What is the difference between scalability and elasticity?

Question

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

SCALABILITY - ability of a softwa

Answer 1

Elasticity is the ability of a system to increase (or decrease) its compute, storage, netowrking, etc. capacity based on specified criteria such as the total load on the system.

For example, you can implement a backend system that initially has 1 server in its cluster but configure it to add an extra instance to the cluster if the average per minute CPU utilization of all the servers in the cluster exceeds a given threshold (e.g. 70%).

Similarly, you can configure your system to remove servers from the backend cluster if the load on the system decreases and the average per-minute CPU utilization goes below a threshold defined by you (e.g. 30%).

As another example, you can configure your system to increase the total disk space of your backend cluster by an order of 2 if more than 80% of the total storage currently available to it is used. If for whatever reason, at a later point, data is deleted from the storage and, say, the total used storage goes below 20%, you can decrease the total available disk space to its original value.

But some systems (e.g. legacy software) are not distributed and maybe they can only use 1 CPU core. So even though you can increase the compute capacity available to you on demand, the system cannot use this extra capacity in any shape or form. Such systems are not scalable. But a scalable system can use increased compute capacity and handle more load without impacting the overall performance of the system.

A scalable system does not depend on elasticity though. Traditionally, IT departments could replace their existing servers with newer servers that had more CPUs, RAM, and storage and port the system to the new hardware to employ the extra compute capacity available to it.

Cloud environments (AWS, Azure, Google Cloud, etc.) offer elasticity and some of their core services are also scalable out of the box. Furthermore, if you build a scalable software, you can deploy it to these cloud environments and benefit from the elastic infrastructure they provide you to automatically increase/decrease the compute resources available to you on-demand.