http://tomcat.apache.org/tomcat-6.0-doc/cluster-howto.html
https://www.digitalocean.com/community/tutorials/how-to-share-php-sessions-on-multiple-memcached-servers-on-ubuntu-14-04
http://www.justincarmony.com/blog/2010/09/15/sharing-sessions-across-multiple-servers-with-memcache/
http://www.bitvolution.com/session-sharing-in-php-the-easy-way
https://blogs.oracle.com/muraliveligeti/entry/coherence_session_sharing_between_applications
https://code.google.com/p/memcached-session-manager/wiki/SetupAndConfiguration
http://stackoverflow.com/questions/16243450/share-a-session-across-multiple-servers-with-different-domains
http://serverfault.com/questions/32421/how-is-session-stickiness-achieved-across-multiple-web-servers
https://technology.amis.nl/2012/01/18/sharing-session-state-between-jee-web-application-through-weblogic-session-descriptor-of-sharing-enabled/
http://blog.gigaspaces.com/global-http-session-sharing/
http://shiro.apache.org/session-management.html
http://msdn.microsoft.com/en-us/library/aa479313.aspx
http://www.codeproject.com/Articles/27090/Sharing-Session-Across-Applications
http://forums.asp.net/t/1234883.aspx
http://blogs.lessthandot.com/index.php/webdev/serverprogramming/aspnet/sharing-asp-net-session-state-between-ap/
http://galratner.com/blogs/net/archive/2011/06/01/sharing-memory-session-between-servers.aspx
http://www.developer.com/net/asp/article.php/3595766/Storing-Session-State-in-a-SQL-Server-Database.htm
http://en.wikipedia.org/wiki/Single_sign-on
http://www.opengroup.org/security/sso/sso_intro.htm
http://www.authenticationworld.com/Single-Sign-On-Authentication/