Understanding Load balancing

Imagine you have a retail web application running on a single application server
instance and that you average 10,000 customers a month. Your company decides to
run a TV advertisement and predicts that you may start getting 100,000 customers a
month. You’re tasked with making sure that the application can support this number
of users. In addition, your application needs to be highly available. To accomplish
these goals, you have to deploy multiple application server instances and balance
requests across them.
Load balancing is a way of balancing incoming load, or concurrent requests, across
multiple application server instances, making your applications scalable and highly
available. Scalability is a term used to describe the ability to make your application
handle more user load by adding hardware and/or creating redundant instances of
your application without having to change code. Load balancing can help scale your
application because you can add more servers for the load balancer to balance the
load across, increasing the amount of traffic your application can handle. High availability
is the ability to continue processing requests in the face of server failure.

The load balancer acts as a single point of entry into an application environment
as well as a traffic director for requests.

TYPES OF LOAD BALANCERS
Load balancing can be done in two primary ways: with a hardware load balancer or
with a software load balancer. Hardware load balancers are typically more expensive
but are also more reliable


Load balancers typically make a single IP address for a cluster visible to clients. The
load balancer maintains a map of internal (or virtual) IP addresses for each machine
in the cluster. When the load balancer receives a request, it rewrites the header to
point to a particular machine in the cluster. If a machine in the cluster is removed or fails, the
hardware load balancer has the ability to recognize the failure and avoid
routing requests to it.