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1<html><head><META http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><title>Apache Tomcat 6.0 - Clustering/Session Replication HOW-TO</title><meta value="Filip Hanik" name="author"><meta value="fhanik@apache.org" name="email"><meta value="Peter Rossbach" name="author"><meta value="pero@apache.org" name="email"></head><body vlink="#525D76" alink="#525D76" link="#525D76" text="#000000" bgcolor="#ffffff"><table cellspacing="0" width="100%" border="0"><!--PAGE HEADER--><tr><td><!--PROJECT LOGO--><a href="http://tomcat.apache.org/"><img border="0" alt="
2      The Apache Tomcat Servlet/JSP Container
3    " align="right" src="./../images/tomcat.gif"></a></td><td><font face="arial,helvetica,sanserif"><h1>Apache Tomcat 6.0</h1></font></td><td><!--APACHE LOGO--><a href="http://www.apache.org/"><img border="0" alt="Apache Logo" align="right" src="./../images/asf-logo.gif"></a></td></tr></table><table cellspacing="4" width="100%" border="0"><!--HEADER SEPARATOR--><tr><td colspan="2"><hr size="1" noshade></td></tr><tr><!--RIGHT SIDE MAIN BODY--><td align="left" valign="top" width="80%"><table cellspacing="4" width="100%" border="0"><tr><td valign="top" align="left"><h1>Apache Tomcat 6.0</h1><h2>Clustering/Session Replication HOW-TO</h2></td><td nowrap="true" valign="top" align="right"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Important Note"><strong>Important Note</strong></a></font></td></tr><tr><td><blockquote>
4<p><b>You can also check the <a href="../config/cluster.html">configuration reference documentation.</a></b>
5</p>
6</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="For the impatient"><strong>For the impatient</strong></a></font></td></tr><tr><td><blockquote>
7  <p>
8    Simply add <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>&lt;Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"/&gt;</pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
9    to your <code>&lt;Engine&gt;</code> or your <code>&lt;Host&gt;</code> element to enable clustering.
10  </p>
11  <p>
12    Using the above configuration will enable all-to-all session replication
13    using the <code>DeltaManager</code> to replicate session deltas. By all-to-all we mean that the session gets replicated to all the other
14    nodes in the cluster. This works great for smaller cluster but we don't recommend it for larger clusters(a lot of tomcat nodes).
15    Also when using the delta manager it will replicate to all nodes, even nodes that don't have the application deployed.<br>
16    To get around this problem, you'll want to use the BackupManager. This manager only replicates the session data to one backup
17    node, and only to nodes that have the application deployed. Downside of the BackupManager: not quite as battle tested as the delta manager.
18    <br>
19    Here are some of the important default values:<br>
20    1. Multicast address is 228.0.0.4<br>
21    2. Multicast port is 45564 (the port and the address together determine cluster membership.<br>
22    3. The IP broadcasted is <code>java.net.InetAddress.getLocalHost().getHostAddress()</code> (make sure you don't broadcast 127.0.0.1, this is a common error)<br>
23    4. The TCP port listening for replication messages is the first available server socket in range <code>4000-4100</code><br>
24    5. Two listeners are configured <code>ClusterSessionListener</code> and <code>JvmRouteSessionIDBinderListener</code><br>
25    6. Two interceptors are configured <code>TcpFailureDetector</code> and <code>MessageDispatch15Interceptor</code><br>
26    The following is the default cluster configuration:<br>
27    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
28        &lt;Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
29                 channelSendOptions="8"&gt;
30
31          &lt;Manager className="org.apache.catalina.ha.session.DeltaManager"
32                   expireSessionsOnShutdown="false"
33                   notifyListenersOnReplication="true"/&gt;
34
35          &lt;Channel className="org.apache.catalina.tribes.group.GroupChannel"&gt;
36            &lt;Membership className="org.apache.catalina.tribes.membership.McastService"
37                        address="228.0.0.4"
38                        port="45564"
39                        frequency="500"
40                        dropTime="3000"/&gt;
41            &lt;Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
42                      address="auto"
43                      port="4000"
44                      autoBind="100"
45                      selectorTimeout="5000"
46                      maxThreads="6"/&gt;
47
48            &lt;Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter"&gt;
49              &lt;Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/&gt;
50            &lt;/Sender&gt;
51            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/&gt;
52            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/&gt;
53          &lt;/Channel&gt;
54
55          &lt;Valve className="org.apache.catalina.ha.tcp.ReplicationValve"
56                 filter=""/&gt;
57          &lt;Valve className="org.apache.catalina.ha.session.JvmRouteBinderValve"/&gt;
58
59          &lt;Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
60                    tempDir="/tmp/war-temp/"
61                    deployDir="/tmp/war-deploy/"
62                    watchDir="/tmp/war-listen/"
63                    watchEnabled="false"/&gt;
64
65          &lt;ClusterListener className="org.apache.catalina.ha.session.JvmRouteSessionIDBinderListener"/&gt;
66          &lt;ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/&gt;
67        &lt;/Cluster&gt;   
68    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
69  </p>
70  <p>Will cover this section in more detail later in this document.</p>
71</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Cluster Basics"><strong>Cluster Basics</strong></a></font></td></tr><tr><td><blockquote>
72
73<p>To run session replication in your Tomcat 6.0 container, the following steps
74should be completed:</p>
75<ul>
76  <li>All your session attributes must implement <code>java.io.Serializable</code></li>
77  <li>Uncomment the <code>Cluster</code> element in server.xml</li>
78  <li>If you have defined custom cluster valves, make sure you have the <code>ReplicationValve</code>  defined as well under the Cluster element in server.xml</li>
79  <li>If your Tomcat instances are running on the same machine, make sure the <code>tcpListenPort</code>
80      attribute is unique for each instance, in most cases Tomcat is smart enough to resolve this on it's own by autodetecting available ports in the range 4000-4100</li>
81  <li>Make sure your <code>web.xml</code> has the <code>&lt;distributable/&gt;</code> element
82      or set at your <code>&lt;Context distributable="true" /&gt;</code></li>
83  <li>If you are using mod_jk, make sure that jvmRoute attribute is set at your Engine <code>&lt;Engine name="Catalina" jvmRoute="node01" &gt;</code>
84      and that the jvmRoute attribute value matches your worker name in workers.properties</li>
85  <li>Make sure that all nodes have the same time and sync with NTP service!</li>
86  <li>Make sure that your loadbalancer is configured for sticky session mode.</li>
87</ul>
88<p>Load balancing can be achieved through many techniques, as seen in the
89<a href="balancer-howto.html">Load Balancing</a> chapter.</p>
90<p>Note: Remember that your session state is tracked by a cookie, so your URL must look the same from the out
91   side otherwise, a new session will be created.</p>
92<p>Note: Clustering support currently requires the JDK version 1.5 or later.</p>
93<p>The Cluster module uses the Tomcat JULI logging framework, so you can configure logging
94   through the regular logging.properties file. To track messages, you can enable logging on the key:<code>org.apache.catalina.tribes.MESSAGES</code></p>
95</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Overview"><strong>Overview</strong></a></font></td></tr><tr><td><blockquote>
96
97<p>To enable session replication in Tomcat, three different paths can be followed to achieve the exact same thing:</p>
98<ol>
99  <li>Using session persistence, and saving the session to a shared file system (PersistenceManager + FileStore)</li>
100  <li>Using session persistence, and saving the session to a shared database (PersistenceManager + JDBCStore)</li>
101  <li>Using in-memory-replication, using the SimpleTcpCluster that ships with Tomcat 6 (lib/catalina-tribes.jar + lib/catalina-ha.jar)</li>
102</ol>
103
104<p>In this release of session replication, Tomcat can perform an all-to-all replication of session state using the <code>DeltaManager</code> or
105   perform backup replication to only one node using the <code>BackupManager</code>.
106   The all-to-all replication is an algorithm that is only efficient when the clusters are small. For larger clusters,  to use
107   a primary-secondary session replication where the session will only be stored at one backup server simply setup the BackupManager. <br>
108   Currently you can use the domain worker attribute (mod_jk &gt; 1.2.8) to build cluster partitions
109   with the potential of having a more scaleable cluster solution with the DeltaManager(you'll need to configure the domain interceptor for this).
110   In order to keep the network traffic down in an all-to-all environment, you can split your cluster
111   into smaller groups. This can be easily achieved by using different multicast addresses for the different groups.
112   A very simple setup would look like this:
113   </p>
114
115<div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
116        DNS Round Robin
117               |
118         Load Balancer
119          /           \
120      Cluster1      Cluster2
121      /     \        /     \
122  Tomcat1 Tomcat2  Tomcat3 Tomcat4
123</pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
124
125<p>What is important to mention here, is that session replication is only the beginning of clustering.
126   Another popular concept used to implement clusters is farming, ie, you deploy your apps only to one
127   server, and the cluster will distribute the deployments across the entire cluster.
128   This is all capabilities that can go into with the FarmWarDeployer (s. cluster example at <code>server.xml</code>)</p>
129<p>In the next section will go deeper into how session replication works and how to configure it.</p>
130
131</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Cluster Information"><strong>Cluster Information</strong></a></font></td></tr><tr><td><blockquote>
132<p>Membership is established using multicast heartbeats.
133   Hence, if you wish to subdivide your clusters, you can do this by
134   changing the multicast IP address or port in the <code>&lt;Membership&gt;</code> element.
135</p>
136<p>
137   The heartbeat contains the IP address of the Tomcat node and the TCP port that
138   Tomcat listens to for replication traffic. All data communication happens over TCP.
139</p>
140<p>
141    The <code>ReplicationValve</code> is used to find out when the request has been completed and initiate the
142    replication, if any. Data is only replicated if the session has changed (by calling setAttribute or removeAttribute
143    on the session).
144</p>
145<p>
146    One of the most important performance considerations is the synchronous versus asynchronous replication.
147    In a synchronous replication mode the request doesn't return until the replicated session has been
148    sent over the wire and reinstantiated on all the other cluster nodes.
149    Synchronous vs asynchronous is configured using the <code>channelSendOptions</code>
150    flag and is an integer value. The default value for the <code>SimpleTcpCluster/DeltaManager</code> combo is
151    8, which is asynchronous. You can read more on the <a href="../tribes/introduction.html">send flag(overview)</a> or the
152    <a href="http://tomcat.apache.org/tomcat-6.0-doc/api/org/apache/catalina/tribes/Channel.html">send flag(javadoc)</a>.
153    During async replication, the request is returned before the data has been replicated. async replication yields shorter
154    request times, and synchronous replication guarantees the session to be replicated before the request returns.
155</p>
156</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Bind session after crash to failover node"><strong>Bind session after crash to failover node</strong></a></font></td></tr><tr><td><blockquote>
157<p>
158    If you are using mod_jk and not using sticky sessions or for some reasons sticky session don't
159    work, or you are simply failing over, the session id will need to be modified as it previously contained
160    the worker id of the previous tomcat (as defined by jvmRoute in the Engine element).
161    To solve this, we will use the JvmRouteBinderValve.
162</p>
163<p> 
164    The JvmRouteBinderValve rewrites the session id to ensure that the next request will remain sticky
165    (and not fall back to go to random nodes since the worker is no longer available) after a fail over.
166    The valve rewrites the JSESSIONID value in the cookie with the same name.
167    Not having this valve in place, will make it harder to ensure stickyness in case of a failure for the mod_jk module.
168</p>
169<p>
170    By default, if no valves are configured, the JvmRouteBinderValve is added on.
171    The cluster message listener called JvmRouteSessionIDBinderListener is also defined by default and is used to actually rewrite the
172    session id on the other nodes in the cluster once a fail over has occurred.
173    Remember, if you are adding your own valves or cluster listeners in server.xml then the defaults are no longer valid,
174    make sure that you add in all the appropriate valves and listeners as defined by the default.
175</p>
176<p>
177    <b>Hint:</b><br>
178    With attribute <i>sessionIdAttribute</i> you can change the request attribute name that included the old session id.
179    Default attribuite name is <i>org.apache.catalina.cluster.session.JvmRouteOrignalSessionID</i>.
180</p>
181<p>
182    <b>Trick:</b><br>
183    You can enable this mod_jk turnover mode via JMX before you drop a node to all backup nodes!
184    Set enable true on all JvmRouteBinderValve backups, disable worker at mod_jk
185    and then drop node and restart it! Then enable mod_jk Worker and disable JvmRouteBinderValves again.
186    This use case means that only requested session are migrated.
187</p>
188
189
190
191</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Configuration Example"><strong>Configuration Example</strong></a></font></td></tr><tr><td><blockquote>
192    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
193        &lt;Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
194                 channelSendOptions="6"&gt;
195
196          &lt;Manager className="org.apache.catalina.ha.session.BackupManager"
197                   expireSessionsOnShutdown="false"
198                   notifyListenersOnReplication="true"
199                   mapSendOptions="6"/&gt;
200          &lt;!--
201          &lt;Manager className="org.apache.catalina.ha.session.DeltaManager"
202                   expireSessionsOnShutdown="false"
203                   notifyListenersOnReplication="true"/&gt;
204          --&gt;       
205          &lt;Channel className="org.apache.catalina.tribes.group.GroupChannel"&gt;
206            &lt;Membership className="org.apache.catalina.tribes.membership.McastService"
207                        address="228.0.0.4"
208                        port="45564"
209                        frequency="500"
210                        dropTime="3000"/&gt;
211            &lt;Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
212                      address="auto"
213                      port="5000"
214                      selectorTimeout="100"
215                      maxThreads="6"/&gt;
216
217            &lt;Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter"&gt;
218              &lt;Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/&gt;
219            &lt;/Sender&gt;
220            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/&gt;
221            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/&gt;
222            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.ThroughputInterceptor"/&gt;
223          &lt;/Channel&gt;
224
225          &lt;Valve className="org.apache.catalina.ha.tcp.ReplicationValve"
226                 filter=".*\.gif;.*\.js;.*\.jpg;.*\.png;.*\.htm;.*\.html;.*\.css;.*\.txt;"/&gt;
227
228          &lt;Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
229                    tempDir="/tmp/war-temp/"
230                    deployDir="/tmp/war-deploy/"
231                    watchDir="/tmp/war-listen/"
232                    watchEnabled="false"/&gt;
233
234          &lt;ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/&gt;
235        &lt;/Cluster&gt;
236    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
237    <p>
238      Break it down!!
239    </p>
240    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
241        &lt;Cluster className="org.apache.catalina.ha.tcp.SimpleTcpCluster"
242                 channelSendOptions="6"&gt;
243    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
244    <p>
245      The main element, inside this element all cluster details can be configured.
246      The <code>channelSendOptions</code> is the flag that is attached to each message sent by the
247      SimpleTcpCluster class or any objects that are invoking the SimpleTcpCluster.send method.
248      The description of the send flags is available at <a href="http://tomcat.apache.org/tomcat-6.0-doc/api/org/apache/catalina/tribes/Channel.html">
249      our javadoc site</a>
250      The <code>DeltaManager</code> sends information using the SimpleTcpCluster.send method, while the backup manager
251      sends it itself directly through the channel.
252      <br>For more info, Please visit the <a href="../config/cluster.html">reference documentation</a>
253    </p>
254    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
255          &lt;Manager className="org.apache.catalina.ha.session.BackupManager"
256                   expireSessionsOnShutdown="false"
257                   notifyListenersOnReplication="true"
258                   mapSendOptions="6"/&gt;
259          &lt;!--
260          &lt;Manager className="org.apache.catalina.ha.session.DeltaManager"
261                   expireSessionsOnShutdown="false"
262                   notifyListenersOnReplication="true"/&gt;
263          --&gt;       
264    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
265    <p>
266        This is a template for the manager configuration that will be used if no manager is defined in the &lt;Context&gt;
267        element. In Tomcat 5.x each webapp marked distributable had to use the same manager, this is no longer the case
268        since Tomcat 6 you can define a manager class for each webapp, so that you can mix managers in your cluster.
269        Obviously the managers on one node's application has to correspond with the same manager on the same application on the other node.
270        If no manager has been specified for the webapp, and the webapp is marked &lt;distributable/&gt; Tomcat will take this manager configuration
271        and create a manager instance cloning this configuration.
272        <br>For more info, Please visit the <a href="../config/cluster-manager.html">reference documentation</a>
273    </p>
274    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
275          &lt;Channel className="org.apache.catalina.tribes.group.GroupChannel"&gt;
276    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
277    <p>
278        The channel element is <a href="../tribes/introduction.html">Tribes</a>, the group communication framework
279        used inside Tomcat. This element encapsulates everything that has to do with communication and membership logic.
280        <br>For more info, Please visit the <a href="../config/cluster-channel.html">reference documentation</a>
281    </p>
282    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
283            &lt;Membership className="org.apache.catalina.tribes.membership.McastService"
284                        address="228.0.0.4"
285                        port="45564"
286                        frequency="500"
287                        dropTime="3000"/&gt;
288    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
289    <p>
290        Membership is done using multicasting. Please note that Tribes also supports static memberships using the
291        <code>StaticMembershipInterceptor</code> if you want to extend your membership to points beyond multicasting.
292        The address attribute is the multicast address used and the port is the multicast port. These two together
293        create the cluster separation. If you want a QA cluster and a production cluster, the easiest config is to
294        have the QA cluster be on a separate multicast address/port combination the the production cluster.<br>
295        The membership component broadcasts TCP adress/port of itselt to the other nodes so that communication between
296        nodes can be done over TCP. Please note that the address being broadcasted is the one of the
297        <code>Receiver.address</code> attribute.
298        <br>For more info, Please visit the <a href="../config/cluster-membership.html">reference documentation</a>
299    </p>
300    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
301            &lt;Receiver className="org.apache.catalina.tribes.transport.nio.NioReceiver"
302                      address="auto"
303                      port="5000"
304                      selectorTimeout="100"
305                      maxThreads="6"/&gt;
306    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
307    <p>
308        In tribes the logic of sending and receiving data has been broken into two functional components. The Receiver, as the name suggests
309        is responsible for receiving messages. Since the Tribes stack is thread less, (a popular improvement now adopted by other frameworks as well),
310        there is a thread pool in this component that has a maxThreads and minThreads setting.<br>
311        The address attribute is the host address that will be broadcasted by the membership component to the other nodes.
312        <br>For more info, Please visit the <a href="../config/cluster-receiver.html">reference documentation</a>
313    </p>
314    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
315
316            &lt;Sender className="org.apache.catalina.tribes.transport.ReplicationTransmitter"&gt;
317              &lt;Transport className="org.apache.catalina.tribes.transport.nio.PooledParallelSender"/&gt;
318            &lt;/Sender&gt;
319    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
320    <p>
321        The sender component, as the name indicates is responsible for sending messages to other nodes.
322        The sender has a shell component, the <code>ReplicationTransmitter</code> but the real stuff done is done in the
323        sub component, <code>Transport</code>.
324        Tribes support having a pool of senders, so that messages can be sent in parallel and if using the NIO sender,
325        you can send messages concurrently as well.<br>
326        Concurrently means one message to multiple senders at the same time and Parallel means multiple messages to multiple senders
327        at the same time.
328        <br>For more info, Please visit the <a href="../config/cluster-sender.html">reference documentation</a>
329    </p>
330    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
331            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.TcpFailureDetector"/&gt;
332            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.MessageDispatch15Interceptor"/&gt;
333            &lt;Interceptor className="org.apache.catalina.tribes.group.interceptors.ThroughputInterceptor"/&gt;
334          &lt;/Channel&gt;
335    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
336    <p>
337        Tribes uses a stack to send messages through. Each element in the stack is called an interceptor, and works much like the valves do
338        in the Tomcat servlet container.
339        Using interceptors, logic can be broken into more managable pieces of code. The interceptors configured above are:<br>
340        TcpFailureDetector - verifies crashed members through TCP, if multicast packets get dropped, this interceptor protects against false positives,
341        ie the node marked as crashed even though it still is alive and running.<br>
342        MessageDispatch15Interceptor - dispatches messages to a thread (thread pool) to send message asynchrously.<br>
343        ThroughputInterceptor - prints out simple stats on message traffic.<br>
344        Please note that the order of interceptors is important. the way they are defined in server.xml is the way they are represented in the
345        channel stack. Think of it as a linked list, with the head being the first most interceptor and the tail the last.
346        <br>For more info, Please visit the <a href="../config/cluster-interceptor.html">reference documentation</a>
347    </p>
348    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
349          &lt;Valve className="org.apache.catalina.ha.tcp.ReplicationValve"
350                 filter=".*\.gif;.*\.js;.*\.jpg;.*\.png;.*\.htm;.*\.html;.*\.css;.*\.txt;"/&gt;
351    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
352    <p>
353        The cluster uses valves to track requests to web applications, we've mentioned the ReplicationValve and the JvmRouteBinderValve above.
354        The &lt;Cluster&gt; element itself is not part of the pipeline in Tomcat, instead the cluster adds the valve to its parent container.
355        If the &lt;Cluster&gt; elements is configured in the &lt;Engine&gt; element, the valves get added to the engine and so on.
356        <br>For more info, Please visit the <a href="../config/cluster-valve.html">reference documentation</a>
357    </p>
358    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
359          &lt;Deployer className="org.apache.catalina.ha.deploy.FarmWarDeployer"
360                    tempDir="/tmp/war-temp/"
361                    deployDir="/tmp/war-deploy/"
362                    watchDir="/tmp/war-listen/"
363                    watchEnabled="false"/&gt;
364    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
365    <p>
366        The default tomcat cluster supports farmed deployment, ie, the cluster can deploy and undeploy applications on the other nodes.
367        The state of this component is currently in flux but will be addressed soon. There was a change in the deployment algorithm
368        between Tomcat 5.0 and 5.5 and at that point, the logic of this component changed to where the deploy dir has to match the
369        webapps directory.
370        <br>For more info, Please visit the <a href="../config/cluster-deployer.html">reference documentation</a>
371    </p>
372    <div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
373          &lt;ClusterListener className="org.apache.catalina.ha.session.ClusterSessionListener"/&gt;
374        &lt;/Cluster&gt;
375    </pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
376    <p>
377        Since the SimpleTcpCluster itself is a sender and receiver of the Channel object, components can register themselves as listeners to
378        the SimpleTcpCluster. The listener above <code>ClusterSessionListener</code> listens for DeltaManager replication messages
379        and applies the deltas to the manager that in turn applies it to the session.
380        <br>For more info, Please visit the <a href="../config/cluster-listener.html">reference documentation</a>
381    </p>
382   
383</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Cluster Architecture"><strong>Cluster Architecture</strong></a></font></td></tr><tr><td><blockquote>
384
385<p><b>Component Levels:</b>
386<div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
387         Server
388           |
389         Service
390           |
391         Engine
392           |  \
393           |  --- Cluster --*
394           |
395         Host
396           |
397         ------
398        /      \
399     Cluster    Context(1-N)                 
400        |             \
401        |             -- Manager
402        |                   \
403        |                   -- DeltaManager
404        |                   -- BackupManager
405        |
406     ---------------------------
407        |                       \
408      Channel                    \
409    ----------------------------- \
410        |                          \
411     Interceptor_1 ..               \
412        |                            \
413     Interceptor_N                    \
414    -----------------------------      \
415     |          |         |             \
416   Receiver    Sender   Membership       \
417                                         -- Valve
418                                         |      \
419                                         |       -- ReplicationValve
420                                         |       -- JvmRouteBinderValve
421                                         |
422                                         -- LifecycleListener
423                                         |
424                                         -- ClusterListener
425                                         |      \
426                                         |       -- ClusterSessionListener
427                                         |       -- JvmRouteSessionIDBinderListener
428                                         |
429                                         -- Deployer
430                                                \
431                                                 -- FarmWarDeployer
432     
433     
434</pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
435</p>
436
437</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="How it Works"><strong>How it Works</strong></a></font></td></tr><tr><td><blockquote>
438<p>To make it easy to understand how clustering works, We are gonna take you through a series of scenarios.
439   In the scenario we only plan to use two tomcat instances <code>TomcatA</code> and <code>TomcatB</code>.
440   We will cover the following sequence of events:</p>
441
442<ol>
443<li><code>TomcatA</code> starts up</li>
444<li><code>TomcatB</code> starts up (Wait that TomcatA start is complete)</li>
445<li><code>TomcatA</code> receives a request, a session <code>S1</code> is created.</li>
446<li><code>TomcatA</code> crashes</li>
447<li><code>TomcatB</code> receives a request for session <code>S1</code></li>
448<li><code>TomcatA</code> starts up</li>
449<li><code>TomcatA</code> receives a request, invalidate is called on the session (<code>S1</code>)</li>
450<li><code>TomcatB</code> receives a request, for a new session (<code>S2</code>)</li>
451<li><code>TomcatA</code> The session <code>S2</code> expires due to inactivity.</li>
452</ol>
453
454<p>Ok, now that we have a good sequence, we will take you through exactly what happens in the session repliction code</p>
455
456<ol>
457<li><b><code>TomcatA</code> starts up</b>
458    <p>
459        Tomcat starts up using the standard start up sequence. When the Host object is created, a cluster object is associated with it.
460        When the contexts are parsed, if the distributable element is in place in web.xml
461        Tomcat asks the Cluster class (in this case <code>SimpleTcpCluster</code>) to create a manager
462        for the replicated context. So with clustering enabled, distributable set in web.xml
463        Tomcat will create a <code>DeltaManager</code> for that context instead of a <code>StandardManager</code>.
464        The cluster class will start up a membership service (multicast) and a replication service (tcp unicast).
465        More on the architecture further down in this document.
466    </p><p></p>
467</li>
468<li><b><code>TomcatB</code> starts up</b>
469    <p>
470        When TomcatB starts up, it follows the same sequence as TomcatA did with one exception.
471        The cluster is started and will establish a membership (TomcatA,TomcatB).
472        TomcatB will now request the session state from a server that already exists in the cluster,
473        in this case TomcatA. TomcatA responds to the request, and before TomcatB starts listening
474        for HTTP requests, the state has been transferred from TomcatA to TomcatB.
475        In case TomcatA doesn't respond, TomcatB will time out after 60 seconds, and issue a log
476        entry. The session state gets transferred for each web application that has distributable in
477        its web.xml. Note: To use session replication efficiently, all your tomcat instances should be
478        configured the same.
479    </p><p></p>
480</li>
481<li><B><code>TomcatA</code> receives a request, a session <code>S1</code> is created.</B>
482    <p>
483        The request coming in to TomcatA is treated exactly the same way as without session replication.
484        The action happens when the request is completed, the <code>ReplicationValve</code> will intercept
485        the request before the response is returned to the user.
486        At this point it finds that the session has been modified, and it uses TCP to replicata the
487        session to TomcatB. Once the serialized data has been handed off to the operating systems TCP logic,
488        the request returns to the user, back through the valve pipeline.
489        For each request the entire session is replicated, this allows code that modifies attributes
490        in the session without calling setAttribute or removeAttribute to be replicated.
491        a useDirtyFlag configuration parameter can be used to optimize the number of times
492        a session is replicated.
493    </p><p></p>
494
495</li>
496<li><b><code>TomcatA</code> crashes</b>
497    <p>
498        When TomcatA crashes, TomcatB receives a notification that TomcatA has dropped out
499        of the cluster. TomcatB removes TomcatA from its membership list, and TomcatA will no longer
500        be notified of any changes that occurs in TomcatB.
501        The load balancer will redirect the requests from TomcatA to TomcatB and all the sessions
502        are current.
503    </p><p></p>
504</li>
505<li><b><code>TomcatB</code> receives a request for session <code>S1</code></b>
506    <p>Nothing exciting, TomcatB will process the request as any other request.
507    </p><p></p>
508</li>
509<li><b><code>TomcatA</code> starts up</b>
510    <p>Upon start up, before TomcatA starts taking new request and making itself
511    available to it will follow the start up sequence described above 1) 2).
512    It will join the cluster, contact TomcatB for the current state of all the sessions.
513    And once it receives the session state, it finishes loading and opens its HTTP/mod_jk ports.
514    So no requests will make it to TomcatA until it has received the session state from TomcatB.
515    </p><p></p>
516</li>
517<li><b><code>TomcatA</code> receives a request, invalidate is called on the session (<code>S1</code>)</b>
518    <p>The invalidate is call is intercepted, and the session is queued with invalidated sessions.
519        When the request is complete, instead of sending out the session that has changed, it sends out
520        an "expire" message to TomcatB and TomcatB will invalidate the session as well.
521    </p><p></p>
522
523</li>
524<li><b><code>TomcatB</code> receives a request, for a new session (<code>S2</code>)</b>
525    <p>Same scenario as in step 3)
526    </p><p></p>
527
528
529</li>
530<li><code>TomcatA</code> The session <code>S2</code> expires due to inactivity.
531    <p>The invalidate is call is intercepted the same was as when a session is invalidated by the user,
532       and the session is queued with invalidated sessions.
533       At this point, the invalidet session will not be replicated across until
534       another request comes through the system and checks the invalid queue.
535    </p><p></p>
536</li>
537</ol>
538
539<p>Phuuuhh! :)</p>
540
541<p><b>Membership</b>
542    Clustering membership is established using very simple multicast pings.
543    Each Tomcat instance will periodically send out a multicast ping,
544    in the ping message the instance will broad cast its IP and TCP listen port
545    for replication.
546    If an instance has not received such a ping within a given timeframe, the
547    member is considered dead. Very simple, and very effective!
548    Of course, you need to enable multicasting on your system.
549</p>
550
551<p><b>TCP Replication</b>
552    Once a multicast ping has been received, the member is added to the cluster
553    Upon the next replication request, the sending instance will use the host and
554    port info and establish a TCP socket. Using this socket it sends over the serialized data.
555    The reason I choose TCP sockets is because it has built in flow control and guaranteed delivery.
556    So I know, when I send some data, it will make it there :)
557</p>
558
559<p><b>Distributed locking and pages using frames</b>
560    Tomcat does not keep session instances in sync across the cluster.
561    The implementation of such logic would be to much overhead and cause all
562    kinds of problems. If your client accesses the same session
563    simultanously using multiple requests, then the last request
564    will override the other sessions in the cluster.
565</p>
566
567</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="Monitoring your Cluster with JMX"><strong>Monitoring your Cluster with JMX</strong></a></font></td></tr><tr><td><blockquote>
568<p>Monitoring is a very important question when you use a cluster. Some of the cluster objects are JMX MBeans </p>
569<p>Add the following parameter to your startup script with Java 5:
570<div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
571set CATALINA_OPTS=\
572-Dcom.sun.management.jmxremote \
573-Dcom.sun.management.jmxremote.port=%my.jmx.port% \
574-Dcom.sun.management.jmxremote.ssl=false \
575-Dcom.sun.management.jmxremote.authenticate=false
576</pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
577</p>
578<p>Activate JMX with JDK 1.4:
579<ol>
580<li>Install the compat package</li>
581<li>Install the mx4j-tools.jar at common/lib (use the same mx4j version as your tomcat release)</li>
582<li>Configure a MX4J JMX HTTP Adaptor at your AJP Connector<p></p>
583<div align="left"><table border="0" cellpadding="0" cellspacing="4"><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#ffffff"><pre>
584&lt;Connector port="${AJP.PORT}"
585   handler.list="mx"
586   mx.enabled="true"
587   mx.httpHost="${JMX.HOST}"
588   mx.httpPort="${JMX.PORT}"
589   protocol="AJP/1.3" /&gt;
590</pre></td><td width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr><tr><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td><td height="1" width="1" bgcolor="#023264"><img border="0" hspace="0" vspace="0" height="1" width="1" src="./../images/void.gif"></td></tr></table></div>
591</li>
592<li>Start your tomcat and look with your browser to http://${JMX.HOST}:${JMX.PORT}</li>
593<li>With the connector parameter <code>mx.authMode="basic" mx.authUser="tomcat" mx.authPassword="strange"</code> you can control the access!</li>
594</ol>
595</p>
596<p>
597List of Cluster Mbeans<br>
598<table border="1" cellpadding="5">
599
600  <tr>
601    <th align="center" bgcolor="aqua">Name</th>
602    <th align="center" bgcolor="aqua">Description</th>
603    <th align="center" bgcolor="aqua">MBean ObjectName - Engine</th>
604    <th align="center" bgcolor="aqua">MBean ObjectName - Host</th>
605  </tr>
606
607  <tr>
608    <td>Cluster</td>
609    <td>The complete cluster element</td>
610    <td><code>type=Cluster</code></td>
611    <td><code>type=Cluster,host=${HOST}</code></td>
612  </tr>
613 
614  <tr>
615    <td>DeltaManager</td>
616    <td>This manager control the sessions and handle session replication </td>
617    <td><code>type=Manager,path=${APP.CONTEXT.PATH}, host=${HOST}</code></td>
618    <td><code>type=Manager,path=${APP.CONTEXT.PATH}, host=${HOST}</code></td>
619  </tr>
620
621  <tr>
622    <td>ReplicationValve</td>
623    <td>This valve control the replication to the backup nodes</td>
624    <td><code>type=Valve,name=ReplicationValve</code></td>
625    <td><code>type=Valve,name=ReplicationValve,host=${HOST}</code></td>
626  </tr>
627
628  <tr>
629    <td>JvmRouteBinderValve</td>
630    <td>This is a cluster fallback valve to change the Session ID to the current tomcat jvmroute.</td>
631    <td><code>type=Valve,name=JvmRouteBinderValve,
632              path=${APP.CONTEXT.PATH}</code></td>
633    <td><code>type=Valve,name=JvmRouteBinderValve,host=${HOST},
634              path=${APP.CONTEXT.PATH}</code></td>
635  </tr>
636
637</table>
638</p>
639</blockquote></td></tr></table><table cellpadding="2" cellspacing="0" border="0"><tr><td bgcolor="#525D76"><font face="arial,helvetica.sanserif" color="#ffffff"><a name="FAQ"><strong>FAQ</strong></a></font></td></tr><tr><td><blockquote>
640<p>Please see <a href="http://tomcat.apache.org/faq/cluster.html">the clustering section of the FAQ</a>.</p>
641</blockquote></td></tr></table></td></tr><!--FOOTER SEPARATOR--><tr><td colspan="2"><hr size="1" noshade></td></tr><!--PAGE FOOTER--><tr><td colspan="2"><div align="center"><font size="-1" color="#525D76"><em>
642        Copyright &copy; 1999-2008, Apache Software Foundation
643        </em></font></div></td></tr></table></body></html>
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