Local Propagation and Rule Triggers

In constraint world, relationships are modelled as constraints. Constraints allow the user to declare a set of relations on a set of objects and the task of finding the values (that satisfy these constraints) is left to a constraint solver. A constraint solver satisfies the constraints by a number of different constraint satisfaction techniques [9].

A commonly used technique for constraint satisfaction, Local Propagation [11], assumes constraints can be modelled as a graph where nodes represent operators and edges represent the possible flow of values. When a node receives sufficient value from its edges, it triggers, calculates one or more values for the edges that do not contain values and sends these new values out. These new values in turn may cause other nodes to trigger but the current node is unaware of such future triggers. Figure 3 describes the possible rules for one such node that models addition of two numbers.

Figure 3: Rules for a plus node.

Note here the rule triggers are local to each node and only involve information that is contained on the edges that connect to this node. Inspired by this approach of deducing unknown values from known values, we will base our formalism to reflect how relationships can deduce unknown form field values from known form field values by means of rule triggers.