System X-ability

Scalability

Scalability - is an ability of the system to operate as the number of users or requests increase.

Metrics:

• Is performance and responsiveness impacted?

  • average response times across users per context

  • max response times across users per context

• Is there an increase in timeouts?

  • number of timeouts across users per context

• Is there an increase in system crashes?

  • number of crashes across users or requests

• Is there enough database capacity?

• Is there enough vm capacity?

Scalable system

NON scalable system

Not scalability issue

Fitness functions

Send an alert when the number of crashes (timeouts, response time duration) increases by 20% and the number of users increases by 8%. (Numbers are just an example)

Scalability vs performance

Scalability - CONSISTENT response time when load increases. And this consistency may cost some performance of the system.

In this example if the systems requires to scale a lot then DB connections are becoming a bottle neck. Solution:

• introduce caching of the data. This will decrease amount of connections and dramatically increase a scalability.

Elasticity

Elasticity is the ability of a system to remain responsive during significantly high instantaneous spikes in user load