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Because of the nature of a clustered environment, ColdFusion variables that are stored in memory or the registry, such as application, session, and server variables, are not sustained by default when a user is balanced, or shifted from one server in a cluster to another in order to balance server load. For example, using server variables in a clustered server environment won't work, since server variables are not carried from one server in a cluster to another when users are shifted to another server. State cannot be maintained correctly. Client variables, however, can be sustained because they can be stored in an external repository. If you are planning to migrate a ColdFusion application that uses session and server variables to a clustered server environment, you need to consider how to deal with this issue.
An option in the ClusterCATS Explorer allows you to override the mechanism that would otherwise balance a session among servers in the cluster. Here's how to do it.
Once enabled, ClusterCATS maintains session state by ensuring the current session remains connected to the server on which state was set. After being balanced on initial connection, the user will not be shifted or balanced to another server in the cluster during the session.
There may be significant site performance costs associated with enabling this feature, since users will never be balanced to other servers in a cluster, regardless how heavy the load may be on the server maintaining state.
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