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Modularity

The rules above for scripts allow scripts to be added freely to our debugging schema without any attention to precedences between scripts as long as they all remain homogeneous, convergent, and aware of their effects. These requirements allow a new form of `phenomenological' software modularity; a modularity based upon effect rather than interface. If scripts can maintain agreement about their overlapping effects, they may be freely mixed and employed toward a common goal. Maelstrom's algorithm is nothing more than a `phenomenological sort' of phenomenologically modular scripts into a total order within which each can function properly.

Traditional ideas of software modularity express modules as being defined by interfaces with specified preconditions and postconditions on the use of each interface. For Maelstrom, instead modularity is based solely upon effect and postconditions; modularity is a requirement of the postconditions of the network after the script finishes. Every script must check its preconditions itself and only proceed if they are appropriate, so all scripts are expected to function correctly for all possible sets of preconditions.

Of course, what has been omitted so far in this discussion is the difficulty of writing scripts with the appropriate properties of environmental awareness, convergence, and homogeneity. In general it is difficult to construct such scripts while assuring script quality and reliability[10]. It is this difficulty that justifies the use of complex tools such as Cfengine[2,3,4] to fulfill that purpose instead.


next up previous
Next: Assuring convergence Up: Phenomenology Previous: Phenomenology
Alva L. Couch
2001-10-02