Clustered environments

One of the things that we failed to mention in our initial diagram of Single Logout was how the logout is performed. Unfortunately, it is implemented by storing a mapping of the service ticket to HttpSession as an in-memory map. This means that Single Logout will not work properly within a clustered environment:

Figure 18.3 – CAS authentication in a clustered environment

Consider the following situation in the context of the preceding diagram:

1. The user logs in to Cluster Member A.
2. Cluster Member A validates the service ticket.
3. It then stores in memory, the mapping of the service ticket to the user’s session.
4. The user requests to log out from the CAS server.

The CAS server sends a logout request to the CAS service, but Cluster Member B receives the logout request. It looks in its memory but does not find a session for Service Ticket A, because it only exists in Cluster Member A. This means...