This article will overview the relation between Apache HTTP Server and tomcat and also the reverse proxy.
For a long time tomcat/apache was a black-box for me because I did not have to manage it. But few years ago I had the opportunity to gain more knowledge on this subject. The aim of this article is to focus on the big picture of apache/tomcat and present the mod reverse proxy for Apache HTTP server.
What is tomcat ?
Tomcat executes Java servlets and renders Webpages JSP ( Java server page). This guide can help you to understand and run tomcat https://tomcat.apache.org/tomcat-3.2-doc/uguide/tomcat_ug.html
Install tomcat on linux :
Why use Apache if tomcat is a web server ?
Apache is more robust for HTML/images static content. For production environment it is necessary to have Apache HTTP web server combined with tomcat for dynamic content(JSP).
How apache and tomcat communicate together ?
I am not going into details since there is a documentation about it in user’s guide and also here https://tomcat.apache.org/tomcat-3.2-doc/tomcat-apache-howto.html
I am just going through the most important steps briefly and give real world example along the way. For information the example i am giving is in with tomcat version 8.0.23 and apache version Apache/2.2.15 (Unix).
What’s required to pull this off?
Answers to the above three questions!
1. Configure Tomcat
2. Install a web server adapter.
3. Modify Apache’s httpd.conf file.
1.1 Modify Tomcat’s server.xml file.
-> Create connectors (HTTP/HTTPS/AJP). A “Connector” represents an endpoint by which requests are received and responses are returned.
->The AJP connector is mechanism by which Tomcat will communicate with Apache.
1.2 Defining a context.
->It is NOT recommended to place elements directly in the server.xml file. Defined in context.xml instead.https://tomcat.apache.org/tomcat-7.0-doc/config/context.html
-> We can also defined here the jdbc configuration used to access the database. Jdbc Example under the context :
The Resources element represents all the resources available to the web application. https://tomcat.apache.org/tomcat-8.0-doc/config/resources.html . Example of resource within the context to mount the web app :
2.Install a web server adapter.
This adapter is not located in apache or tomcat configuration. It answers the question : “How will Apache forward these requests to Tomcat?”.
mod_jk requires two entities:
mod_jk.xxx – The Apache HTTP Server module, depending on your operating system, it will be mod_jk.so, mod_jk.nlm or MOD_JK.SRVPGM (see the build section).For example in our linux machine.
find / -iname ‘*MOD_JK*’ -print 2>/dev/null
workers.properties – A file that describes the host(s) and port(s) used by the workers (Tomcat processes). A sample workers.properties can be found under the conf directory in the source download.
Also as with other Apache modules, mod_jk should be first installed on the modules directory of your Apache HTTP Server, ie: /usr/lib/apache and you should update your httpd.conf file.
Mod_jk.conf – It is not necessarily needed to make custom changes of this file. There are situation where we need to make changes.
For information workers.properties and Mod_jk.conf are located under our module apache/conf.d
3.Modify Apache’s httpd.conf file.
We need to tell Apache how to load and initialize our adapter, and that certain requests should be handled by this adapter and forwarded onto Tomcat.Tomcat does most of the work for you.
Each time you start Tomcat, after it loads Contexts (both from the server.xml and automatically from $TOMCAT_HOME/webapps), it automagically generates a number of files for you. The two that we’re concerned with are:
tomcat-apache.conf (should really be named mod_jserv.conf-auto)
For example on my latest project our httpd.file we have simply a line to include all configuration files for apache including mod_jk.conf. :
NOTE : For this application we do not use mod_jk.conf-auto but our own custom configuration file mod_jk.conf.
More information at chapter “httpd.conf – Apache’s main configuration file” https://tomcat.apache.org/tomcat-3.2-doc/tomcat-apache-howto.html#httpd
Reverse Proxy in Apache
A reverse proxy (or gateway), appears to the client just like an ordinary web server. No special configuration on the client is necessary. The client makes ordinary requests for content in the namespace of the reverse proxy. The reverse proxy then decides where to send those requests and returns the content as if it were itself the origin.
We wanted to set up a reverse proxy in order to access to a remote web server running on different machine but on the same network. The idea was to access a servlet running on different application. It would save us duplicating the servlet and a database for our application. Instead of creating something existing we could just reuse an existing web server.
As you have seen previously we have loaded all .conf files in conf.d directory including our reverse_proxy.conf file.
Our configuration file for the reverse proxy is the following :
# Load the proxy module
LoadModule proxy_http_module modules/mod_proxy_http.so
Apache module mod_proxy :