Clusters

Clustered servers.  One of the most important reason for having clustered servers is so that in the event that one server fails the other servers will pick up the load and take over for the failed server.  However, there are some things you need to know before everyone goes rushing off to say "Clusters will solve all of our problems."

Applications must be built to function in a cluster.  So, what does this mean?  It means that if you are going to store a file being exchanges with the client, the file needs to be stored in a location that all servers can access.  It means that if session state is stored it needs to use SQL Server as the storage mechanism as this is used across all servers.  Essentially, nothing should be stored on the local machine.  Nothing.

Connections are "sticky", but don't depend on it.  By default the load balancer will try to send the user back to the same server that they were on previously.  However, in the event that the server is out of the cluster or there is a particularly heavy peak load on the server, the load balancer may send the user to another server.  This is expected and desired from a load balancing perspective, so your application better not rely on the user always going back to the same server.

Clusters do not protect against soft failures.  If a server dies, blows up, is incinerated by aliens from a distant galaxy, is cryogenically frozen in a block of nitrogen, or has it's network connection severed, the load balancing software will automatically move users to one of the other servers in the cluster.  However, in the event that the error is a soft error, an error in which the application responds, but does so incorrectly, the load balancer does not know that there is a problem and will continue to send users to that server.  Indeed, if the error is pervasive and actually causes all pages to fail, and fail quickly, the load balancer may be confused to the point where it thinks that the server is not under a large load and that it can handle more connections.

If you keep these things in mind you'll understand that while clusters do help us out a lot, there is some work to ensuring that they can help and that the help is what we are expecting.  Overall, however, the benefits of a clustered environment far outweigh the disadvantages.