How to manage session variables in a web cluster?

Question

Session variables are normally keept in the web server RAM memory.

In a cluster, each request made by a client can be handled by a different cluster node. right?!

So, in this case...

• What happens with session variables? Aren't they stored in the nodes RAM memory?
• How the other nodes will handled my request correctly if it doesn't have my session variables, or at least all of it?
• This issue is treated by the web server (Apache, IIS) or by the language runtime (PHP, ASP.NET, Ruby, JSP)?

EDIT: Is there some solution for Classic ASP?

Answer 1

As Will said, most load-balancing approaches will use some sort of stickiness in the way the distribute forthcoming requests from the same client, meaning, a unique client will hit the same server unless that actual server goes down.

That minimizes the need of distribution of session-data, meaning that only in the eventual failure of a server, a client would loose his session. Depending on your app, this is more or less critical. In most cases, this is not a big issue.

Even the simplest way of loadbalacing (round-rubin the DNS-lookups) will do some sort of stickiness since most browsers will cache the actual lookup and therefor keep going to the first record it received, AFAIK.

It's usually the runtime that is responsible for the sessiondata, in for exampla PHP it's possible to define your own session-handler, which can persist the data into a database for instance. By default PHP stores sessiondata on files, and it might be possible to share these files on a SAN or equivalent in order to share session-data. This was just a theory I had but never got around to test since we decided that loosing sessions wasn't critical and didn't want that single point of failure.

Answer 2

Get a Linux machine and set up http://www.danga.com/memcached . Its speed is unbeatable compared to other approaches. (for example, cookies, form hidden variables, databases)