Filtering Hypermedia Network
by Using View Mechanism

Georges GARDARIN and Shim YOON

PRiSM Laboratory

University of Versailles Saint-Quentin

45 Avenue des Etats-Unis

78035 Versailles Cedex, FRANCE

email: Georges.Gardarin@prism.uvsq.fr

http://www.prism.uvsq.fr/english/databases/geotropics/people/gardarin_us.html

email: Shim.Yoon@prism.uvsq.fr

http://www.prism.uvsq.fr/english/databases/geotropics/people/yoon_us.html

ACM Copyright Notice

Abstract

Table of Contents

• Introduction
• Hypermedia Database Model and HyOQL
• HyView Definition
• HyView Functional Description
  • Multiple Views
  • Automatic Selective Type Closing
  • Object and Relationship Creation
  • New User-defined Methods
• Conclusion
• Bibliography

Introduction

While these research efforts have extended the hypermedia query facilities, the hypermedia network filtering ability is often too restricted. Regarding a query language as a supplementary access mechanism to the navigation, we need to keep the hypermedia sub-network satisfying query criteria and filter out irrelevant information. In this sub-network, the users could locate the information of interest to them by navigation. The work presented in this paper shows that the hypermedia network filtering facility can be provided by using a view mechanism by integrating the view concept in the HyOQL.

The remainder of this paper is organized as follows. The next section presents briefly the hypermedia database model and the HyOQL query language, and in the following two sections, the view mechanism is explained by giving its definition language and by giving its functional description. The last section concludes the paper.

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Hypermedia Database Model and HyOQL

Definition 1: A database Schema is a triple S = (C, R, P) where,

• C is a set of classes where a class c is a pair (T, M). T is the type of the class of the form [a1: t1, a2: t2,..., an: tn] where an is a name of an attribute and tn is a type of the attribute an. M is a set of methods.
• R is a set of Is_a relationships that is a subset of { c < c' | c, c'E C} where c < c' denotes that c is a c' meaning that c is a sub class of c' and c' is a super class of c.
• P is a set of persistence entry points.

• C is a set of classes. C = {c | Exist c=HyNode, Exist c=HyLink, Exist c=HyAnchor, c E C}. This means that there exist three predefined classes, HyNode, HyLink and HyAnchor in a set of classes of a schema.
• HyNode is a class of a pair of (T, M) where, T = [inlinks: {HyLink}, outlinks: {HyLink}] where the attribute inlinks is a set of incoming hyperlinks and the attribute outlinks is a set of outgoing hyperlinks from this class.
• HyLink is a class of a pair of (T, M) where, T = [srcanchors: HyAnchor, desanchors: HyAnchor] where the attribute srcanchor is a source anchor and the attribute desanchor is a destination anchor ,being both of type HyAnchor.
• R is a set of Is_a relationship. R = {c < c' | Forall c E C : (c' < HyNode)}. It means that the root class of any application class should be the HyNode class, providing for application classes to inherit from HyNode class capabilities.

For example, the database schema of the Figure 1 can be represented as S = (C, R, P) where, C = { Paper, Author, Abstract, Para, Comment }, Paper = ([title: string, author: {Author}, abstract: {Para}], M), Author = ([name: string], M), Para = ([text: string], M), Comment = ([result: string, text: {Para}], M), R = {}, and P = {Papers, Comments}.

The hypermedia database schema of this example is HS = (C, R, P) where, C = {HyNode, HyLink, HyAnchor, Paper, Author, Abstract, Para, Comment }, R = { Paper < HyNode, Author < HyNode, Abstract < HyNode, Para < HyNode, Comment < HyNode}, and P = {Papers, Comments}.

Figure 1: A Database Schema vs. A Hypermedia Database Schema

HyOQL query facilities are achieved by providing special link operators, additional quantifiers and the methods dealing with both content-based and structure-based access. The operations supporting queries are implemented in HyNode class. HyNode class is the class from which the user document classes inherit their common properties. This class models a node consisting of outgoing and incoming links to/from another node in a hypermedia document network. The following HyOQL query is to find all the paper nodes having at least two comments of "accept" and the comment nodes not having any incoming link. We used the backnodes and backlinks operations defined in the HyNode class to get the incoming nodes.

	select 	p
	from 	p in Papers
	where	atleast 2 c in p.backnodes: c.result = "accept"
		and
		not exists l in c.backlinks: true

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HyView Definition

• VC is a set of virtual classes where a virtual class vc is a pair (vT, vM) where vT is the type of the virtual class and vM is a set of virtual methods.
• VR is a set of virtual Is_a relationship. VR = {vc < vc' | vc, vc' E VC}.

	view	view_name	
		[ inherit  	super_class_name]+
		[ type		type_definition]
		[ methods	method_list]]+
		as
		select	select_clause
		from	from_clause
		[where	where_clause]
	end

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HyView Functional Description

• Multiple views can be defined with a unique query.
• Selective automatic type closing is chosen to complete schema reduction.
• Object and relationship creation permit to create new hypermedia nodes and hyperlinks in the basis of existing nodes.
• A view may be equipped with new user-defined methods. This feature allows the user to define specific methods for a view, including presentation methods.

Multiple Views

Multiple views are provided for filtering hypermedia network and keeping hyperlinks between nodes in the filtered portion of network. Figure 3 illustrates a multiple view. The arrows represent hyperlinks between nodes. A user interested only in comments and papers can define a multiple view by referring the names of nodes, that is, comment and paper nodes. The user ignores the other nodes and his navigation scope will be in these two nodes.

Suppose that we want to extract a subnetwork of all papers having at least two "strongly accept" comments, and their associated comments. The appropriate view definition is as follows.

view	PCview	
	type  	paper(p), comment(c)
	as 	select p,c 
		from p in Papers,c in p.backnodes
		where atleast 2 in c: c.note = "strongly accept"
end

Figure 3: Multiple View

< -- HyView Functional Description

Automatic Selective Type Closing (Schema reduction)

Figure 4: Selective Type Closing

If we apply this type closure checking for any referenced nodes, all the hyperlinks outgoing from the nodes must be visible. The hyperlinking structure is expressed by the two attributes, inlinks and outlinks, in the HyNode class from which application classes inherit its properties. If the type closure is done automatically for this structure, all the outgoing nodes referenced by the outlinks attributes could be reached from the chosen node in the view. For example, in the Figure 4, Report, Thesis, and Note nodes could be accessed from Paper node. This choice, however, is not adequate to filter a sub-network since the hypermedia network can not be restrained in this way.

In our view mechanism, we do not enforce the closure property for hyperlinks so that the hypermedia network is limited by the boundary of the view. In the above example, the only hyperlinks permitted are the links between paper nodes and comment nodes. It means that even if there exist other hyperlinks from a paper node to a Report, a Thesis, and a Note, the access to these hyperlinks is not permitted.

We adopt in our view mechanism the automatic selective type closure checking, meaning that the choice whether the type closure should be ensured or not depends on the two types of links.

• For hierarchical links, the type closure is ensured. In the example of Figure 4, even the class Para is not explicitly defined in the view definition, this class must be accessible from the Paper or the Comment classes. If not, users can not display any contents of paper documents since no attribute of the class Paper is defined. To ensure type closure for the hierarchical structure, if there are the classes not defined explicitly in the view definition but used as types of another class, these missed classes are inserted in the view schema automatically. It means that the class Para will be inserted implicitly in the view schema even though they are not defined.

• For hyperlinks, the type closure is not ensured. No type closing for the hyperlinking structure makes invisible the classes that are not included in the view definition. With this choice, we achieve the goal of reducing hypermedia network.

< -- HyView Functional Description

Object and Relationship Creation

These two views, AcceptedPaper and ConfInfo, defined with the base class Paper and Conf respectively do not have newly created objects because they restrict only part of the extent of the base classes. The objects of these views have the same object identifiers as the objects of base classes.

The new relationship is created from the view AcceptedPaper to the view ConfInfo. This is a hyperlink relationship. In the view definition, we have to redefine the attribute outlinks of the view by using a query that gets information about the conference where the paper is accepted.

Figure 5: Object and Relationship Creation

< -- HyView Functional Description

New User-defined Methods

To respond to the needs of multiple representation of a hypermedia network or complex objects, we can define new presentation methods in a view. We can define the methods in order to represent listing, tree, graph, and tabular form of presentation.

< -- HyView Functional Description

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Conclusion

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Bibliography

[Beeri90]

C. Berri and Y. Koranatzky. "A Logical Query Language for Hypertext Systems." The First European Conference on Hypertext (ECHT), Pages 67-80, INRIA, France, November 1990.

[Consens89]

M. P. Consens and A. O. Mendelzon. "Expressing Structural Hypertext Queries in GraphLog." Hypertext '89, Pages 269-292, November 1989.

[Cruz89]

I. F. Cruz, A. O. Mendelzon, and P. T. Wood. "G+: Recursive Queries Without Recursion." In 2nd International Conf. on Expert Database Systems, Pages 645-666. The Benjamin/Cummings Publishing Company Inc. 1989.

[Gardarin94]

G. Gardarin and S. Yoon. "Modeling and Querying Structured Hypermedia Documents." Technical Report, PRiSM Laborotory, University of Versailles, Online document available at URL file://ftp.prism.uvsq.fr/pub/reports/TR-1994/1994.051.ps.gz, December 1994.

[Gardarin95]

G. Gardarin and S. Yoon. "Object-Oriented Modeling and Querying of Hypermedia Documents." DASFAA '95, Pages 441-448, Singapore, April 1995.

[Heiler90]

S. Heiler and S. Zdonik. "Object Views: Extending the Vision." Data Engineering, Pages 86-93, 1990.

[ODMG93]

Obect Database Management Group. "The Object Database Standard", Pre-Publication Version, July 1993.

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