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For objects to be replicated from one node to another, programs libraries use the Java writeObject statement to "Serialize" the object to a stream of bytes that can be transmitted over the network and then restored in the receiving JVM. Ehcache and JBoss Cache use writeObject on individual tickets (although it turns out they also end up serializing copies of all the other ticket replication systems operate on individual ticketsobjects the ticket points to, including the TGT when attempting to replicate a ST). However, writeObject can operate just as well on the entire contents of the TicketRegistry. This TicketRegistry. Making a "checkpoint" copy of the entire collection of tickets to disk (at shutdown for example) and then restoring this collection (after a restart) is very simple to code. Since Java does all the work, it is guaranteed to work, but it might not be efficient enough to use. Still, once you have the idea the code starts to write itselfbehave correctly. It is a useful additional function. However, you can be more aggressive in the use of this approach, and that suggests the design of an entirely different type of TicketRegistry.
Start with the DefaultTicketRegistry source that CAS uses to hold tickets in memory on a single CAS standalone server. Then add the writeObject statement (surrounded by the code to open and close the file) to create a checkpoint copy of all the tickets, and a corresponding readObject and surrounding code to restore the tickets to memory. The first thought was to do the writeObject to a network socket, because that was what all the other TicketRegistry implementations were doing. Then it became clear that it was simpler, and more generally useful, and a safer design, if the data was first written to a local disk file. The disk file could then optionally be transmitted over the network in a completely independent operation. Going first to disk created code that was useful for both standalone and clustered CAS servers, and it guaranteed that the network operations were completely separated from the Ticket objects and therefore the basic CAS function.
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