Drivers for the session failover solution pattern include:

• Failures in software or hardware may well occur while the user is in the middle of interacting with the system, but we cannot allow that failure to interrupt the user's interaction or the server to lose valuable data.
• Having either a redundant or load-balanced, functionally-identical element that can take over from a failed element means that the user can continue interacting with the system, but that element needs to be able to get at the state of the user's interaction with the user to be able to continue that interaction uninterrupted.
• Storing the state information either as part of the request URL or one or more cookies means that any element involved in fulfilling the request can get the information, but the nature of the interaction means the amount and sensitivity of the information make this choice inappropriate.