An Internet Information Services (IIS) application pool is a grouping of URLs that is routed to one or more worker processes. Because application pools define a set of Web applications that share one or more worker processes, they provide a convenient way to administer a set of Web sites and applications and their corresponding worker processes. Process boundaries separate each worker process; therefore, a Web site or application in one application pool will not be affected by application problems in other application pools. Application pools significantly increase both the reliability and manageability of a Web infrastructure.

IIS Worker Process
An Internet Information Services (IIS) worker process is a windows process (w3wp.exe) which runs Web applications, and is responsible for handling requests sent to a Web Server for a specific application pool.

IIS Application Pool Availability
Web sites and Web applications depend on the availability of Internet Information Services (IIS) application pools. IIS application pools in turn depend on the Windows Process Activation Service (WAS). If WAS is not running or errors occur during the startup or shutdown of an application pool, Web sites and Web applications may not be available.
 
IIS Application Pool Configuration
To ensure Web site and Web application isolation, Internet Information Services (IIS) application pools must be configured to have unique names and correctly configured identities. If errors occur during the configuration of an application pool, the application pool may not be available to serve the Web sites and Web applications that are assigned to it.
 
IIS Application Pool Recycling
Internet Information Services (IIS) application pools can be periodically recycled to avoid unstable states that can lead to application crashes, hangs, or memory leaks. By default, application pool recycling is overlapped, which means that the worker process that is to be shut down is kept running until after a new worker process is started. After a new worker process starts, new requests are passed to it. The old worker process shuts down after it finishes processing its existing requests, or after a configured time-out, whichever comes first. This way of recycling ensures uninterrupted service to clients. However, if an application in the application pool cannot run more than one instance of itself at a time, overlapping rotation can be disabled.


Before Giving the Definition : you can say like this, Concept of Application pool has from IIS 6.0 .
Application pools are used to separate sets of IIS worker processes that share the same configuration and application boundaries. Application pools used to isolate our web application for better security, reliability, and availability and performance and keep running with out impacting each other . The worker process serves as the process boundary that separates each application pool so that when one worker process or application is having an issue or recycles, other applications or worker processes are not affected.
One Application Pool can have multiple worker process Also.

응용프로그램 풀은 IIS의 (동일한 구성 과 응용프로그램 경계를 공유하는) 작업자 프로세스들의 집합을 구분하는데에 사용된다.

Main Point to Remember:
1. Isolation of Different Web Application (다른 웹 응용프로그램과의 분리)
2. Individual worker process for different web application (서로 다른 웹 응용프로그램에 대한 개별 작업 프로세스)
3. More reliably web application (더욱 안정적인 웹 응용프로그램)
4. Better Performance (성능 향상)


<출처>
http://technet.microsoft.com/en-us/library/cc735247(v=ws.10).aspx
http://www.dotnetfunda.com/interview/exam1414-what-is-application-pool-in-iis.aspx

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