#ektron

I was talking with someone and in passing, they mentioned that IIS has a timeout for application pool recycling. At first I’m thinking the idle timeout, which defaults to 20 minutes and which I think most .NET developers know about and alter to suit their needs (or put in a keep-alive function). But they actually meant a real full recycling of the application pool, as if you did iisreset. It’s happening by default. It might even be happening now!

Here’s an article about it: http://blogs.iis.net/owscott/why-is-the-iis-default-app-pool-recycle-set-to-1740-minutes. Basically, every 29 hours (because Microsoft didn’t want it set to a strict 24 hours and 29 was the first prime number past 24), your application pool recycles. Great for health, but think about what this means. Every 29 hours, whether you’re in the middle of the work day or late at night, your application pool loses session state, application state, and caching state. And now, I think, you’ll see why I’m bringing this up...

I have a client who has, at times, said the site seems slower to them at certain times of the day. I know their network isn’t the best in the world (heck, they know it too) and I chalked it up to that. But what if this was an instance of a recycle event happening? There’s a lot of cached data in play here...user profiles, menus, news articles, etc...and to have to go back to the server not when I told it to, but because of an arbitrary system reset, is potentially a pain in the butt. I imagine if you’re running in the cloud, or running load-balanced servers, you’re more immune to this. But we all know some projects aren’t that distributed.

So, what can you do? Fortunately, there is an override. Head into IIS, go to your application pool, and choose advanced settings. Scroll down to the bottom, and you’ll see that “1740″ setting that represents 29 hours. Make that a big fat 0, and your application pool won’t recycle at all due to time. However, there’s probably value in doing it periodically, so if you look a little lower, you’ll see where you can set specific times to recycle. Aim for some off-peak hours, like the middle of the night.