Modeling Visibility in Hierarchical Systems Debmalya Biswas INRIA, France K. Vidyasankar Memorial University, Canada

Dec 2006DASI, Italy2 Hierarchical Systems Rooted trees Nodes represent entities Edges represent binary relationships between entities Motivational scenario – Hierarchical Web Services Compositions

Dec 2006DASI, Italy3 Web Services Composition Composition relates to dealing with the assembly of autonomous components so as to deliver a new service out of the existing services.

Dec 2006DASI, Italy4 Hierarchical Composition Hierarchical composition refers to the ability to form a composite service by combining already existing services, which themselves might be composed of other composite/primitive services. Composite Travel & Shipping Service Composite Travel Booking Service Shipping Service Flight Booking ServiceHotel Booking Service

Dec 2006DASI, Italy5 Hierarchical Systems Rooted trees Nodes represent entities Edges represent binary relationships between entities Motivational scenario – Hierarchical Web Services Compositions Nodes are services Edges are parent-child relationship of a service invoking another service

Dec 2006DASI, Italy6 Need for Nontrivial Visibility Not just between parent and children Arbitrary visibility, without any restriction, may not be acceptable In dynamic and heterogeneous environments, trust, autonomy, etc. force selective visibility Very important in large scale systems with hundreds of entities

Dec 2006DASI, Italy7 Fig. courtesy of “Y. Brave, M. Heymann. Control of Discrete Event Systems Modeled as Hierarchical State Machines. In proceedings of 30 th Conference on Decision & Control, 91.”

Dec 2006DASI, Italy8 Visibility For a pair of providers X-Y in the hierarchy, we would like to capture if X can see Y, i.e., if X has visibility over Y. In a general setting, X has visibility over Y if − X wishes to see Y: X may be interested in Y due to functional (get input, send output/notification) or non-functional (transactions, monitoring, end-user interaction) requirements. − Y does not have any objections to X seeing it: Security, privacy, confidentiality, etc. issues play an important role in determining the visibility allowed by a provider. − Remaining nodes in the hierarchy do not have any objections to X seeing Y: Contractual agreements between Y and another node Z may have a bearing on X seeing Y.

Dec 2006DASI, Italy9 E-shopping scenario

Dec 2006DASI, Italy10 E-shopping scenario (contd.) The store S has visibility over its parent and all its children. It does not have visibility over (its descendents) the courier companies C-A and C-B used for the shipment.

Dec 2006DASI, Italy11 E-shopping scenario (contd.) The courier company C-B has visibility of all its ancestors, namely, S-B, S and U, to keep them informed of the delivery status – strong visibility.

Dec 2006DASI, Italy12 E-shopping scenario (contd.) However, the bonus air miles processing unit B has visibility over only the card company H and customer U. It is only concerned with the customer's credit card number and the purchase amount without any need to know the context, namely, the goods purchased and the store. - weak visibility

Dec 2006DASI, Italy13 E-shopping scenario (contd.) Partial visibility

Dec 2006DASI, Italy14 Spheres of Visibility (SoV) In the SoV of X, in addition to the nodes visible to X, we capture their "type of visibility". Let V [X,Y] denotes the type of visibility X has over Y. If V [X,Y] has some edges, then X has a partial strong reference to Y. If V [X,Y] is H[X,Y], that is, it has all the nodes and edges in the path from X to Y, then X has a strong reference to Y. X has a weak visibility (or weak reference) to any node Y that is visible to X.

Dec 2006DASI, Italy15 Coherence For each pair of nodes X and Z, for every node Y in the path from X to Z, Coherence: the strength of visibility of X over Y is at least as much as the strength used for visibility of X over Z. X Y Z

Dec 2006DASI, Italy16 Correlation For each pair of nodes X and Z, for every node Y in the path from X to Z, Coherence: the strength of visibility of X over Y is at least as much as the strength used for visibility of X over Z. Correlation: the strength of visibility of Y over Z is at least as much as the strength used for visibility of X over Z. X Y Z

Dec 2006DASI, Italy17 Coherence – Correlation We show that the two properties, the SoV’s of all the nodes in a hierarchy H are (i) coherent and (ii) correlated, are orthogonal, i.e., independent of each other.

Dec 2006DASI, Italy18 Sphere of Noticeability (SoN) SoN(X) captures: Which nodes have visibility over X? What type (strength) of visibility they have of X? An obvious application of SoN is for change management. For example, a provider X notifying the providers, who have visibility over X, when there is some change in the provider URI (provider details), metrics used to compute the service (service details), log format (execution details), etc.

Dec 2006DASI, Italy19 Relationship - SoV and SoN Weak reference, partial strong reference, strong reference, coherence, correlation properties for SoN can be defined analogous to those for SoV. Property: In a hierarchy H, a Visibility assignment is coherent if and only if the corresponding Noticeability assignment is correlated, and vice versa.

Dec 2006DASI, Italy20 THANK YOU References D. Biswas, K. Vidyasankar, “Modeling Visibility in Hierarchical Systems”, In proc. of ER 06, LNCS 4215, pp D. Biswas, K. Vidyasankar, “Spheres of Visibility”, In proc. of the 3 rd IEEE European Conf. on Web Services (ECOWS) 05, pp Contact: