Gartner just published an important Cloud/PaaS-related research note “Application Infrastructure for Cloud Computing: a Growing Market, 2010”. Here I just want to share a footnote from that paper and see if some might want to disagree, add-on or comment in any other manner. This text is trying to deal with the nuance of some flavors of elasticity and tenancy — all-important to enabling cloud computing.
Elasticity, Horizontal Scalability and Multitenancy
Elasticity, horizontal scalability and multi-tenancy are often mentioned here and elsewhere as key distinguishing characteristics (or requirements) of cloud-computing technologies. Without attempting to provide a full-proof definition, the common-sense nature of these terms is as follows:
- Elasticity is about the ability of software to meet changing capacity demands, deploying and releasing relevant necessary resources on-demand.
- Horizontal scalability is about the ability of software to automatically expand onto additional underlying resources, or contract onto reduced resources, without interrupting operations. (Typically, horizontal scalability refers to the ability of software to automatically spread over added, or contract over reduced, number of physical or virtual machines.)
- Multi-tenancy refers to the ability of software to be offered to multiple user entities (tenants) in a way so that each tenant operates as logically isolated, while, in fact, using physically shared resources. A tenant can be an organization co-using an application with other tenants. It can also be an application co-using underlying resources with other applications.
Multi-tenant elasticity is achieved when resources are allocated and accounted to tenant application instances. Multitasking elasticity occurs at lower levels when resources of one tenant are dynamically allocated to multiple tasks inside an application instance. Multitasking elasticity improves the performance of some applications. It does not amount to a new business model in business use of the software. Only multi-tenant elasticity is the definitive attribute of cloud computing.
… and photography is my hobby. Here is Long Beach Island, NJ on a less than bright and sunny day