Modeling Visibility in Hierarchical Systems

Debmalya BiswasINRIA, France

K. VidyasankarMemorial University, Canada

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Hierarchical Systems Rooted trees

Nodes represent entities

Edges represent binary relationships between entities

Motivational scenario – Hierarchical Web Services Compositions

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Web Services CompositionComposition relates to dealing with the assembly of autonomous

components so as to deliver a new service out of the existing services.

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Hierarchical CompositionHierarchical 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 Service

Hotel Booking Service

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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

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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

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

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VisibilityFor 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.

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E-shopping scenario

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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.

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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.

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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

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E-shopping scenario (contd.)

Partial visibility

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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.

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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

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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

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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.

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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.

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Relationship - SoV and SoNWeak 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.

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THANK YOUReferences

D. Biswas, K. Vidyasankar, “Modeling Visibility in Hierarchical Systems”, In proc. of ER 06, LNCS 4215, pp. 155-167.

D. Biswas, K. Vidyasankar, “Spheres of Visibility”, In proc. of the 3rd IEEE European Conf. on Web Services (ECOWS) 05, pp. 2-13.

Contact: debmalya.biswas@inria.fr