A load balancer is a worker that does not directly communicate with Tomcat. Instead it is responsible for the management of several "real" workers, called members or sub workers of the load balancer.
This management includes:
- Instantiating the workers in the web server.
- Using the worker's load balancing factor, perform weighted load balancing (distributing load according to defined strengths of the targets).
- Keeping requests belonging to the same session executing on the same Tomcat (session stickyness).
- Identifying failed Tomcat workers, suspending requests to them and instead failing over on other workers managed by the load balancer.
- Providing status and load metrics for the load balancer itself and all members via the status worker interface.
- Allowing to dynamically reconfigure load balancing via the status worker interface.
Workers managed by the same load balancer worker are load balanced (based on their configured balancing factors and current request or session load) and also secured against failure by providing failover to other members of the same load balancer. So a single Tomcat process death will not "kill" the entire site.
Some of the features provided by a load balancer are even interesting, when only working with a single member worker (where load balancing is not possible).
Basic Load Balancer Properties
A worker is configured as a load balancer by setting its worker
The following table specifies some properties used to configure a load balancer worker:
- balance_workers is a comma separated list of names of the member workers of the
load balancer. These workers are typically of type ajp13. The member workers do
not need to appear in the
worker.listproperty themselves, adding the load balancer to it suffices.
- sticky_session specifies whether requests with SESSION ID's should be routed back to the same Tomcat instance that created the session. You can set sticky_session to false when Tomcat is using a session manager which can share session data across multiple instances of Tomcat - or if your application is stateless. By default sticky_session is set to true.
- lbfactor can be added to each member worker to configure individual
strengths for the members. A higher
lbfactorwill lead to more requests being balanced to that worker. The factors must be given by integers and the load will be distributed proportional to the factors given. Higher factors lead to more requests.
# The load balancer worker balance1 will distribute # load to the members worker1 and worker2 worker.balance1.type=lb worker.balance1.balance_workers=worker1, worker2 worker.worker1.type=ajp13 worker.worker1.host=myhost1 worker.worker1.port=8009 worker.worker2.type=ajp13 worker.worker1.host=myhost2 worker.worker1.port=8009
jvmRouteattribute in the Engine element of each Tomcat's server.xml. The name of the Tomcat needs to be equal to the name of the corresponding load balancer member. In the above example, Tomcat on host "myhost1" needs
jvmRoute="worker1", Tomcat on host "myhost2" needs
For a complete reference of all load balancer configuration attributes, please consult the worker reference.
Advanced Load Balancer Worker Properties
The load balancer supports complex topologies and failover configurations.
Using the member attribute
distance you can group members.
The load balancer will always send a request to a member of lowest distance.
Only when all of those are broken, it will balance to the members of the
next higher configured distance. This allows to define priorities between
Tomcat instances in different data center locations.
When working with shared sessions, either by using session replication
or a persisting session manager (e.g. via a database), one often splits
up the Tomcat farm into replication groups. In case of failure of a member,
the load balancer needs to know, which other members share the session.
This is configured using the
domain attribute. All workers
with the same domain are assumed to share the sessions.
For maintenance purposes you can tell the load balancer to not
allow any new sessions on some members, or even not use them at all.
This is controlled by the member attribute
The value Active allows normal use of a member, disabled
will not create new sessions on it, but still allow sticky requests,
and stopped will no longer send any requests to the member.
Switching the activation from "active" to "disabled" some time before
maintenance will drain the sessions on the worker and minimize disruption.
Depending on the usage pattern of the application, draining will take from
minutes to hours. Switching the worker to stopped immediately before
maintenance will reduce logging of false errors by mod_jk.
Finally you can also configure hot spare workers by using
activation set to disabled in combination with
redirect added to the other workers:
# The advanced router LB worker worker.list=router worker.router.type=lb worker.router.balance_workers=worker1,worker2 # Define the first member worker worker.worker1.type=ajp13 worker.worker1.host=myhost1 worker.worker1.port=8009 # Define preferred failover node for worker1 worker.worker1.redirect=worker2 # Define the second member worker worker.worker2.type=ajp13 worker.worker2.host=myhost2 worker.worker2.port=8009 # Disable worker2 for all requests except failover worker.worker2.activation=disabled
redirect flag on worker1 tells the load balancer
to redirect the requests to worker2 in case that worker1 has a problem.
In all other cases worker2 will not receive any requests, thus acting
like a hot standby.
A final note about setting
activation to disabled:
The session id coming with a request is send either
as part of the request URL (
;jsessionid=...) or via a cookie.
When using bookmarks or browsers that are running since a long time,
it is possible to send a request carrying an old and invalid session id
pointing at a disabled member.
Since the load balancer does not have a list of valid sessions, it will
forward the request to the disabled member. Thus draining takes longer than
expected. To handle such cases, you can add a Servlet filter to your web
application, which checks the request attribute
This attribute contains one of the strings "ACT", "DIS" or "STP". If you
detect "DIS" and the session for the request is no longer active, delete the
session cookie and redirect using a self-referential URL. The redirected
request will then no longer carry session information and thus the load
balancer will not send it to the disabled worker. The request attribute
JK_LB_ACTIVATION has been added in version 1.2.32.
Status Worker properties
The status worker does not communicate with Tomcat. Instead it is responsible for the worker management. It is especially useful when combined with load balancer workers.
# 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 HTTP Servers use:
# 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 Require ip 127.0.0.1 </Location>