Most interactive web applications today which allow you to manipulate data in some form require authentication and authorization mechanisms to control access. In the good old days "Basic Authentication" was the most commonly used authentication mechanism. Once authenticated, the browser would send the credentials to the server with every subsequent HTTP request. Over time, cookie and URL based session tracking mechanisms took over, which is what we are using in most of our web based applications today. In a one-server farm, you probably don't need to pay attention to how these authentication, authorization or tracking mechanisms works. But once you have multiple servers in the farm, you do need to verify that a clients authenticated to one of the server is considered authenticated by all of the servers. Since, in "Basic Authentication", the browser sends the authentication credentials with every request it doesn't matter which server your session requests goes to. Each request is treated as a new session and credentials are verified every single time.
Transition to sessions
As traffic and security concerns grew, some of the smarter web applications switched to using "sessionids" in cookies and URLs. In this design, servers issue a sessionid (like a train/plane ticket) for every successful authentication and requests the client to send the sessionid instead of the authentication credentials for every subsequent request. This not only reduces the chances of someone sniffing your credentials, it also reduces the work done on the server because all it has to do now is check if the sessionid was issued by it in the last few minutes/hours (and is active), instead of validating the user against the all the users in the database for every single request.
To understand which HTTP requests are part of which session, the server has to issue a unique HTTP session identifier to each of the user sessions. As it happens most web application servers also maintain an internal "sessionid" for each session, so inserting this sessionid in the cookie/url seemed to be the most natural thing to do. Unfortunately, if you have done any web programming, you will notice that unless you do something special, these default sessionids are local to each application server. Without doing special magic, like setting up application server in clustering mode, or persisting session information on shared resource (database/NFS) it would be difficult for one server to know which sessions are active on other application servers. This is where the scalability problem comes in. Replicating a lot of session information between 2 nodes might be simple, and 10 nodes might be possible.. but this architecture is not very scalable without a little extra work.
Anatomy of a session
"Sessions", have three primary purposes.
- It helps group together HTTP requests coming from same browser.
- It can cache authentication credentials and key user info after user authenticates.
- It allows caching of other information in session which might be temporary in nature.
If you have ever tried to create a new user account on a website, chances are that you would have gone through a couple of pages before the registration was completed. Most web applications keep the information from intermediate pages in temporary session variables within the application server. Developers love to use temporary session variables for such information and this approach of storing temporary information can grow into a beast which cannot be replicated or shared using shared resource. To design a scalable website now, not only would one have to persist the sessionid and authentication credentials onto some kind of shared resource, one would also have to figure out what to do with these temporary session variables.
Since sessions are created only once per session, and since authentication happens only once too, it should be possible to keep that kind of information in a central database or shared network cache like memcached. The temporary session variables on the other hand can be very noisy (changes frequently) and may need not be critical enough to be available to the other servers. One needs to find a balance between good performance and reliability. I know organizations which have decided to accept some level of data loss with temporary session variables to improve over all performance.
But the key thing to remember here is that every time a session is created or authenticated it should be copied over to some shared scalable resource. It could be something like a cluster of memcached servers, or a cluster of replicated database servers. This will guarantee that if the user does switch application servers the requests can be authenticated by checking this shared resource before continuing with what they were doing.
Session vs State
The information about where the user is or was within an application at any given moment is called the "state". If that information is kept only on the server and is not replicated you will probably see errors of some kind when servers fail or switches user to another server. If the application server does persist the sessionid and user credentials in a central database, then, as far as the session is considered it wouldn't matter which server the user goes to. Most of the new web applications today try to maintain the state information within the browser itself which frees up the server from the responsibility of storing, maintaining and replicating state information across all the servers.
Loadbalancers: If you are doing what I did, you'd be calculating the probability of a user switching servers mid-session. In our environment we noticed about one in 50 servers fail every month. Such disasters will force all sessions created on that server to another server. Occasionally we also see issues with loadbalancer, which can reset all sticky sessions across all servers. This isn't that bad. But, if you are running a CPU intensive web application, session stickiness can at times make some servers more loaded than others and if your traffic grows enough to need multiple loadbalancers, or have multiple global sites running in active-active mode, you will eventually need the ability handle server switching/failovers.
REST (Representational State Transfer): The REST architecture does recommend some of the ideas I addressed above, but in its strictest interpretation one is not allowed to create Cookies. In my personal opinion a Cookieless world, though ideal, is not pragmatic.