Application pools are made up of a listening and routing structure in HTTP.sys and one or more ready-to-start worker processes that are waiting to process incoming requests. In worker process isolation mode, you can configure IIS to periodically restart worker processes under certain conditions. This automatic restarting of worker processes is called recycling.
Recycling an application pool causes the WWW service to shut down all running worker processes that are serving the application pool, and then start new worker processes. Whether the WWW service starts the new worker processes before it stops the existing one depends on the DisallowOverlappingRotation property in the metabase. Recycling an application pool does not alter any state in HTTP.sys or change any configuration in the metabase.
When an application pool is serviced by more than one worker process, the recycle operation is staggered in some scenarios. Only when a worker process is recycled on demand or when all of the worker processes hit their memory limits at the same time would they be restarted simultaneously.
For information about recycling an application pool on demand, see Configuring Worker Processes for Recycling.