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-<html><head><META http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><title>The Apache Tomcat Connector - Generic HowTo - LoadBalancer HowTo</title><meta name="author" value="Mladen Turk"><meta name="email" value="mturk@apache.org"><link href="../../style.css" type="text/css" rel="stylesheet"></head><body bgcolor="#ffffff" text="#000000" link="#525D76" alink="#525D76" vlink="#525D76"><table border="0" width="100%" cellspacing="4"><!--PAGE HEADER--><tr><td colspan="2"><!--TOMCAT LOGO--><a href="http://tomcat.apache.org/"><img src="../../images/tomcat.gif" align="left" alt="Apache Tomcat" border="0"></a><!--APACHE LOGO--><a href="http://www.apache.org/"><img src="http://www.apache.org/images/asf-logo.gif" align="right" alt="Apache Logo" border="0"></a></td></tr><!--HEADER SEPARATOR--><tr><td colspan="2"><hr noshade size="1"></td></tr><tr><!--RIGHT SIDE MAIN BODY--><td width="80%" valign="top" align="left"><table border="0" width="100%" cellspacing="4"><tr><td align="left" valign="top"><h1>The Apache Tomcat Connector - Generic HowTo</h1><h2>LoadBalancer HowTo</h2></td><td align="right" valign="top" nowrap="true"><img src="../../images/void.gif" width="1" height="1" vspace="0" hspace="0" border="0"></td></tr></table><table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td bgcolor="#525D76"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Introduction"><strong>Introduction</strong></a></font></td></tr><tr><td><blockquote>
-<br>
-<p>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.</p>
-<p>
-This management includes:
-</p>
-<ul>
-<li>
-Instantiating the workers in the web server.
-</li>
-<li>
-Using the worker's load-balancing factor, perform weighted load balancing
-(distributing load according to defined strengths of the targets).
-</li>
-<li>
-Keeping requests belonging to the same session executing on the same Tomcat
-(session stickyness).
-</li>
-<li>
-Identifying failed Tomcat workers, suspending requests to them and instead
-falling-back on other workers managed by the load balancer.
-</li>
-<li>
-Providing status and load metrics for the load balancer itself and all
-members via the status worker interface.
-</li>
-<li>
-Allowing to dynamically reconfigure load-balancing via the status worker
-interface.
-</li>
-</ul>
-<p>
-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.
-</p>
-<p>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).</p>
-
-<table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td bgcolor="#828DA6"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Basic Load Balancer Properties"><strong>Basic Load Balancer Properties</strong></a></font></td></tr><tr><td><blockquote>
-<p>A worker is configured as a load balancer by setting its worker <b class="code">type</b>
-to <b>lb</b>.
-</p>
-<p>
-The following table specifies some properties used to configure a load balancer worker:
-</p>
-<ul>
-<li><b>balance_workers</b> is a comma separated list of names of the member workers of the
-load balancer. These workers are typically of type <b>ajp13</b>. The member workers do
-not need to appear in the <b class="code">worker.list</b> property themselves, adding the
-load balancer to it suffices.</li>
-<li><b>sticky_session</b> 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
-<b>False</b> 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 <b>True</b>.</li>
-<li><b>lbfactor</b> can be added to each member worker to configure individual
-strengths for the members. A higher <b class="code">lbfactor</b> will 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.</li>
-</ul>
-
-<div class="example"><pre>
- # 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
-</pre></div>
-
-<p><font color="#ff0000">
-Session stickyness is not implemented using a tracking table for sessions.
-Instead each Tomcat instance gets an individual name and adds its name at
-the end of the session id. When the load balancer sees a session id, it
-finds the name of the Tomcat instance and sends the request via the correct
-member worker. For this to work you must set the name of the Tomcat instances
-as the value of the <b class="code">jvmRoute</b> attribute 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 <b class="code">jvmRoute="worker1"</b>, Tomcat on host "myhost2"
-needs <b class="code">jvmRoute="worker2"</b>.
-</font></p>
-
-<p>For a complete reference of all load balancer configuration
-attributes, please consult the worker <a href="../../reference/workers.html">reference</a>.
-</p>
-</blockquote></td></tr></table>
-
-<table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td bgcolor="#828DA6"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Advanced Load Balancer Worker Properties"><strong>Advanced Load Balancer Worker Properties</strong></a></font></td></tr><tr><td><blockquote>
-<p>The load balancer supports complex topologies and failover configurations.
-Using the member attribute <b class="code">distance</b> 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.
-</p>
-<p>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 <b class="code">domain</b> attribute. All workers
-with the same domain are assumed to share the sessions.</p>
-<p>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 <b class="code">activation</b>.
-The value <b>Active</b> allows normal use of a member, <b>disabled</b>
-will not create new sessions on it, but still allow sticky requests,
-and <b>stopped</b> 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.</p>
-<p>Finally you can also configure hot spare workers by using
-<b class="code">activation</b> set to <b>disabled</b> in combination with
-the attribute <b class="code">redirect</b> added to the other workers:</p>
-
-<div class="example"><pre>
- # 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
-</pre></div>
-
-<p>
-The <b class="code">redirect</b> 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.
-</p>
-
-<p>A final note about setting <b class="code">activation</b> to <b>disabled</b>:
-The session id coming with a request is send either
-as part of the request URL (<b class="code">;jsessionid=...</b>) 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 <b class="code">JK_LB_ACTIVATION</b>.
-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
-<b class="code">JK_LB_ACTIVATION</b> has been added in version 1.2.32.</p>
-</blockquote></td></tr></table>
-
-<table border="0" cellspacing="0" cellpadding="2" width="100%"><tr><td bgcolor="#828DA6"><font color="#ffffff" face="arial,helvetica.sanserif"><a name="Status Worker properties"><strong>Status Worker properties</strong></a></font></td></tr><tr><td><blockquote>
-<p>
-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.
-</p>
-<div class="example"><pre>
- # Add the status worker to the worker list
- worker.list=jkstatus
- # Define a 'jkstatus' worker using status
- worker.jkstatus.type=status
-</pre></div>
-<p>Next thing is to mount the requests to the jkstatus worker. For Apache
-web servers use the:</p>
-<div class="example"><pre>
- # Add the jkstatus mount point
- JkMount /jkmanager/* jkstatus
-</pre></div>
-<p>To obtain a higher level of security use the:</p>
-<div class="example"><pre>
- # 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>
-</pre></div>
-
-</blockquote></td></tr></table>
-
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