These 2 terms – clustering and load balancing – are used in the same sense by a majority of IT people with relative impunity.
Clustering has a formal meaning. A cluster is a group of resources that are trying to achieve a common objective, and are aware of one another. Clustering usually involves setting up the resources (servers usually) to exchange details on a particular channel (port) and keep exchanging their states, so a resource’s state is replicated at other places as well. It usually also includes load balancing, wherein, the request is routed to one of the resources in the cluster as per the load balancing policy.
Load balancing can also happen without clustering when we have multiple independent servers that have same setup, but other than that, are unaware of each other. Then, we can use a load balancer to forward requests to either one server or other, but one server does not use the other server’s resources. Also, one resource does not share its state with other resources.
Each load balancer basically does following tasks:
- Continuously check which servers are up.
- When a new request is received, send it to one of the servers as per the load balancing policy.
- When a request is received for a user who already has a session, send the user to the *same* server (This part is important, as otherwise user would keep going between different servers, but not able to really do any work). This part is not required for serving static pages, in that case, there are no user sessions.
What does it mean from a user’s perspective? Which one is better?
Every time some one asks a generic question – which one is better, the answer is invariably “It depends”. This isn’t political equivocation, but simply restating the fact that if one was better than the other in all circumstances, then the other wouldn’t exist.
Clustering saves the user’s state, and is more transparent to the user, but is harder to setup, and is very resource specific. Different application servers have different clustering protocols, and don’t necessarily work out of the box (don’t you believe any of that marketing fluff). Load balancing is comparatively more painless, and relatively more independent of application servers.
From a user’s perspective, it means that if the user is doing something on the application, and that server goes down, then depending upon whether the system is doing clustering or load balancing, the user observes different behavior. If the system is clustered, the user may be able to continue doing the transaction, and may not even realize that the server has gone down. If the system is load balanced without clustering, that means that the user’s state will likely be lost, and the user will be simply sent to the other server(s) to restart transaction. The user has lost some work.