Save the session data in the database !

Question

Hello everybody,
I read an article on JavaWorld website about saving the session data of an application in the database instead of mantaing them in memory.
In this way, the web application is more scalable and in case the application server crashed the session data can be taken by another application server.
I was sure that this was a good motivation of writing a class to save the session data of my application in the database, but i read another article where a good methodology to
create a scalable web application was to save the session data in more application server so if one application server crashed another application server can take the control of the entire application.
I think that at the moment i have just a bit of confusion in my mind.
Any suggestion ?
Cheers.
Stefano

Answer 1

What you've read was a method of "load balancing" where "state" is maintained by a backend server instead of the web servers themselves. Yes, if done properly, this would achive true web server independance, and if one crashed, another would pick up where it left off. Of course, you'll still need an HTTP load balancer in front of the web server farm, so it can dynamically shift the load.
From an architecture standpoint, it's a good way to go, but only IF you really need 7x24 with zero downtime. Reason being twofold. 1) It's a huge performance hit, becase each GET/POST requires a trip to the DB, because it's holding state. 2) It's a lot of work, because you're now taking on the role of "managing" sessions (e.g. when to expire, reading the cookie/URL for session IDs.
I was once responsible for a 7x24 Intranet that ran core business applications. My service level agreement with my customers was 7x24 alright, but with the risk of a possible "blip" (meaning, if one server crashed, simply reconnect and continue your work). Worst case, one lost just that one page worth of work.
I hope that helps explain!
bRi