A web server delivers static web content—for example, HTML pages, files, images, video—primarily in response to hypertext transfer protocol (HTTP) requests from a web browser.
An application server typically can deliver web content too, but its primary job is to enable interaction between user clients and server-side application code—the code representing what is often called business logic—to generate and deliver dynamic content, such as transaction results, decision support or real-time analytics.
The client for an application server can be the application’s own user UI, a web browser or a mobile app and the client-server interaction can occur via any number of communication protocols. However, in practice the line between web servers and application servers has become fuzzier, particularly as the web browser has emerged as the application client of choice and as user expectations of web applications and web application performance have grown.
Most web servers support plug-ins for scripting languages (for example, ASP, JSP, PHP, Perl) that enable the web server to generate dynamic content based on server-side logic. An increasing number of application servers not only incorporate web server capabilities, but use HTTP as their primary protocol and support other protocols (for example, CGI and CGI variants) for interfacing with web servers.
They also allow web applications to use services like reverse proxy, clustering, redundancy and load balancing—services that improve performance and reliability and allow developers to focus less on infrastructure and more on coding. To make matters more confusing, many web servers and some application servers are referred to or refer to themselves, as web application servers.
The bottom line is that today’s most popular web servers and application servers are hybrids of both. Most of the increasingly rich applications you use today feature a combination of static web content and dynamic application content, delivered through a combination of web server and application server technologies.
