While trying to define Grid may well be a hopeless task, it is certainly useful and feasible to talk about the different elements of the service-oriented ecosystem. That is what Steve Tuecke and I did in a recent article, "Describing the Elephant: The Different Faces of IT as Service."
The introduction to this article explains what it is about:
In a well-known story, a group of blind men are asked to describe an elephant. Each encounters a different part of the animal, and not surprisingly provides a different description.
We see a similar degree of confusion in the IT industry today, as terms like service-oriented architecture, Grid, utility computing, on-demand, adaptive enterprise, data center automation, and virtualization are bandied about. As when listening to the blind men, it can be hard to know what reality lies behind the words, whether and how the different pieces fit together, and what we should be doing about the animal(s) that are being described. (Of course, in the case of the blind men, we did not also have marketing departments in the mix!)
Our goal in this article is to shed some light on these matters and provide, in effect, a description of the elephant. More specifically, we describe what we see as a major technology trend that is driving many related efforts, namely the transformation from vertically integrated silos to horizontally integrated, service-oriented systems. We explain how various popular terms relate to this overarching trend, and describe the technology required to realize this transformation.
As does the summary:
We have argued that SOA, grid, on-demand, utility computing, software as service, and other related terms all represent different perspectives on the same overall goal—namely, the restructuring of enterprise IT as a horizontally integrated, service-oriented architecture. If successfully realized, that goal will see in-house, third-party, and outsourced applications all operating in a uniform environment, with on-demand provisioning of both in-house and outsourced hardware resources—and also, of course, high degrees of security, monitoring, auditing, and management.
This Holy Grail of open, standards-based, autonomically managed software and dynamically provisioned hardware has certainly not yet been achieved. That does not mean, however, that enterprises cannot start today to create horizontally integrated, service-oriented infrastructures. Solid Web services products allow for the creation of service-oriented applications. Mature commercial and open source virtualization and workload management products and open source grid infrastructure software provide what is needed to create horizontally integrated infrastructure to sit behind those applications. Integration remains more of an exercise for the customers (or their services vendors) than is desirable, but that situation should change as independent software vendors start to grid-enable their products. Meanwhile, progress on further standards is accelerating as experience is gained with deployments and pressure builds from end users for interoperable solutions.