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giovedì 18 novembre 2010

tomcat mod_jk tutorial

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

In this tutorial, we assume that a stable Apache Web Server 2.X has been installed on your host. The next step in the checklist is downloading the latest stable release of Tomcat mod_jk, available at


Once downloaded, the module mod_jk.so should be copied in your Apache module directory (usually located in the APACHE_ROOT/modules directory). Check your Apache documentation if you cannot locate it.

Windows users are encouraged to rename the binary file to mod_jk.dll if the downloaded Windows module bears the .so extension. This way you will not confuse this library with a compiled library for Unix.
The configuration of mod_jk can be included into the Apache httpd.conf file or held in an external file, which is a good practice:

# Load mod_jk module
LoadModule jk_module modulesc/mod_jk.so # UNIX
# LoadModule jk_module modules/mod_jk.dll # WINDOWS
# Where to find workers.properties
JkWorkersFile /etc/httpd/conf/workers.properties
# Where to put jk shared memory
JkShmFile /var/log/httpd/mod_jk.shm

# Where to put jk logs
JkLogFile /var/log/httpd/mod_jk.log
# Set the jk log level [debug/error/info]
JkLogLevel info
# Select the timestamp log format
JkLogStampFormat "[%a %b %d %H:%M:%S %Y] "
# Send everything for context /yourApplication to mod_jk loadbalancer
JkMount /yourApplication/* loadbalancer

The module is loaded in memory by the LoadModule directive; the configuration of the single nodes is contained in a separate file named workers.properties, which will be examined in a moment.

The JkMount directive tells Apache which URLs it should forward to the mod_jk module. Supposing we have deployed a web application reachable at the web context yourApplication, with the above JKMount directive all requests with URL path /yourApplication/* are sent to the mod_jk load balancer. This way, you actually split the requests either on Apache directly (static contents) or on the load balancer for Java applications.

So, if you want your web application served directly by Tomcat Web Server you would need to point the browser to this location: http://localhost:8080/yourApplication

The same web context, proxied by Apache Web server can be reached at:

Additionally, you can use the JkMountFile directive that allows dynamic updates of mount points at runtime. When the mount file is changed, mod_jk will reload its content.

# Load mount points
JkMountFile conf/uriworkermap.properties

The format of the file is /url=worker_name. To get things started, paste the following example into the file you created:

# Mount the Servlet context to the ajp13 worker

This will configure mod_jk to forward requests to /yourApplication Apache web container.
Next, you need to configure the workers file conf/workers.properties. A worker is a process that defines a communication link between Apache and the Tomcat container.

This file specifies where the different nodes are located and how to balance the calls between the hosts. The configuration file is made up of global directives (that are generic for all nodes) and the individual worker's configuration. This is a sample two-node configuration:

# Define list of workers that will be used
# Define Node1
# Define Node2
# Load-balancing behaviour
# Status worker for managing load balancer

In this file, each node is defined using the worker.XXX naming convention where XXX represents an arbitrary name you choose for each of the target servlet containers. For each worker, you must specify the host name (or IP address) and the port number of the AJP13 connector running in the servlet container.

balance_workers is a comma-separated list of workers that the load balancer need to manage.
sticky_session specifies whether requests with SESSION IDs should be routed back to the same Tomcat worker. If sticky_session is set to true or 1, sessions are sticky, otherwise sticky_session is set to false. (The default is true.)

Finally, we must configure the Web instances on all clustered nodes so that they can expect requests forwarded from the mod_jk load balancer. Edit the server.xml file; locate the element and add an attribute jvmRoute:

<engine defaulthost="localhost" jvmroute="node1" name="Catalina">

The same attribute is required on node2:

<engine defaulthost="localhost" jvmroute="node2" name="Catalina">

You also need to be sure the AJP Connector definition is uncommented. By default, it is enabled.

<Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />