a. The elastic behavior of cloud may be defined as quantifiable ability to manage, measure, predict and adapt responsiveness of an application based on real time demands placed on an infrastructure using a combination of local and remote computing sources.

b. Elasticity refers to cloud computing’s ability to add or remove resources at a fine grain (one server at a time with E(2) and with a lead time of minutes rather than weeks allows matching resources to workload much more closely.

c.Two fundamental elements of elasticity are:

• TIME: resources are available as soon as request is made

• COST: Pay-as-you-go charging

d. Example:

Assume a service has predictable daily where peak requires 500 servers at noon, 100 servers at night. Therefore on average 300 servers per day.

Actual utilization over day = 300 * 24 server-hours.

But since we must provision to the peak of 500 servers we pay for = 500 * 24 = 12000 server-hours, a factor 1.7 more than what we needed.

Therefore as pay-as-you-go cost is less than 1.7 times cost of buying the servers. We can save money.

e. Thus elastic behavior can remove risk from a startup enterprise by allowing pay-as-you-go infrastructure.

** DNS, load balancers, caching server, application server and the databases all run on multiple servers in different availability zones such that if failures occur there will be no or little downtime.