By strict definition, a web server is a common subset of an application server.
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.
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The market is flooded with web servers and application servers—too many to list here. Instead, we thought it might be more valuable to list the most popular free, open source options available:
Nginx
Nginx (link resides outside ibm.com) is an open source web server that includes reverse proxy, load balancing, mail proxy and HTTP cache capabilities. Commercial, supported versions of Nginx are also available, at Nginx, Inc. (link resides outside ibm.com).
According to the internet research and cybercrime prevention company Netcraft (link resides outside ibm.com), Nginx served or proxied nearly 38% of all the world’s websites and over 25% of the million busiest sites as of December 2019. World-known enterprise Nginx users include Dropbox, Netflix and Zynga.
Apache HTTP Server
First released in 1995, Apache HTTP Server (also known as ‘Apache’) is another popular free, open source web server that, until recently, powered more websites than any other web server—71% at its peak—before being overtaken by Nginx in April 2019. As of December 2019, Apache served over 24% of all sites worldwide and 31% of the million busiest sites.
Apache Tomcat
Apache Tomcat (link resides outside ibm.com) is an open source application server that runs Java Servlets, renders and delivers web pages that include JavaServer Page code, and serves Java Enterprise Edition (Java EE) applications. Released in 1998, Tomcat is the most widely used open source Java application server.
Glassfish
Glassfish (link resides outside ibm.com) is an open source Java EE application server launched by Sun Microsystems in 2006, and it is hosted today by the Eclipse Foundation (link resides outside ibm.com). Like most Java application servers, Glassfish supports Java Servlets, Enterprise JavaBeans (EJB) and more, but it can also function as a web server, serving up web content in response to HTTP requests.
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