Application Isolation Modes
Applies To: Windows Server 2003, Windows Server 2003 with SP1
IIS 6.0 runs a server in one of two distinct request processing models, called application isolation modes. In each isolation mode, IIS functions differently, although both application isolation modes rely on HTTP.sys as the HTTP listener.
Worker process isolation mode
Worker process isolation mode takes advantage of the redesigned architecture for IIS 6.0. In this application isolation mode, you can group Web applications into application pools, through which you can apply specific configuration settings to groups of applications and to the worker processes servicing those applications. By using application pools, you can run all application code in an isolated environment; however, unlike earlier versions of IIS, IIS 6.0 provides isolation without a performance penalty, because there are no added process hops.
Worker process isolation mode provides application (or site) compatibility for most existing applications. This application isolation mode is specified by a false value for the metabase property IIs5IsolationModeEnabled.
IIS 5.0 isolation mode
IIS 5.0 isolation mode is provided for applications that depend upon specific features and behaviors of IIS 5.0. Use this mode only if an application has a compatibility issue when it runs in worker process isolation mode and you cannot resolve the problem.
In this application isolation mode, you can isolate applications as you did in IIS 5.0: in Low isolation (in-process), Medium isolation (pooled out-of-process), or High isolation (out-of-process). This application isolation mode is specified by a true value for the IIs5IsolationModeEnabled metabase property.
Whenever possible, run your server in worker process isolation mode, which is more secure and reliable than IIS 5.0 isolation mode and is also the default application isolation mode for a clean installation of IIS 6.0. For more information about IIS 6.0 application isolation mode defaults, see Application Isolation Mode Defaults.