The Open Grid Services Architecture (OGSA), developed by The Global Grid Forum, aims to define a common, standard, and open architecture for grid-based applications. The goal of OGSA is to standardize practically all the services one finds in a grid application (job management services, resource management services, security services, etc.) by specifying a set of standard interfaces for these services.

However, when the powers-that-be undertook the task of creating this new architecture, they realized they needed to choose some sort of distributed middleware on which to base the architecture. In other words, if OGSA (for example) defines that the JobSubmisionInterface has a submitJob method, there has to be a common and standard way to invoke that method if we want the architecture to be adopted as an industry-wide standard. This base for the architecture could, in theory, be any distributed middleware (CORBA, RMI, or even traditional RPC). For reasons that will be explained further on, Web Services were chosen as the underlying technology.

However, although the Web Services Architecture was certainly the best option, it still had several shortcomings which made it inadequate for OGSA's needs. OGSA overcame this obstacle by defining an extended type of Web Service called Grid Service (as shown in the diagram: Grid Services are defined by OGSA). A Grid Service is simply a Web Service with a lot of extensions that make it adequate for a grid-based application (and, in particular, for OGSA). In the diagram: Grid Services are an extension of Web Services. Finally, since Grid Services are going to be the distributed technology underlying OGSA, it is also correct to say that OGSA is based on Grid Services.

OGSA alone doesn't go into much detail when describing Grid Services. It basically outlines what a Grid Service should have (that Web Services don't) but little else. That is why OGSA spawned another standard called the Open Grid Services Infrastructure (OGSI, also developed by The Global Grid Forum) which gives a formal and technical specification of what a Grid Service is. In other words, for a high-level architectural view of what Grid Services are, and how they fit into the next generation of grid applications, OGSA is the place to go. For an excruciatingly detailed specification of how Grid Services work, OGSI is the place to go. In the diagram: Grid Services are specified by OGSI (as opposed to simply 'being defined' by OGSA). The Related Documents section has a link to the Grid Service Specification.

The Globus Toolkit is a software toolkit, developed by The Globus Alliance, which we can use to program grid-based applications. The third version of the toolkit (GT3) includes a complete implementation of OGSI (in the diagram GT3 implements OGSI). However, it's very important to understand that GT3 isn't only an OGSI implementation. It includes a whole lot of other services, programs, utilities, etc. Some of them are built on top of OGSI and are called the WS (Web Services) components, while other are not built on top of OGSI and are called the pre-WS components. We'll take a closer look at the GT3 architecture shortly.

Consider the following simple example. Suppose you want to build a new house. The first thing you need to do is to hire an architect to draw up the blueprints, so you can get an idea of what your house will look like. Once you're happy with the architect's job, it's time to hire an engineer who will plan all the construction details (where to put the master beams, the power cables, the plumbing, etc.). The engineer then passes all his plans to qualified professional workers (construction workers, electricians, plumbers, etc) who will actually build the house.

We could say that OGSA (the definition) is the blueprints the architect creates to show what the building looks like, OGSI (the specification) is the structural design that the engineer creates to support the architect's vision for the building, and GT3 is the bricks, cement, and beams used to build the building to the engineer's specifications.