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The Apache Tomcat Connector - Generic HowTo

Workers HowTo

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Introduction

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

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.

Workers Type

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):

TypeDescription
ajp12This worker knows how to forward requests to out-of-process Tomcat workers using the ajpv12 protocol.
ajp13This worker knows how to forward requests to out-of-process Tomcat workers using the ajpv13 protocol.
lbThis is a load-balancing worker; it knows how to provide round-robin based sticky load balancing with a certain level of fault-tolerance.
statusThis 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.
  worker.local.type=ajp12
  # Defines a worker named "remote" that uses the ajpv13 protocol to forward requests to a Tomcat process.
  worker.remote.type=ajp13
  # Defines a worker named "loadbalancer" that loadbalances several Tomcat processes transparently.
  worker.loadbalancer.type=lb
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
  worker.worker1.host=www.x.com
  worker.worker1.port=8007
  worker.worker1.lbfactor=2

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.host=www2.x.com
  worker.worker2.port=8009
  worker.worker2.lbfactor=3
  # worker "worker2" uses connections, which will stay no more than 10mn in the connection pool
  worker.worker2.connection_pool_timeout=600
  # worker "worker2" ask operating system to send KEEP-ALIVE signal on the connection
  worker.worker2.socket_keepalive=1
  # 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
  worker.balance1.balance_workers=worker1, 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
  worker.list=jkstatus
  # Define a 'jkstatus' worker using status
  worker.jkstatus.type=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
 <Location /jkmanager/>
    JkMount jkstatus
    Order deny,allow
    Deny from all
    Allow from 127.0.0.1
 </Location>
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
  mynet=194.226.31
  # Using the above macro to simplify the address definitions
  # for a farm of workers.
  worker.node1.host=$(mynet).11
  worker.node2.host=$(mynet).12
  worker.node3.host=$(mynet).13
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
  worker.toe.type=ajp13
  worker.toe.socket_keepalive=true
  worker.toe.connect_timeout=10000
  worker.toe.recovery_options=7
  # workers tic and tac inherit those values
  worker.tic.reference=worker.toe
  worker.tac.reference=worker.toe

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
  worker.list=lb
  # Let's define some defaults
  worker.basic.port=8009
  worker.basic.type=ajp13
  worker.basic.socket_keepalive=true
  worker.basic.connect_timeout=10000
  worker.basic.recovery_options=7
  # And we use them in two groups
  worker.lb1.domain=dom1
  worker.lb1.distance=0
  worker.lb1.reference=worker.basic
  worker.lb2.domain=dom2
  worker.lb2.distance=1
  worker.lb2.reference=worker.basic
  # Now we configure the load balancer
  worker.lb.type=lb
  worker.lb.method=B
  worker.lb.balanced_workers=w11,w12,w21,w22
  worker.w11.host=myhost11
  worker.w11.reference=worker.lb1
  worker.w12.host=myhost12
  worker.w12.reference=worker.lb1
  worker.w21.host=myhost21
  worker.w21.reference=worker.lb2
  worker.w22.host=myhost22
  worker.w22.reference=worker.lb2
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)
  worker.worker1.type=ajp12
  worker.worker1.host=localhost
  worker.worker1.port=8007
  worker.worker1.lbfactor=1
  # Set properties for worker2 (ajp13)
  worker.worker2.type=ajp13
  worker.worker2.host=localhost
  worker.worker2.port=8009
  worker.worker2.lbfactor=1
  worker.worker2.connection_pool_timeout=600
  worker.worker2.socket_keepalive=1
  worker.worker2.socket_timeout=60
  # Set properties for worker3 (lb) which use worker1 and worker2
  worker.worker3.balance_workers=worker1,worker2

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