A tale of two servers

The Apache httpd web server and the Apache Tomcat server are two very popular products in use for delivering web content.

About the Apache httpd server

The Apache httpd web server grew out of the NCSA web server. At its core, httpd implements the HTTP protocol specification. It was primarily designed to serve static files to browsers. Over the years, many features have been added and the core web server can be extended through the use of modules. Some popular modules include:

• mod_rewrite : this module provides a powerful mechanism for the administrator to rewrite URL requests that come in or get sent out of httpd
• mod_cache : this module provides a top-level module framework for caching web content
  
• mod_ssl : this module adds strong encryption to httpd using the OpenSSL toolkit
• mod_dbd : this module adds a database connectivity layer to httpd
• mod_mime : this module allows the administrator to control actions within the server based on MIME types and file extensions, it allows for the defining of handlers and filters for output

The Apache httpd web server ships with over 70 modules and there is a repository of 3rd party modules to extend it even further (currently with 490 modules.)

About the Apache Tomcat server

The Apache Tomcat server is a project that started at Sun Microsystems as the reference implementation of the Java Servlet and Java Server Pages Specifications. The Tomcat code base was donated by Sun to the Apache Software Foundation in 1999. As the Java Servlet and JSP specifications develop and mature, Tomcat's versatility grows and expands. As implied, the Tomcat server is run in a Java virtual machine and requires, at least, a JRE (Java Runtime Environment) to run although it is more commonly run using a JDK (Java Development Kit.) Tomcat should run anywhere a JDK is available (Linux, Windows, UNIX, etc.).

Tomcat also implements the HTTP protocol and can be extended to execute non-Java scripts, such as Python, PHP and Ruby.

For the remainder of the article, the Apache httpd web server will be referred to as "Apache" and the Apache Tomcat server will be referred to as "Tomcat."

The relationship between Apache and Tomcat

Both Apache and Tomcat implement the HTTP protocol for serving files to browsers. Both can be extended to serve dynamic content using many popular scripting languages. So what are the differences? Why would you choose one over the other? To answer that, let's look at the strengths.

• There is no module to allow Apache to directly execute Java code. Since Tomcat is a Java application executing in a Java virtual machine, Java becomes the native language of the web applications it serves.
  
• It can be argued that the integration of scripting languages like PHP and Python is better with Apache than with Tomcat but this isn't easily proven or refuted.
  
• Apache generally has a smaller memory footprint than Tomcat and serves static content faster using fewer resources.
  
• Virtual hosting, aliasing and URL rewriting are, generally, easier to configure in Apache.

Because of these points, people usually use Apache for supporting scripting languages and serving static content and use Tomcat for Java applications. Where they merge together is in the use of an Apache module, mod_jk for the 1.3.x and 2.0.x versions of Apache and mod_proxy_ajp for the 2.2.x version of Apache. The module allows Apache to proxy requests between the browser and a Tomcat server seamlessly through the use of a binary protocol, with persistent connections, to streamline communications, making it the preferred mechanism for connecting the two servers.

Another way to connect Apache and Tomcat is to use the mod_proxy_http module in Apache 2.2.x. This is a generic HTTP proxy and does not provide the benefits of the mod_jk/mod_proxy_ajp modules.

The combination of Apache and Tomcat provides for a powerful and scalable web server architecture