Let’s face it, as an administrator your biggest concern with IIS is this: will it keep running your sites and applications reliably? Have you ever had a misbehaving application, one that leaks memory and starts to slow down your computer? Without dealing with the situation, your computer will eventually crash and need to be rebooted. Websites and applications are no different. Some of them are well-behaved little children running on your web server. Others are nightmares eating up memory and hogging processing. Without the ability to separate the good from the bad, you’d find yourself going to the office in the middle of the night to restart your web servers.
Application pools provide isolation to each website on a server, preventing one site from harming (crashing) another. Using them increases the web server’s reliability and the availability of each website. Think of the virtual machines you’re using for the labs for this book; each one has its own memory and processing allocation. If one VM crashes, it has no effect on the others, nor does it crash the host operating system.
You can use application pools to isolate websites and applications in several scenarios, such as the following: