Introducing Scalability

When they launch a new website, many developers have visions of having created the next Google, Amazon, or Facebook. Unfortunately, the developers have no way of knowing who the actual user demand will be. Should the site become widely used, it may require additional servers or disk-storage capacity.

Scalability defines a site or application’s ability to use additional resources on demand. The site or application may scale up to utilize additional resources when the system is experiencing high user demand and may later scale down its resource usage when the user demand declines. The ability for the cloud to scale up and down in this way, on demand, is described using the terms cloud elasticity. In other words, the cloud is said to be elastic based upon its ability to scale.

Applications that run within the cloud are normally highly scalable. An application administrator can manually add or remove resources, or the application can be configured to scale automatically. As shown in FIGURE 1-9, applications scale through the use of additional servers (physical or virtual) or through the addition of disk-storage space.