Scalability is a measure of how an application would react when the resources at its disposal are scaled up. If an application supports 10,000 users with a given infrastructure, can it support at least 20,000 users with double the infrastructure?

If a web application does not respond within a couple of seconds, there is a very high chance that the user of your application will be disappointed. Performance usually refers to the ability of a system to provide an agreed-upon response time for a defined number of users.

In the world of Cloud, the scalability of applications becomes even more important. It's difficult to guess how successful a startup might be. Twitter or Facebook might not have expected such success when they were incubated. Their success, for large measure, depends on how they were able to adapt to a multi-fold increase in their user base without affecting the performance.

We will discuss building highly scalable microservices using Spring Cloud and Spring Data Flow in Chapter 9, Spring Cloud and Chapter 10, Spring Cloud Data Flow.