AJP Protocol Reference
A Tomcat worker is a Tomcat instance that is waiting to execute servlets on behalf of some web server.
For example, we can have a web server such as Apache forwarding servlet requests to a
Tomcat process (the worker) running behind it.
The scenario described above is a very simple one;
in fact one can configure multiple Tomcat workers to serve servlets on
behalf of a certain web server.
The reasons for such configuration can be:
We want different contexts to be served by different Tomcat workers to provide a
development environment where all the developers share the same web server but own a Tomcat worker of their own.
We want different virtual hosts served by different Tomcat processes to provide a
clear separation between sites belonging to different companies.
We want to provide load balancing, meaning run multiple Tomcat workers each on a
machine of its own and distribute the requests between them.
There are probably more reasons for having multiple workers but I guess that this list is enough...
Tomcat workers are defined in a properties file dubbed workers.properties and this tutorial
explains how to work with it.
This document was originally part of Tomcat: A Minimalistic User's Guide written by Gal Shachor,
but has been split off for organisational reasons.
Defining workers to the Tomcat web server plugin can be done using a properties file
(a sample file named workers.properties is available in the conf/ directory).
the file contains entries of the following form:
worker.list=<a comma separated list of worker names>
# the list of workers
worker.list= worker1, worker2
When starting up, the web server plugin will instantiate the workers whose name appears in the
worker.list property, these are also the workers to whom you can map requests. The directive can be used multiple times.
Each named worker should also have a few entries to provide additional information on his behalf.
This information includes the worker's type and other related worker information.
Currently the following worker types that exists are (JK 1.2.5):
|ajp12||This worker knows how to forward requests to out-of-process Tomcat workers using the ajpv12 protocol.|
|ajp13||This worker knows how to forward requests to out-of-process Tomcat workers using the ajpv13 protocol.|
|lb||This is a load-balancing worker; it knows how to provide round-robin based sticky load balancing with a certain level of fault-tolerance.|
|status||This is a status worker for managing load balancers.|
Defining workers of a certain type should be done with the following property format:
worker.worker name.type=<worker type>
Where worker name is the name assigned to the worker and the worker type is one of the four types defined
in the table (a worker name may only contain any space the characters [a-zA-Z0-9\-_]).
# Defines a worker named "local" that uses the ajpv12 protocol to forward requests to a Tomcat process.
# Defines a worker named "remote" that uses the ajpv13 protocol to forward requests to a Tomcat process.
# Defines a worker named "loadbalancer" that loadbalances several Tomcat processes transparently.
|Setting Worker Properties|
After defining the workers you can also specify properties for them.
Properties can be specified in the following manner:
worker.<worker name>.<property>=<property value>
Each worker has a set of properties that you can set as specified in the following subsections:
|ajp12 Worker properties|
The ajp12 has been deprecated with Tomcat 3.3.x and you should use instead
ajp13 which is the only ajp protocol known by all Tomcat 4 and above.
The ajp12 typed workers forward requests to out-of-process Tomcat workers
using the ajpv12 protocol over TCP/IP sockets.
the ajp12 worker properties are :
host property sets the host where the Tomcat worker is listening for ajp12 requests.
port property sets the port where the Tomcat worker is listening for ajp12 requests
lbfactor property is used when working with a load balancer worker, this is the load-balancing factor for the worker.
We'll see more on this in the lb worker section.
# worker "worker1" will talk to Tomcat listening on machine www.x.com at port 8007 using 2 lb factor
Notes: In the ajpv12 protocol, connections are created, used and then closed at each request.
The default port for ajp12 is 8007
|ajp13 Worker properties|
The ajp13 typed workers forward requests to out-of-process Tomcat workers using the ajpv13 protocol over TCP/IP sockets.
The main difference between ajpv12 and ajpv13 are that:
ajpv13 is a more binary protocol and it tries to compress some of the request data by coding
frequently used strings as small integers.
ajpv13 reuses open sockets and leaves them open for future requests (remember when you've got a Firewall between your
web server and Tomcat).
ajpv13 has special treatment for SSL information so that the container can implement
SSL related methods such as isSecure().
You should note that Ajp13 is now the only out-process protocol supported by Tomcat 4 and above.
# worker "worker2" will talk to Tomcat listening on machine www2.x.com at port 8009 using 3 lb factor
# worker "worker2" uses connections, which will stay no more than 10mn in the connection pool
# worker "worker2" ask operating system to send KEEP-ALIVE signal on the connection
# mount can be used as an alternative to the JkMount directive
worker.worker2.mount=/contexta /contexta/* /contextb /contextb/*
Notes: In the ajpv13 protocol, the default port is 8009
|lb Worker properties|
The load-balancing worker does not really communicate with Tomcat workers.
Instead it is responsible for the management of several "real" workers.
This management includes:
Instantiating the workers in the web server.
Using the worker's load-balancing factor, perform weighed-round-robin load balancing where
high lbfactor means stronger machine (that is going to handle more requests)
Keeping requests belonging to the same session executing on the same Tomcat worker.
Identifying failed Tomcat workers, suspending requests to them and instead falling-back on
other workers managed by the lb worker.
The overall result is that workers managed by the same lb worker are load-balanced (based on their lbfactor and current user session) and also fall-backed so a single Tomcat process death will not "kill" the entire site.
The following table specifies some properties that the lb worker can accept:
- balance_workers is a comma separated list of workers that the load balancer need to manage.
As long as these workers should only be used via the load balancer worker,
there is no need to also put them into the worker.list property.
This directive can be used multiple times for the same load balancer.
- sticky_session specifies whether requests with SESSION ID's should be routed back to the same
Tomcat worker. Set sticky_session to False when Tomcat is using a Session Manager which
can persist session data across multiple instances of Tomcat. By default sticky_session is set to True.
# The worker balance1 while use "real" workers worker1 and worker2
|Status Worker properties|
The status worker does not communicate with Tomcat.
Instead it is responsible for the load balancer management.
# Add the status worker to the worker list
# Define a 'jkstatus' worker using status
Next thing is to mount the requests to the jkstatus worker. For Apache
web servers use the:
# Add the jkstatus mount point
JkMount /jkmanager/* jkstatus
To obtain a higher level of security use the:
# Enable the JK manager access from localhost only
Deny from all
Allow from 127.0.0.1
|Property file macros|
You can define "macros" in the property files.
These macros let you define properties and later on use them while
constructing other properties.
# property example, like a network base address
# Using the above macro to simplify the address definitions
# for a farm of workers.
|Hierarchical property configuration|
Workers can reference configurations of other workers.
If worker "x" references worker "y", then it inherits all
configuration parameters from "y", except for the ones
that have explicitly been set for "x".
# worker toe defines some default settings
# workers tic and tac inherit those values
Please note, that the reference contains
the full prefix to the referenced configuration attributes,
not only the name of the referenced worker.
References can be nested. Be careful to avoid loops!
Attributes which are allowed multiple times for a single worker
can not be merged from a worker and a reference. An attribute
is only inherited from a reference, if it is not already set
for the referring worker.
References are especially useful, when configuring load balancers.
Try to understand the following two stage references:
# We only use one load balancer
# Let's define some defaults
# And we use them in two groups
# Now we configure the load balancer
|A sample worker.properties|
Since coping with worker.properties on your own is not an easy thing to do,
a sample worker.properties file is bundled along JK.
You could also find here a sample workers.properties defining :
An ajp12 worker that used the host localhost and the port 8007
An ajp13 worker that used the host localhost and the port 8008
An lb worker that load balance the ajp12 and ajp13 workers
# Define 3 workers, 2 real workers using ajp12, ajp13, the last one being a loadbalancing worker
worker.list=worker1, worker2, worker3
# Set properties for worker1 (ajp12)
# Set properties for worker2 (ajp13)
# Set properties for worker3 (lb) which use worker1 and worker2