalable or re-write the code to do some simple session filtering or the like (and not being a developer I get dirty looks when I point developers at information like your site ... I can hear them thinking "how dare you suggest I don't know how to code a web site you lowly infrastructure cretin").

Before developer on-line lynch me I should point out that sometimes the cause of not being able to scale a site is that I can't get in new hardware quick enough, but then who knows when you will get slashdotted right ?. So my question applies even when a developer of genius level brilliance has built a unsurpasibly scalable web site for me to run the infrastructure for.

My best guess so far is using something like HAProxy to load balance sessions, and then use it's more advanced total session count, and cookie issuing abilities to track users and bounce some at a given "heavy load" point. This isn't ideal as the heavy load point would have to be based on connection counts not server load or server response times, but it's the best I can come up with so far.

Also, having mentioned brilliant developers writing great sites not always making my question redundant, could I ask, do people normally think about coping with overload when designing scalable solution - surely they should but I don't see much talk about it. Couldn't a simple Java filter or the equivalent for other things be built into applications ? It'd be nice to have a site that not only scales, but "is nice" when waiting for the infrastructure it runs on to be scaled, which could be several days when you have to purchase new hardware.