What is the difference between scalability and ela

Question

I've heard many people using both terms interchangeably. However, in my opinion there is difference between them:

SCALABILITY - ability of a system to increase the workload on its current hardware resources (scale up);

ELASTICITY - ability of a system to increase the workload on its current and additional (dynamically added on demand) hardware resources (scale out);

Elasticity is strongly related to deployed-on-cloud applications.

Have I got the difference between these two non-functional software characteristics right? May someone give specific examples, so we could draw more distinctive border between these two terms?

Answer 1

From my limited understanding of those concepts, an example:

Say we have a system of 5 computers that does 5 work units, if we need one more work unit to be done we we'll have to use one more computer. That is a scalable system but it is not elastic. Somebody going to have to go and get that other computer. Also, if a new computer is purchased and the extra work unit is not needed any more, the system get stuck with a redundant resource.

Now, lets say that the same system uses, instead of it's own computers, a cloud service that is suited for it's needs. Ideally, when the workload is up one work unit the cloud will provide the system with another "computing unit", when workload goes back down the cloud will gracefully stop providing that computing unit. That is a situation where a system is both scalable and elastic.