Formalizing visibility characteristics in hierarchical systems

摘要

We consider hierarchical systems where nodes represent entities and edges represent binary relationships among them. An example is a hierarchical composition of Web services where the nodes denote services and the edges represent the parent–child relationship of a service invoking another service. A fundamental issue to address in such systems is, for two nodes X and Y in the hierarchy, whether X can see Y, that is, whether X has visibility over Y. The visibility could be with respect to certain attributes like operational details, execution logs and security related issues. In a general setting, X seeing Y may depend on (i) X wishing to see Y, (ii) Y wishing to be seen by X, and (iii) other nodes not objecting to X seeing Y. In this paper, we develop a generic conceptual model to express visibility. We study two complementary notions: sphere of visibility of a node X that includes all the nodes in the hierarchy that X can see; and sphere of noticeability of X that includes all the nodes that can see X. We also identify dual properties, coherence and correlation, that relate the spheres of different nodes in special ways and also relate the visibility and noticeability notions. We study some variants of coherence and correlation also. These properties give rise to interesting and useful visibility and noticeability assignments, and their representations.