Both elasticity and scalability refer to the ability of an environment to serve changing volumes of workloads by adjusting the capacity of the system, whether through expansion or contraction. This is an important concern when choosing a cloud provider. After all, you want to be sure that it is capable of scaling up to meet your needs as more resources are needed. Often, the two terms get used interchangeably, but they really indicate two related, but different things.
Scalability is the ability of software and hardware that is being used to continue to function properly with increased workflow volume, in other words to “ramp up” to meet demand. A typical situation where scalability would be an issue is when a company adds new employees and additional capacity is needed for all those new workstations. Elasticity is the ability to deal with sudden increases or decreases in demand through shifting and pooling resources to meet needs without over provisioning. A situation where elasticity would be an issue is an ecommerce site that has a product suddenly “blow up” or a site gets the “Reddit hug of death,” and there is a sudden and dramatic increase in bandwidth requirements.