what is the difference between Web server VS Application Server.where application server is used and where web server is used. please tell me in
detail about web server and application server and their purpose.

Web servers are used to manage and make available web pages. The web server holds the web application and controls access to it and ensures security to a given extent. The web server recieves requests from its clients(Web browsers) and responds to those requests via mostly HTTP. Examples of web serves include IIS and Apache.

Application servers perform similar functions but rather serve mostly non web based clients. However application servers are used  in a variety of scenarios. In enterprise systems application servers can hold the business logic tier in a n-tier system. Application servers provide an environment for the application they hold to execute in.
They provide services such as multithreading, connection pooling, and scalability scope, security enforcement etc.

Application servers can be highly specialised in the functionality allowing one create a complete system based on the services that these servers provide. Examples include. IBM' web sphere, BEA Weblogic etc

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