A web server is computer software and underlying hardware that accepts requests via HTTP, the network protocol created to distribute web pages, or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiates communication by making a request for a specific resource using HTTP, and the server responds with the content of that resource or an error message. The server can also accept and store resources sent from the user agent if configured to do so.
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A server can be a single computer, or even an embedded system such as a router with a built-in configuration interface, but high-traffic websites typically run web servers on fleets of computers designed to handle large numbers of requests for documents, multimedia files and interactive scripts. A resource sent from a web server can be a preexisting file available to the server, or it can be generated at the time of the request by another program that communicates with the server program. The former is often faster and more easily cached for repeated requests, while the latter supports a broader range of applications. Websites that serve generated content usually incorporate stored files whenever possible.
Technologies such as REST and SOAP, which use HTTP as a basis for general computer-to-computer communication, have extended the application of web servers well beyond their original purpose of serving human-readable pages.