Cloud computing overlaps some of the concepts of distributed, grid and utility computing, however it does have its own meaning if contextually used correctly. The conceptual overlap is partly due to technology changes, usages and implementations over the years.
Trends in usage of the terms from Google searches shows Cloud Computing is a relatively new term introduced in the past year. There has also been a decline in general interest of Grid, Utility and Distributed computing. Likely they will be around in usage for quit a while to come. But Cloud computing has become the new buzz word driven largely by marketing and service offerings from big corporate players like Google, IBM and Amazon.
The term cloud computing probably comes from (at least partly) the use of a cloud image to represent the Internet or some large networked environment. We don’t care much what’s in the cloud or what goes on there except that we depend on reliably sending data to and receiving data from it. Cloud computing is now associated with a higher level abstraction of the cloud. Instead of there being data pipes, routers and servers, there are now services. The underlying hardware and software of networking is of course still there but there are now higher level service capabilities available used to build applications. Behind the services are data and compute resources. A user of the service doesn’t necessarily care about how it is implemented, what technologies are used or how it’s managed. Only that there is access to it and has a level of reliability necessary to meet the application requirements.
In essence this is distributed computing. An application is built using the resource from multiple services potentially from multiple locations. At this point, typically you still need to know the endpoint to access the services rather than having the cloud provide you available resources. This is also know as Software as a Service. Behind the service interface is usually a grid of computers to provide the resources. The grid is typically hosted by one company and consists of a homogeneous environment of hardware and software making it easier to support and maintain. Once you start paying for the services and the resources utilized, well that’s utility computing.
Cloud computing really is accessing resources and services needed to perform functions with dynamically changing needs. An application or service developer requests access from the cloud rather than a specific endpoint or named resource. What goes on in the cloud manages multiple infrastructures across multiple organizations and consists of one or more frameworks overlaid on top of the infrastructures tying them together. Frameworks provide mechanisms for:
resource registration and discovery
service level agreement definitions
The cloud is a virtualization of resources that maintains and manages itself. There are of course people resources to keep hardware, operation systems and networking in proper order. But from the perspective of a user or application developer only the cloud is referenced. The Assimilator project is a framework that executes across a heterogeneous environment in a local area network providing a local cloud environment. In the works is the addition of a network overlay to start providing an infrastructure across the Internet to help achieve the goal of true cloud computing.