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10.4 The Concept of a Design Space
As with any complex taxonomy, it is useful to classify both dependencies and coordination processes using multidimensional design spaces (Bell 1972; Lane 1990). Each dimension of the design space describes variation in some design choice. Values along a dimension are called design alternatives. They correspond to alternative requirements or implementation choices. For example, when selecting a data transportation mechanism, the number of data readers could be one design dimension; the location of readers relative to the writer could be another. Figure 10.2 illustrates a tiny design space for selecting a data transportation mechanism. Specific designs are described by points in the design space, identified by the dimensional values that correspond to their design choices.
Figure 10.2: Simple design space for selecting a data transportation mechanism
Successful design spaces reduce the problem of design to that of answering a simple set of questions. They also organize related design alternatives ''close''to each other and expose correlations between various aspects of design. Finally they can be easily translated into computerized knowledge bases that can help semi-automate the design task.Our problem requires the construction of two, related, design spaces:
A dependency design space. Dependency design dimensions represent interaction requirements that are significant for choosing a coordination processes. For example, the number of users of a resource and the degree of concurrency allowed by a resource are two dependency design dimensions.Each point in the dependency design space defines a different dependency type.
A coordination design space. Coordination design dimensions represent design alternatives available to satisfy interaction requirements. For example, the protocol used to share a nonconcurrent resource is an implementation design dimension.Each point in the coordination design space defines a different coordination process.
In addition, each point in the dependency design space (dependency type) must be associated to a coordination design space for managing it. Our objective in the following sections is to define related dependency and coordination design spaces for each family of dependencies.
Figure 10.3: Taxonomy of resources
The success of a design-space description of design alternatives clearly lies in the choice of dimensions and specific dimensional values (design alternatives). There is no obvious rigorous way of defending a particular set of choices. Neither Bell and Newell (1972) nor Lane (1990) have offered any justification for their dimensions and alternatives, except for their own intuition and the usefulness of the resulting description. I will follow the same path, simply proposing a set of dimensions and trying to show empirically that they form a useful description of design alternatives.
