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# Apache Tomcat Configuration Reference ## The ClusterManager object | print-friendly version
Introduction
A cluster manager is an extension to Tomcat's session manager interface, `org.apache.catalina.Manager` A cluster manager must implement the `org.apache.catalina.ha.ClusterManager` and is solely responsible for how the session is replicated. There are currently two different managers, the `org.apache.catalina.ha.session.DeltaManager` replicates deltas of session data to all members in the cluster. This implementation is proven and works very well, but has a limitation as it requires the cluster members to be homogeneous, all nodes must deploy the same applications and be exact replicas. The `org.apache.catalina.ha.session.BackupManager` also replicates deltas but only to one backup node. The location of the backup node is known to all nodes in the cluster. It also supports heterogeneous deployments, so the manager knows at what locations the webapp is deployed. We are planning to add more managers with even more sophisticated backup mechanism to support even larger clusters. Check back soon!
The <Manager>
The `<Manager>` element defined inside the `<Cluster>` element is the template defined for all web applications that are marked `<distributable/>` in their `web.xml` file. However, you can still override the manager implementation on a per web application basis, by putting the `<Manager>` inside the `<Context>` element either in the `context.xml` file or the `server.xml` file.
Attributes
> Common Attributes
> > Attribute | Description
> > --- | ---
> > `className` |
> > `name` | The name of this cluster manager, the name is used to identify a session manager on a node. The name might get modified by the `Cluster` element to make it unique in the container.
> > `defaultMode` | Deprecated since 6.0.0
> > `notifyListenersOnReplication` | Set to `true` if you wish to have session listeners notified when session attributes are being replicated or removed across Tomcat nodes in the cluster.
> > `expireSessionsOnShutdown` | When a webapplication is being shutdown, Tomcat issues an expire call to each session to notify all the listeners. If you wish for all sessions to expire on all nodes when a shutdown occurs on one node, set this value to `true`. Default value is `false`.
> org.apache.catalina.ha.session.DeltaManager Attributes
> > Attribute | Description
> > --- | ---
> > `domainReplication` | Set to true if you wish sessions to be replicated only to members that have the same logical domain set. If set to false, session replication will ignore the domain setting the `<Membership>` element.
> > `expireSessionsOnShutdown` | When a webapplication is being shutdown, Tomcat issues an expire call to each session to notify all the listeners. If you wish for all sessions to expire on all nodes when a shutdown occurs on one node, set this value to `true`. Default value is `false`.
> org.apache.catalina.ha.session.BackupManager Attributes
> > Attribute | Description
> > --- | ---
> > `mapSendOptions` | The backup manager uses a replicated map, this map is sending and receiving messages. You can setup the flag for how this map is sending messages, the default value is `8`(asynchronous).
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