Do you run a very profitable website? Do you have an thriving on-line business? Are you experiencing a huge amount of traffic to your servers and planning to buy multiple servers? Are you an enterprise company serving applications to a huge number of users? You ought to know about application load balancers then.
An application load balancer is a hardware device that is used in data centres to balance the load (incoming connection requests) to an application across multiple servers. Normally people start with one server, then they find that one server is not able to handle the load. So, they go for another server and so on. But when one has multiple servers like that to manage (either for an e-commerce application or an in-house application like enterprise management software), they can employ hardware based application load balancers.
They sit in the data centres in between the incoming lines and the host servers. All the requests for connections are generally sent to these application load balancers. They receive the packets, check which server is functioning and in the best position to accept a connection (handling least load at that point of time, etc.) and then send the request to that server. The server responds back to the load balancer and the load balancer forward the response to the user. The user and the server think that they are talking to each other.
Application load balancers are required for two main reasons: High Availability (They keep checking if the server is functioning properly before sending connection request. If certain servers are not functioning, they take them off their list, and send the connection requests to other servers till they start functioning again). The second reason is to load balance the traffic: You don't want a single small server to handle all the incoming requests while a secondary big server is idle right? So, it distributes the load across several servers/ applications.
While they have been around for quite sometime, they have always been a quiet contributor. They have evolved in to Application Deliver Controllers of today which does more tasks than just load balancing (Like SSL offloading, caching etc). But load balancing is an important function performed by them.
