The Jakarta Project The mighty Tomcat - Meow!

Working with mod_jk

By Gal Shachor <shachor@il.ibm.com>

Table of Contents

What is mod_jk?

mod_jk is a replacement to the elderly mod_jserv. It is a completely new Tomcat-Apache plug-in that handles the communication between Tomcat and Apache.

Why mod_jk?

Several reasons:

What does it mean to me?

You will need to get to know a new simplified configuration mechanism. The advantage is that learning this mechanism will give you a head start if you want to deploy Tomcat on Apache and other web servers, such as Microsoft's Internet Information Server (IIS) and the iPlanet Enterprise Web Server.

Definitions and Terminology

In this document I am going to use a few terms, so let's define them:
 
Term Meaning
Worker Process A worker is a tomcat instance that is running to serve servlet requests coming from the web server. In most cases there is only a single worker (the one and only tomcat process) but sometimes you will run multiple workers to achieve load balancing or site partitioning. Each worker is identified to the web server by the host were it is located, the port where it listens and the communication protocol used to exchange messages.
In-Process Worker This is a special worker. Instead of working with a Tomcat process residing on another process, the web server opens a JVM and executes Tomcat inside the web server process address space. Our discussion in this document is not going to get into this special worker.
Web Server Plug-in/Tomcat Redirector For Tomcat to cooperate with any web server it needs an "agent" to reside in the web server and send him servlet requests. This is the web server plug-in, and in our case the web server plug-in is mod_jk. The redirector usually comes in the shape of a DLL or shared object module that you plug into the web server.
Plug-in Configuration We need to configure the web server plug-in so that it knows where the different Tomcat workers are and to which of them it should forward requests. This information, accompanied with some internal parameter, such as the log level, comprises the plug-in configuration. 
Web Server Configuration Each web server has some configuration that defines its behavior, e.g. on which port to listen, what files to serve, what web server plug-ins to load, etc. You will need to modify your web server configuration to instruct it to load the Tomcat redirector mod_jk.

Obtaining mod_jk

mod_jk can be obtained in two formats - binary and source.  Depending on the platform you are running your web server on, a binary version of mod_jk may be available.  It is recommended to use the binary version if one is available.  If the binary is not available, follow the instructions for building mod_jk from source.  Notes at the end of this section offer recommendations for specific platforms.

mod_jk Binaries

Binaries for mod_jk are available for several platforms in the same area as the Tomcat Binary Release.  The binaries are located in subdirectories by platform.  For some platforms, such as Windows, this is the typical way of obtaining mod_jk since most Windows systems do not have C compilers.  For others, the binary distribution of mod_jk offers simpler installation.

For example, the Tomcat 3.3 Release at http://jakarta.apache.org/builds/jakarta-tomcat/release/v3.3/bin/ contains the following:
 

linux/i386 Contains mod_jk.so for Apache 1.3 for the standard API as well as EAPI and mod_jk.so for Apache 2.0
netware/i386 Contains the mod_jk.nlm and nsapi.nlm
win32/i386 Contains the mod_jk.dll for Windows as well as other useful binaries.

Check the site for the latest binaries.

Note: The version of mod_jk is not dependent on the version of Tomcat.  The Tomcat 3.3 distribution of mod_jk will function correctly with other 3.x versions of Tomcat, such as Tomcat 3.2.1.

Building mod_jk

mod_jk is available in source distribution for all Windows and most Unix platforms.  The source for mod_jk is included in the Binary Distribution of Tomcat in the TOMCAT_HOME/native/mod_jk/ directory.  This directory is organized by Web Server name and version.  Each directory contains the source as well as the appropriate build scripts, make files, or project files.

Building mod_jk for NT

The redirector was developed using Visual C++ version 6.0, so having this environment is a prerequisite if you want to perform a custom build.

The steps that you need to take are:

  1. Change directory to the apache1.3 or apache2.0 source directory depending on your version of Apache.
  2. Set an APACHE1_HOME environment variable which points to where your Apache is installed.
  3. Execute the following command:


    4. MSDEV mod_jk.dsp /MAKE ALL

    If msdev is not in your path, enter the full path to msdev.exe. Also, ApacheCore.lib is expected to exist in the APACHE1_HOME\src\Debug or APACHE1_HOME\src\Release (or the older APACHE1_HOME\src\CoreD or APACHE1_HOME\src\CoreR) directories before linking will succeed. You will need to build enough of the Apache source to create the required librarie(s).

  4. Copy mod_jk.dll to Apache's modules directory.
This will build both release and debug versions of the redirector plug-in (mod_jk).

An alternative will be to open mod_jk.dsp in msdev and build it using the build menu.

Building mod_jk for Unix

Apache

  1. Make sure your Apache has DSO support. You can check this with $APACHE_HOME/bin/httpd -l. If you see "mod_so.c" in the output, DSO support is available. If it's missing, you may have to recompile or reinstall Apache.
  2. Find out whether your Apache has EAPI support. If you compiled it yourself from source, EAPI is probably not compiled in, unless you added it yourself (perhaps with mod_ssl). You need to build mod_jk.so with or without EAPI to match your Apache configuration. If you install a mismatched mod_jk.so, $APACHE_HOME/bin/apachectl configtest will warn you.
  3. Make sure you have Perl 5 installed. The apxs script used to build the module is written in Perl.
  4. Change directory to TOMCAT_HOME/native/mod_jk/apache1.3 (or apache2.0).


    5. Build mod_jk.so. Following are three techniques you can try, in order of simplicity:

    1. Run the build script for your platform.  If a build script is not available for your platform, you may be able to build mod_jk using ./build-unix.sh. This script will set some variables, call apxs as below, and try to copy mod_jk.so to $APACHE_HOME/libexec. If it fails, you need to do the following manually:
    2. If build-unix.sh fails, you may have better luck with the Makefiles in the same directory, e.g. "make -f Makefile.linux mod_jk.so"


      3. Finally, you can try to build it manually. Run the apxs command that came with your apache distribution (hint: look in /usr/local/apache/bin, /usr/sbin, or wherever you installed apache). Type the command all on one line.

      For Linux:

      apxs -o mod_jk.so -I../jk -I/usr/local/jdk/include -I/usr/local/jdk/include/linux -c *.c ../jk/*.c
      Your build may fail because the object files from the ../jk directory have been compiled to the current directory, rather than their source directory. Running gcc -shared -o mod_jk.so *.o should finish the build.

      For Solaris:

      Use the build-solaris.sh script as follows:

      # sh build-solaris.sh
      This will build and install mod_jk.so in your apache/libexec directory.  This script contains settings for your Java and Apache home locations.  Make sure that these are set according to your installation.  The default settings are JAVA_HOME=/usr/java and APACHE_HOME=/usr/local/apache.  If your installation is different, you will need to edit the build-solaris.sh script and change these values appropriately.

      See README.solaris located in TOMCAT_HOME/native/mod_jk/apache1.3 for more information.

      If the build script does not work, you can also build mod_jk as follows:

      $APACHE_HOME/bin/apxs -o mod_jk.so -DSOLARIS -I../jk -I/usr/java/include -I/usr/java/include/solaris -c *.c ../jk/*.c

      For HP-UX 11.00:

      Use the build-hpux.sh script as follows:

      # sh build-hpux.sh
      This will build and install mod_jk.so in your apache/libexec directory.  This script contains settings for your Java and Apache home locations.  Make sure that these are set according to your installation.  The default settings are JAVA_HOME=/opt/java1.3 and APACHE_HOME=/usr/local/apache.  If your installation is different, you will need to edit the build-hpux.sh script and change these values appropriately.

      Also note that there are two HP-UX build scripts.  One script was written to build mod_jk without JNI support using GNU GCC.  The other script builds mod_jk with JNI support, however, this script requires the HP ANSI C Compiler (not the stripped down C compiler included with HP-UX to rebuild the kernel).  The HP Compiler is required because the dlopen() and related shared libraries are only available for 64-bit applications and reliable 64-bit compilation is not available with the current version of GCC.

      The build-hpux.sh script should also work for HP-UX 10.00.

      See README.hpux located in TOMCAT_HOME/native/mod_jk/apache1.3 for more information.

      For other Unixes (including FreeBSD):

      The build-hpux-cc.sh script should be modifiable for IRIX and AIX.  Edit the script and change the APACHE_HOME and JAVA_HOME locations as required.

      If you are using EAPI, try adding -DEAPI to the apxs command after mod_jk.so.

      If apxs fails with apxs:Break: Command failed with rc=255, it may have been damaged by mod_ssl. Search for: 

      my $CFG_LD_SHLIB      = q();          # substituted via Makefile.tmpl
my $CFG_LDFLAGS_SHLIB = q(); # substituted via Makefile.tmpl
      and change to: 
      my $CFG_LD_SHLIB      = q(ld);          # substituted via Makefile.tmpl
my $CFG_LDFLAGS_SHLIB = q(-G); # substituted via Makefile.tmpl
      If you've installed Java in another directory, adjust accordingly.

      For other Unixes you should be able to work it out, but remember that the order of the arguments to apxs is important!.

  5. Now, copy the mod_jk library.  # cp mod_jk.so $APACHE_HOME/libexec. (Note that the build scripts attempt to do this, but you may have to su first.)

Other Webservers

There are several Makefiles in the other directories under the TOMCAT_HOME/native/mod_jk/ directory.  You should also check the Tomcat documentation for specific information related to other web servers.

Configuring Apache

This section details the configuration that is required for the Apache Web Server to support mod_jk.

Removing mod_jserv directives

If you've previously configured Apache to use mod_jserv, remove any ApJServMount directives from your httpd.conf. If you're including tomcat-apache.conf or tomcat.conf, you'll want to remove them as well - they are specific to mod_jserv.  The mod_jserv configuration directives are not compatible with mod_jk!

Configure Apache to use mod_jk

The simplest way to configure Apache to use mod_jk is to use Tomcat to generate the mod_jk configuration file and put the following include directive at the end of your Apache httpd.conf file (make sure you replace TOMCAT_HOME with the correct path for your Tomcat installation:

Include TOMCAT_HOME/conf/auto/mod_jk.conf

Example:

Include /usr/local/jakarta-tomcat/conf/auto/mod_jk.conf

This will tell Apache to use directives in the mod_jk.conf file in the Apache configuration.  This file is created by starting Tomcat with the "jkconf" option. Tomcat will initialize, write the configuration file and then exit. This may be done while an instance of Tomcat is running. Options for controlling how the mod_jk configuration file is generated are described in the configuring Tomcat section below [Configuring Tomcat].

NOTE:  If you plan to use the Tomcat generated configuration, skip the rest of this section and continue with the Configuring Tomcat section.

Custom configurations can be created by enabling the auto-configuration and copying the TOMCAT_HOME/conf/auto/mod_jk.conf file to your own configuration file, such as TOMCAT_HOME/conf/jk/mod_jk.conf.

The basic configuration is as follows:

A simple example would be to include the following lines in your httpd.conf file: 
LoadModule    jk_module  libexec/mod_jk.so
AddModule     mod_jk.c
JkWorkersFile /usr/local/jakarta-tomcat/conf/workers.properties
JkLogFile     /usr/local/apache/logs/mod_jk.log
JkLogLevel    info
JkLogStampFormat "[%a %b %d %H:%M:%S %Y] "

Assigning URLs to Tomcat

If you have created a custom or local version of mod_jk.conf as noted above, you can change settings such as the workers or URL prefix.

Use mod_jk's JkMount directive to assign specific URLs to Tomcat. In general the structure of a JkMount directive is: 

JkMount <URL prefix> <Worker name>
For example the following directives will send all requests ending in .jsp or beginning with /servlet to the "ajp13" worker, but jsp requests to files located in /otherworker will go to "remoteworker". 
JkMount /*.jsp ajp13
JkMount /servlet/* ajp13
JkMount /otherworker/*.jsp remoteworker
You can use the JkMount directive at the top level or inside <VirtualHost> sections of your httpd.conf file.

Configuring Tomcat

Enabling Tomcat's Apache Auto-Config

In most simple cases Tomcat can generate the needed Apache configuration. The default installation of Tomcat comes configured to generate a configuration file on demand for Apache to use mod_jk. Most of the time you don't need to do anything but include this file (appending "Include TOMCAT_HOME/conf/auto/mod_jk.conf") in your httpd.conf, as shown in the previous section (Configuring Apache).

To have Tomcat generate the configuration file, start Tomcat with the "jkconf" option. Tomcat will initialize, write the configuration file, and then exit. This may be done while an instance of Tomcat is running.

The generation of the mod_jk configuration file is controlled by the ApacheConfig module. The default installation of Tomcat 3.3 comes with the ApacheConfig directive already present in the TOMCAT_HOME/conf/server.xml.

Note: Generating the mod_jk configuration file on demand is new in Tomcat 3.3.  Older versions of Tomcat create the auto-config file without a directive in server.xml and generate it each time Tomcat is started.  The new ApacheConfig directive in Tomcat 3.3 allows for additional configuration options as detailed later in this section.  For older versions of Tomcat, refer to the documentation that came with that version.

If you have special needs, for example mounting URL prefixes that are not the default, you can use this file as a base for your customized configuration and save the results in another file. If you manage the Apache configuration yourself you'll need to update it whenever you add a new context.

Note that you must restart tomcat and apache after adding a new context; Apache doesn't support configuration changes without a restart. Also the file TOMCAT_HOME/conf/auto/mod_jk.conf is generated when tomcat starts, so you'll need to start Tomcat before Apache. Tomcat will overwrite TOMCAT_HOME/conf/auto/mod_jk.conf each startup so customized configuration should be kept elsewhere.  For example, copy TOMCAT_HOME/conf/auto/mod_jk.conf to TOMCAT_HOME/conf/jk/mod_jk.conf before making changes.  You'll need to start Tomcat with the "jkconf" option once to generate this file with your configuration for the first time.

It is also possible to specify the location of the auto generated files by setting options in the <ApacheConfig /> block.  The following details the syntax: 

< ContextManager ... >
  ...
  <ApacheConfig options />
  ...
< /ContextManager >
 where options can include any of the following attributes:  Example: 
...

<ApacheConfig configHome="/home/mydir" />

...

Note: The default mod_jk configuration file generated by: 

<ApacheConfig />
differs from the file generated by Tomcat 3.2.x. In Tomcat 3.3, the default is to forward all requests to Tomcat for Tomcat's contexts (i.e. forwardAll="true"), excluding the root context. This helps ensure that behavior configured in web.xml files will function properly. The configuration that best matches Tomcat 3.2.x behavior is: 
<ApacheConfig forwardAll="false" noRoot="false"
        jkConfig="conf/mod_jk.conf-auto" jkDebug="error" />
With this configuration, JSP and servlet requests to the root context will be served from Tomcat's ROOT webapp. Other requests, such as for static files, will be served from Apache's DocumentRoot directory. If Apache's DocumentRoot isn't updated to point to Tomcat's ROOT webapp, then the root context will malfunction. For example, JSP's that reference ".gif" files in Tomcat's ROOT webapp will not be able to display them since they won't be found under Apache's DocumentRoot directory.

(Optional) Configuring Tomcat to use the Ajpv13 protocol

mod_jk can use either the original Ajpv12 protocol or the newer Ajpv13 protocol. Both protocols are enabled by default. The "Ajp13" Connection Handler in Tomcat will give you the benefit of a faster protocol and the ability to identify requests made via HTTPS.

The default installation of Tomcat 3.3 comes with connectors for both protocols in the TOMCAT_HOME/conf/server.xml. The default choice by the ApacheConfig directive will be to using the "ajp13" worker which uses the Ajpv13 protocol. Even though mod_jk uses the Ajpv13 by default, you shouldn't remove the Ajpv12 connector present in the server.xml file. The Ajpv12 connector is also used for shutting down Tomcat.

(Optional) Defining "workers"

Configuring workers manually.

Workers are configured using the file TOMCAT_HOME/conf/jk/workers.properties. There is a great deal of information in the workers.properties howto document, and you should really look at that first. If you're in a hurry however, you can probably get away with editing the file workers.properties and setting the workers.tomcat_home, workers.java_home and ps variables to the correct values for your system.

Example Configuration

Here's an example configuration which probably reflects many real-world setups. A site is using Tomcat and Apache with two virtual hosts (one of them using HTTPS as well, which we're assuming is being handled by mod_ssl).

URLs ending in .jsp and beginning with /servlet are handled by Tomcat, the rest are handled by Apache. The files for each Host are server out of /web/host1 and /web/host2 respectively.

The example are over-simplified and incomplete but should get you started. Also note, this form virtual host setup is new in Tomcat 3.3 and later - this example won't work with Tomcat 3.2 and earlier..
  
.
.
<Ajp12Connector port="8007" />

<Ajp13Connector port="8009" />
.
Table 1 - Excerpt from server.xml showing the Ajp12 and Ajp13 Connectors.
  
<?xml version="1.0" encoding="ISO-8859-1"?>
<Server>
    <Host name="host1.apache.org">
      <Context path="" docBase="/web/host1" debug="0"/>
    </Host>
    <Host name="host2.apache.org">
      <Context path="" docBase="/web/host2" debug="0"/>
    </Host>
</Server>
Table 2 - Apps-vhosts.xml file showing two virtual hosts, each with a default context defined.
  
# Setup for Solaris system
#
workers.tomcat_home=/usr/local/jakarta-tomcat
workers.java_home=/usr/java
ps=/
worker.list=ajp12, ajp13

# Definition for Ajp13 worker (Ajp12 left to readers imagination)
#
worker.ajp13.port=8009
worker.ajp13.host=localhost
worker.ajp13.type=ajp13
Table 3 - Excerpt from workers.properties showing the Ajp13 worker
  
# Load mod_jk
#
LoadModule    jk_module  libexec/mod_jk.so
AddModule     mod_jk.c

# Configure mod_jk
#
JkWorkersFile /usr/local/jakarta-tomcat/conf/jk/workers.properties
JkLogFile     /usr/local/apache/logs/mod_jk.log
JkLogLevel    info

# First Virtual Host.
#
<VirtualHost 10.0.0.1:80>
  DocumentRoot /web/host1
  ServerName host1.apache.org
  JkMount /*.jsp ajp13
  JkMount /servlet/* ajp13
</VirtualHost>

# Second Virtual Host. Also accessible via HTTPS
#
<VirtualHost 10.0.0.2:80>
  DocumentRoot /web/host2
  ServerName host2.apache.org
  JkMount /*.jsp ajp13
  JkMount /servlet/* ajp13
</VirtualHost>

<VirtualHost 10.0.0.2:443>
  DocumentRoot /web/host2
  ServerName host2.apache.org
  SSLEngine On
  JkMount /*.jsp ajp13
  JkMount /servlet/* ajp13
</VirtualHost>
Table 4 - Excerpt from Apaches httpd.conf showing JK directives.

Troubleshooting and F.A.Q.s

Q. Where can I get help/support for mod_jk?

A. The primary mechanism for support is through the Tomcat Documentation included in the TOMCAT_HOME/doc directory.  These documents are viewable via browser through Tomcat http://localhost:8080/doc/index.html.  Documentation is also available on the Apache Jakarta web site for Tomcat at http://jakarta.apache.org/tomcat/tomcat-3.3-doc/index.html.

For additional help, the best resource is the Tomcat Users Discussion list.  You should start by searching the mail list archives located at http://www.mail-archive.com/tomcat-user@jakarta.apache.org/ or http://mikal.org/interests/java/tomcat/index.jsp before you post questions to the list.  If you are unable to locate the answer to your question in the archive, you can post questions about Tomcat or mod_jk to the user list for assistance.  Make sure that you include the version of Apache and Tomcat that you are using as well as the platform you are running on. http://jakarta.apache.org/site/mail.html

Q. I can't find mod_jk anywhere. Where is it?

A. Starting with Tomcat 3.3, the source for mod_jk is included with the binary distribution, in the native/mod_jk directory.  You can also download the Source Distribution of Tomcat to obtain the source for mod_jk, which is how it was obtained in versions prior to Tomcat 3.3. The Binary Distributions of mod_jk are available at the same location as the Binary Distribution of Tomcat.  The mod_jk binaries are located in subdirectories by platform. But in May 2001, the jakarta-tomcat-connectors was started and you'll find here up to date featured mod_jk (ie: new protocols AJP14/WARP)

Q. Which protocol should I use? Ajp12 or Ajp13?

A. Ajp13 is a newer protocol, it's faster, and it works better with SSL. You almost certainly want to use it. There is more information in the workers.properties howto document.

Q. Whenever I restart Tomcat, Apache locks up!

A. The Ajp13 protocol keeps an open socket between Tomcat and Apache. The latest release of mod_jk (the one found since Tomcat 3.3-m2 and J-T-C) handle the network failure. But with previous release of mod_jk, you may have to restart Apache as well.

Q. Why do two mod_jk.so files exist (-eapi ad -noeapi) in download dir for Linux ?

A. Many versions of Apache use a modified API, known at Extended API. For example, Apache using mod_ssl and the Apache present in certain recent Linux distributions. So if you have an 'Extended Apache', you need to use mod_jk.so-eapi, and use mod_jk.so-noeapi for standard Apache. It's wise to avoid using EAPI modules on standard API Apache and standard API modules on EAPI Apache. Always be sure to have the mod_jk.so for your version of Apache.

Q. What's that message about 'garbled DSO ?'

A. It's related to Apache EAPI. The message 'mod_jk.so is garbled - perhaps this is not an Apache module DSO ?' just told you are trying to install a mod_jk.so DSO module that was compiled on an Apache using EAPI, such as Apache-mod_ssl or Apache from Redhat distro 6.2/7.0. However, your system is using the standard Apache with the normal API.

Q. And the message about 'module might crash under EAPI! '

A. Also related to EAPI, the message '[warn] Loaded DSO /usr/lib/apache/mod_jk.so uses plain Apache 1.3 API, this module might crash under EAPI! (please recompile it with -DEAPI)', the mod_jk.so was compiled under normal Apache with standard API and you are trying to install the module on an Apache using EAPI.

Q. Where can I get more information?

A. The workers.properties howto document has considerably more in-depth information than this one, and is worth a look. You could also try searching the mailing list archives for "mod_jk" or look at the source.

Q. APXS is getting an error during the build of mod_jk, like rc=0 or rc=255.  I tried all of the steps in the build section, what do I do now?

A. APXS is a Perl script that is created when you build the Apache web server from source.  Chances are that if you are getting these errors and you obtained Apache as a binary distribution, that APXS is not configured correctly for your system.  Your best bet is to get the Apache source from http://httpd.apache.org and build it yourself.  Use the following for a basic build (read the Apache docs for other options): 
# cd /usr/local/src
# gzip -dc apache_1.3.19.tar.gz|tar xvf -
# cd apache_1.3.19
# ./configure --prefix=/usr/local/apache \
              --enable-module=most \
              --enable-shared=max
# make
# make install
Note: The above steps assume that you downloaded the Apache source and placed it in your /usr/local/src directory.

Q. Why doesn't my form authentication work? I'm getting similar error messages from Apache:

Not Found
The requested URL /login/j_security_check was not found on this server.
A. If you used manual configuration such as: 
JkMount /*.jsp ajp13
or 
JkMount /*.vm ajp13
mod_jk will pass requests for all JSP or Velocity pages to Tomcat. If form authentication is configured like this: 
<login-config>
  <auth-method>FORM</auth-method>
  <realm-name>Protected</realm-name>
  <form-login-config>
    <form-login-page>/login/login.vm</form-login-page>
    <form-error-page>/login/error.vm</form-error-page>
  </form-login-config>
</login-config>
then there is also another, somewhat hidden request involved in the authentication process: /login/j_security_check. It is the 'action' of the form specified within login.vm and error.vm, usually: 
<form method="post" action="j_security_check">
The request /login/j_security_check is not mapped, therefore mod_jk doesn't know that it should pass it to Tomcat. The following fixes the problem: 
JkMount /*.vm ajp13
JkMount /login/j_security_check ajp13

Credits

This document was originally created by Gal Shachor

Revisions by (Alphabetical)

Mike Braden <mikeb@mwbinc.com>
Mike Bremford
Chris Pepper

With help from countless others on the tomcat-dev and tomcat-user lists!
 
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