This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Tomcat using Session Persistence to help retreive a session after it shutdowns or crashes. It's also used within clustered environments. I think that it's enabled by default, but uou can disable it. I don't think that it's part of the specification (except the part on distributed environments).