In general, a reference architecture is a generic and somewhat abstract blueprint-type view of a system that includes the system’s major components, the relationships among them, and the externally visible properties of those components. A reference architecture is not usually designed for a highly specialized set of requirements. Rather, architects tend to use it as a starting point and specialize it for their own requirements.
Models are abstract, and you can’t implement, or have an instance of, an abstract thing. Reference architectures are more concrete. They have aspects that abstract models do not, including cardinality (a measure of the number of elements in a given set), infrastructure, and possibly the concept of spatial-temporal variance (adding time as a concept).
Consider again the example of residential architecture. The domain of residential dwellings has both an implied model and a reference architecture for a class of things called “houses.” The implied model for a house is composed of or aggregated from the following components:
A foundation and/or other subfloor structure to connect the house to the underlying environment, whether it is earth or a body of water
Floors to stand on
Exterior walls to keep out the elements of nature and to provide structural support for the roof
A roof to protect the dwelling’s contents and occupants from the elements of nature and to provide privacy
Some form of entry and exit (possibly implemented as a doorway)
External links to some form of consumable energy (an interface to connect to the electricity grid, a windmill, or some other electricity-generating device)
This model for a house is minimalist and very abstract. It doesn’t detail such things as the type or number of floors, the height of the ceilings, the type of electrical system, and other things that become relevant only when a more concrete architecture (perhaps expressed as a set of blueprints) is made based on a specific set of requirements, such as “a residential dwelling in Denver for a family of five” or “a one-bedroom apartment in Berlin for an elderly person.”
Although this model is very abstract, we can add details to each item in the reference architecture. For example, we can specify that the foundation be built in the shape of a rectangle. Other aspects in the reference architecture become more concrete if we then specialize the model for human inhabitants. For example, the floor must be edged by exterior walls of sufficient height to allow humans to walk through the house without stooping, and interior walls must be constructed to separate the rooms of the house based on their purposes.
A reference architecture expressed as a set of generic blueprints based on the model discussed at the beginning of this section will be insufficient to actually build a modern residential dwelling. It doesn’t contain sufficient detail to serve as a set of builder’s plans. However, an architect can take the reference architecture and specify additional details to create the kind of plan that builders need. For example, the architect can specify the correct energy conduit to account for North American electricity delivery standards, design the interior floor plan based on the requirements of the house’s inhabitants, and so on. In sum, a reference architecture plays an important role as a starting point upon which more specialized instances of a class of thing can be built, with particular purposes and styles addressed as needed.
The Web 2.0 Reference Architecture, therefore, can provide a working framework for users to construct specialized Web 2.0 applications or infrastructures from specific sets of requirements. We’ll explore that architecture next.