Benefits of Open Hypermedia Systems Using Advanced Database Concepts

Informatik ‘96, GI/OCG Jahrestagung, Klagenfurt, Austria. Oldenbourg, September 1996.The Benefits of Open Hypermedia Systems Using AdvancedDatabase Concepts1G. Kappel, S. Rausch-Schott, S. Reich, W. RetschitzeggerDept. of Information Systems,Johannes Kepler University of Linz,A-4040 Linz, AustriaE-Mail: {gerti | stefan | sre | werner}@ifs.uni-linz.ac.atAbstractLarge engineering enterprises are just one application area of open hypermedia systems (OHS), which are characterized by both, open hyperbase and link server functionality. An open hyperbase allows besides others to store and retrieve an extensible set of hypermedia objects. A link server allows to store and retrieve connectivity information and supports consistency management thereof. To provide these functionalities, open hypermedia systems have to tackle several important issues such as interoperability, extensibility, and computation. Using advanced database concepts as OHS baseline architecture these issues can be elegantly resolved. In particular, this paper proposes an OHS architecture based on an active object-oriented database system as well as SGML/HyTime as standardized interchange formats. By using active object-oriented database systems extensibility and computation including consistency management are ensured. By using international standards inter-operability of hypermedia information systems is reached. The proof-of-concept prototype TriGS SGML based on the commercial object-oriented database system GemStone and its active exten-sion TriGS is currently under development.grant GZ 140 1/4-I/A/8/95.1.IntroductionOpen hypermedia systems (OHS) are among the most important and most challenging applications of modern information system technology today. Application domains of OHS are manifold like large engineering enterprises, electronic publishing environments, heterogeneous workflow systems, and integrated software development environments, to mention just a few. In all these domains, profes-sionals are manipulating heterogeneous hypermedia documents using proprietary editing tools. Hypermedia documents refer to inter-linked multimedia objects, and multimedia objects cover data, structured and unstructured text, images, video, and voice information. These documents consist also of links between each other representing various kinds of dependencies. If a new kind of document and a new kind of link is needed they have to be integrated seamless. Thus, what has to be supported is the collaborative manipulation of hypermedia objects in an extensible setting.So, what should open hypermedia systems provide? In short, an OHS should provide the link man-agement between and the integration of an extensible set of tools as well as mechanisms for the stor-age, retrieval and manipulation of an extensible set of hypermedia documents. The former is also referred to as link server functionality, the latter as open hyperbase functionality [36]. Representatives of the former are Microcosm [15] and Chimera [3], representatives of the latter are DeVise Hyperme-dia (DHM) [23], HyperForm [40], and SP3/4 [33]. Without going into details of these systems — the interested reader is referred to the literature [36, 41] — one can observe that OHS technology would gain from incorporating advanced database concepts, such as extensible object-oriented data models, event-based monitoring and scheduling, and collaborative transaction management. Various issues which have to be considered by an OHS such as interoperability, extensibility, and computation have also been the driving forces behind advanced database systems. Thus, there is a great potential in com-bining the two. To provide advanced database concepts as OHS baseline architecture is the main con-tribution of this paper.The paper is structured as follows. Section 2 discusses requirements of OHS such as scalability, open-ness, distribution, heterogeneity, interoperability, extensibility, and computation. They have to be considered both, for the open hyperbase functionality and for the link server functionality of an OHS. Considering these requirements, Section 3 proposes an architectural framework for OHS which in particular takes interoperability, extensibility, and computation into account. This framework is based on TriGS SGML(Tri gger System for G em S tone), an SGML2-aware active object-oriented database sys-tem as extension of the commercial object-oriented database system GemStone. We then discuss our approach in Section 4. Finally, Section 5 focuses on further research issues.2.Requirements of Open Hypermedia SystemsOpen hypermedia systems provide the storage, retrieval and manipulation of an extensible set of hypermedia documents, and they provide the link management between them and the integration of an extensible set of tools working with these hypermedia documents. To provide these functionalities, OHS have to tackle seven important issues, namely scalability, openness, distribution, heterogeneity, interoperability, extensibility, and computation. These are discussed in detail in [41], and the follow-ing is a summary thereof:Scalability. Scale refers to the number of simultaneous users and tools, and to the size of the hyper-base effectively handled by the OHS.Openness. Openness refers to the ease of integrating new tools into the OHS and is reached if the OHS provides a protocol that allows any tool to participate in hypermedia services.Distribution.Distribution refers to both, distributed tools and a distributed hyperbase. Concerning the former, distribution refers to a situation in which all processes can execute on different machines in a network. Concerning the latter, distribution refers to one or all of the three dimensions of distri-bution of data, distribution of control, and heterogeneity (see also next issue).Heterogeneity. Heterogeneity describes whether a single or whether different hypermedia data mod-els are supported by the underlying hyperbase. The analogy with multi database systems is obvious (see also [40]). A multi hyperbase system should support distribution and heterogeneity.Interoperability. Interoperability refers to the support of several protocols for inter-linking and to the support of exchange formats, i.e., import/export formats, for the underlying hyperbase. Ideally, com-monly accepted standards for inter-linking and data exchange should be supported.as well as adding new concepts to the data model of the hyperbase.Computation. Computation refers to the possibility of computing data model elements such as anchors, nodes, links, and composite structures. This also includes the consistency management of these elements.In this paper we will consider especially interoperability, extensibility, and computation when dis-cussing an OHS baseline architecture. Interoperability is reached by supporting SGML/HyTime3 as interchange formats. Extensibility is reached by providing an object-oriented data model which takes metaclasses into account. For example, new SGML element types can be added dynamically by instantiating respective metaclasses. Finally, and in our opinion most importantly, computation is reached by using ECA-rules for describing all kinds of (re-)active behaviour. This includes the spec-ification of dynamic links which are computed at runtime as well as their consistency management. In some systems, ECA-rules are also first class objects and, thus, may be changed at runtime.3.An OHS Architecture based on Active Object-Oriented Database ConceptsThis section describes the architecture of a prototypical open hypermedia system currently being developed at our department. The architecture mainly tackles the above mentioned issues interopera-bility, extensibility, and computation, with the focus lying on computation.3.1Interoperability as an IssueOpen Hypermedia Systems demand for standardized as well as extensible document interchange for-mats. The Standard Generalized Markup Language (SGML) [27] — a well established ISO-standard for structured documents4 — and its hypermedia 'extension' Hypermedia/Time-based Structuring Language (HyTime) [28] clearly are standards which allow the platform independent exchange of time-dependent structured data. Thus, we consider the support of these two standards as crucial for open hypermedia systems.In open hypermedia systems, cooperation is mainly based on the exchange of documents, but most4.For a more detailed presentation of SGML we refer the reader to [22, 37, 38].make document exchange possible, a system- and device-independent data representation format for hypermedia-documents is required. Besides the aspect of device independent storage it is important that the information being handled is of high quality in terms of query, markup and structuring. Another aspect of hypermedia publications concerns the multi-mediality of information, i.e., the doc-uments representing the information are compound documents containing, besides normal text, addi-tional types of media, such as graphics, images, video, and sound.With respect to the above mentioned issue of interoperability SGML is considered the most signifi-cant standard for information exchange. It allows the system independent specification and handling of a document's contents and structure. SGML standardizes the syntax for tagging documents. Thus, it can be used to represent arbitrary document types, e.g., a biography of an artist, a spreadsheet with references to other documents, or a hypermedia presentation. For example,Figure 1 shows portions of an SGML document representing a biography of Picasso [26].<!DOCTYPE BIOGRAPH SYSTEM “biography.dtd.nov6”><BIOGRAPH><HEAD><NAME TYPE="LAST">Picasso</NAME><FRSTNAME>Pablo</FRSTNAME><MIDNAME>Ruiz</MIDNAME><BIOGDATA><BIRTH><PLACE>Malaga</PLACE><DATE><DAY>25</DAY><MONTH>Oct</MONTH><YEAR>1881</YEAR></DATE></BIRTH><DEATH><PLACE>Mougins, France,</PLACE><DATE><DAY>10</DAY><MONTH>April</MONTH><YEAR>1973</YEAR></DATE></DEATH> </BIOGDATA><LABEL><NATIONAL>Spanish</NATIONAL><ROLE>painter, sculptor,draughtsman, print-maker, decorative artist and writer</ROLE>,active in <ACTIVE>France </ACTIVE></LABEL><SHORTDES> He dominated 20th-century ...Figure 1: Biography on Picasso encoded in SGMLA so-called document-type-definition (dtd) describes the structure of a set of documents, and there-fore can be seen as equivalent to an object class within object-oriented systems. The documents them-selves may be interpreted as instances of this class. Another very prominent example of a dtd is HTML, the Hypertext-Markup Language, provided as a standard for exchanging hypermedia infor-mation in the World-Wide Web[5].However, SGML suffers from some impediments in link management, multi-mediality, and data location, which are needed in open hypermedia systems. The Hypermedia/Time-based Structuring Language HyTime5 extends SGML by addressing the problems of:•Locating data of any type by using a flexible standard notation that is independent of the processing system and the data itself.•Describing links within and between documents.•Structuring of contents.•Describing relations between temporal and spatial events occurring in documents.HyTime is characterized by its “meta” definition capabilities. It consists of a set of basic element types called architectural forms.These may be re-used for specific applications. Architectural forms can be compared to abstract classes in object-oriented programming.Considering interoperability issues of documents' contents, such as heterogeneous information, struc-ture, and links between (parts of) documents, the need for SGML/HyTime as standards for informa-tion interchange is obvious. The modelling of SGML/HyTime using advanced object-oriented concepts is subject to the next section.3.2Extensibility as an IssueSGML — and the more HyTime — clearly are standards for information interchange in open hyper-media systems. However, the usage of SGML/HyTime as interchange formats demands for advanced modelling possibilities. Various approaches have been made to make databases — especially object-oriented databases — SGML aware, i.e., to allow them to deal with SGML conforming documents (see e.g. [1, 14, 39]). In addition, as is shown below, for elegantly and efficiently modelling SGML advanced concepts such as metaclass mechanisms are indispensable.SGML can be considered as a meta language or metaclass, respectively. The instances of a metaclass,classes, therefore are the document-type-definitions being defined in SGML. The actual documents then are instances of classes, i.e., they can be compared to objects.Figure 2 depicts this relationship 6.Figure 2: MetaClass-Class-InstancesOur implementation is based on previous experience made by two approaches we have done by using the C++ based SGML library HyMinder and by doing an implementation in VisualWorks\Small-talk [9]. The main advantage of the latter approach is that for each element type a dedicated class is automatically generated during the parsing process, thus, resulting in type awareness for each instance of an element type. By that, queries on a certain element type are more efficiently processed since they are inherently restricted to a single class.We are currently developing a system called TriGS SGML (Tri gger System for G em S tone). To be able to intuitively handle SGML documents in TriGS we are extending it with functionality for dealing with structured documents encoded in SGML. Thereby, we make use of GemStone's metaclass mech-anism. This allows us to generate classes for element types at runtime and thus to add specific type information to each instance of an element type.Figure 3 expresses these different approaches.Rounded elements depict instances, rectangular boxes are classes, and metaclasses are displayed as shaded rectangular boxes. The main difference is that by the use of metaclass mechanisms we are able to dynamically generate a GemStone class for each corresponding SGML element type during runt-ime (see Figure 3a).SGMLdocuments dtd ISO 8879book.dtd newspaper.dtd HTML.dtd ...charlesGoldfarb.book times.newspaper faz.newspaper home.html (1)some many Metaclass ClassInstancesOur research framework allows us to deal with arbitrary types of information because we are able to deal with any SGML document-type-definition. By defining a meta dtd which is a dtd defining dtds we are able to treat dtd’s like “normal'' documents. The meta dtd has been developed by Hüser [26].Figure 3: Different Approaches, with and without MetaClass MechanismIn addition to that, our implementation forms the basis for a future HyTime engine. A “HyTime engine'' is “a program (or portion of a program or a combination of programs) that recognizes HyTime constructs in documents and performs application-independent processing of them.” [28, page 5]. In a first step we are currently working on the implementation of parts of the HyTime “hyperlinks” mod-ule which we believe is especially important for our purposes. The implementation is based on previ-ous experience we have made with the development of HyTime architectural forms in the application area of workflow management [8].3.3Computation as an IssueComputation is a very important issue in Open Hypermedia Systems. It refers to the possibility of computing data model elements — especially links — on demand. By that, not only predefined links but also links generated on the fly by a user's request are possible [25].In the following subsections we introduce active object-oriented databases and propose their use to elegantly support the issue of computation.3.3.1Active Concepts for OHSActive database systems have been widely accepted as a powerful means to increase both, flexibility and modularization of non-standard applications like CIM, workflow management systems, and hypermedia systems.Metaclass InstanceClassa)b)Our basic research vehicle which is used in order to realize active link management is TriGS (= Trig-ger system for GemStone [29, 31]). TriGS has been implemented on top of the object-oriented data-base system GemStone [10]. The GemStone data model is based on the GemStone Smalltalk programming language, which is a derivative of Smalltalk-80 [21].Like most active systems, TriGS is designed according to the ECA paradigm [17]. Rules and its com-ponents are implemented as first-class objects allowing both the definition and modification of rules during runtime. They provide a means to naturally and explicitly specify “what if ...” policies and their reaction (i.e., event based-behaviour) apart from the basic functionality of an application. An active database is not seen as inferior to the application but instead cooperates with the application in a peer-to-peer communication based on knowledge stored centrally inside the database system. This know-ledge determines when and how to react [19]. In almost all active systems the representation of know-ledge is based on Event/Condition/Action rules (ECA rules) [17]. Hence, the database system is able to monitor the situation represented by an event and by one or more conditions, and to execute the corresponding actions when the event occurs and the conditions evaluate to true [4]. TriGS supports primitive events comprising message events, time events, and explicit events, as well as composite events. A message event can be any message sent to an object. Concerning time events, TriGS distin-guishes between absolute, relative and periodical ones. By means of explicit events, the application designer is able to specify new kinds of events and to explicitly notify the active system about their occurrence which may be outside the active database system, too. Finally, composite events allow to construct more complex events out of primitive events and/or again other composite events. This is done by establishing different kinds of relationships between their component events such as conjunc-tion, disjunction and sequence.Keeping these benefits of active concepts in mind and considering the issue of computation in open hypermedia systems, ECA rules are very promising. Link management in OHS is mainly concerned with traversal of links, consistency management of links and above all, making applications link aware. Thus, we consider link management in OHS to take advantage of the following active mech-anisms:•In many hypermedia applications, the only semantics associated with links is to traverse a link and thus to load another piece of information. On the basis of ECA rules it is possible to enrich this link semantics by specifying, for example, different traversal policies, and authorizationpolicies (see also examples in Section 3.3.3 below).•Link validation is an important issue of hypermedia systems. By means of ECA rules, it is pos-sible to enrich link semantics by specifying consistency constraints and maintenance policies for links.•There exists a large number of legacy applications which do have just basic link functionalities or even no link mechanisms at all. By means of ECA rules it is feasible to make these applica-tions link aware with only slight adoptions which can be done, for example, by means of macro-programming [16]. Thus, also these applications are able to provide link functionality.3.3.2Mapping Hyperlinks to ECA RulesThe specification of the semantics of a link comprises two issues: Firstly, to which situations (in which context) should the 'link policy' be applied, and secondly, if that situation arises, what should exactly be done, i.e., which policy should be applied. These issues can naturally be mapped to the event and condition parts as well as to the action part of the rule, respectively, as follows.The event part of a rule specifies the point in time, when a certain policy has to be applied. This event could be a traversal request from a user. Let's for example assume that the application is link aware and fully integrated into our OHS. A link traversal request would then result in simple method calls (e.g., “followLink”) within the object-oriented database schema. Furthermore, it might happen that links are followed on request of an (external) application program. This application request could be realized by specification of an external event as application request. In addition to that, also triggering of different link policies by a combination of arbitrary events is possible. All these link related events are realized by means of the different kinds of events supported by TriGS.The condition of the link rule can be a simple boolean expression or an arbitrary database query spec-ifying either restrictions for traversal, e.g., by checking the authorization of a user, or a computation for the target location of the traversal.The action of a link rule specifies what should be done in addition to navigation or instead of it if the condition evaluates to true. Possible actions could be to display, print, or save the referenced docu-ment(s), to extend the referencing document with the referenced one, to merge the documents in case of a multi-location traversal, or to notify the user about a certain state.In the following, we will describe different application scenarios for using TriGS rules for active link management.3.3.3Enriching the Semantics of HyperlinksAs already mentioned above, the activity which has to be done when triggering a link, e.g., by clicking at the graphical representation of a link, need not simply be navigation along the path specified by the link. By means of ECA rules, arbitrary behaviour (policies) could be specified to be executed upon triggering the link. Therefore, when realizing the traversal of links in terms of ECA rules, the usually fixed semantics of built-in link types can be enhanced in several directions:•The first example shows the usage of time events for dealing with the expiration of links. Imag-ine for example a link to the WWW server of “Informatik 96 in Klagenfurt”. As soon as the conference is over, it could make sense to automatically delete the link and notify the webmas-ter in order to ensure consistent links. The specification in TriGS SGML is as follows:DEFINE RULE expire ASON(27-09-1996-6:00:00 p.m.) DOIF TRUE THEN /* could be omitted */EXECUTE klafu96-link delete.webmaster notify: 'link to Informatik96 deleted!'END RULE expire.The rule is named 'expire' and is based on a time event. The condition is true and the actions performed include the deletion of the link and notification of the webmaster.•Another example deals with authorization. The following rule defines that in case a link from document aDoc.html to page bDoc.html or page cDoc.html is followed and the user is not authorized, a notification of the user is done.DEFINE RULE authorization ASON PRE ((aDoc.html, gate-leg: bDoc.html) ORPRE (aDoc.html, gate-leg: cDoc.html)) DOIF ((user status) <> 'manager') THENEXECUTE INSTEAD Transcript displayPage: 'not authorized!'END RULE authorization.Note that TriGS allows the constructs PRE and POST for specifying when the action in relationto the event shall be executed. PRE indicates, that before the standard reaction to the event the execution of the action part of a rule shall be performed; the semantics of POST is vice versa.Also note that the keyword INSTEAD allows the execution of the action part instead of the execution of the message raised by the event. Thus, in the above example, the link is not resolved but the error message is displayed to the user — provided the fact that the user is not a manager.•The third example deals with update notification mechanisms. The rule below specifies that incase document aDoc.html has been edited, all users having subscribed to an update event for this document are notified.DEFINE RULE update ASON POST (aDoc.html, edited) DOIF (queryResult:= UserList getSubscribedUsersFor: aDoc.html) THENEXECUTE queryResult do:[:user | user notify: aDoc.html message: 'has changed']END RULE update.Note that the condition in this rule is a database query. The condition evaluates to true if the result of the query is not empty.3.4Proposed Architectural Framework based on TriGS SGMLFigure 4 shows the proposed architecture of our prototype system. The right side of the figure refers to the layers of the Dexter Hypertext Reference Model [24].Figure 4: Prototype System Architecture of TriGS SGMLWithin-Component LayerRun-time LayerStorage LayerOn top we have different applications which require link, constraint, and scheduling management such as the already mentioned workflow management system [8] or a hypermedia interaction system such as a World-Wide Web client. These applications are based on a HyTime engine with dedicated modules for each application area. The engine is implemented on top of our active object-oriented database management system TriGS SGML. A public domain Perl-SGML-Parser is integrated into GemStone and allows the import and export of SGML documents. By that, the system is able to man-age both, link information and information contents.The Event Handler component shown in Figure 4 is used for dealing with external events. TriGS, so far, handles all kinds of (message) events occurring inside the database. For Open Hypermedia Sys-tems, however, it is crucial to be aware of events being generated by external applications. For appli-cations not broadcasting their events, the event handler makes use of a polling mechanism. This implies certain efficiency drawbacks, but ensures integration of legacy applications. Within TriGS, however, event based monitoring and scheduling is guaranteed.4.Discussion of the ApproachIn this section we want to critically reflect our approach. As shown in our paper, rule-based (re)active systems seem to be a promising technique for extending link management in OHS into several direc-tions. Nevertheless, it has to be emphasized that our approach of using active capabilities is only a first step towards a more sophisticated link management system. There still exist a lot of unsolved issues arising from the area of active systems. These include scalability, an adequate transaction model for the execution of rules, the problem of termination of rule execution as well as collaboration issues, which are discussed in the following.•Scalability: We have not tested our system with respect to scalability so far. We conjecture, that performance might be a problem in a wide area network. This has to be further investigated for using TriGS SGML in globally distributed Open Hypermedia Systems.•Transaction management: Concerning transactions, most of current active approaches [7, 20] allow to decouple the execution of rules from the application by building upon the nested trans-action model [34]. However, in the context of composite events, which are a natural way to express the event part of links, the nested transaction model is too restricted. What is actually required is that the transaction, wherein the rule is executed, can be made dependent on allcomponent events which are part of a certain composite event, and these component events are raised in different transactions. We have done one step into this direction by developing a so-called multi-parent subtransaction model (for further details see [30]).•Termination: The problem of termination is especially eminent in the context of link traversal.Termination guarantees that rules do not trigger each other forever. That is, if a link is fol-lowed, a rule is triggered which in turn is allowed to trigger another rule and so on. More syn-tactically, this means that the action of a rule covers the triggering event of other rules or even of the same rule again. In this case, there is always the problem of proper termination. There are some different possible solutions for this problem, which have to be considered in further work [2, 13, 32]. First, it may be the responsibility of the rule designer to specify only termi-nating rule sets. Second, as is done in some commercial active relational database systems there may be a user-defined or system-defined limit for the number of forward-chaining rules which are checked at runtime. A third approach is to check at rule definition time, whether there is no cycle, i.e., a rule cannot directly or indirectly trigger itself. Last but not least, the most restrictive approach would be to disallow cascaded rules at all.•Collaboration: Collaborative issues arise by both rule modelling and rule enactment when sev-eral users are involved. For such collaborative environments also the above mentioned issues transactions, termination, and scalability have to be considered.5.Summary and Future WorkThe paper introduced TriGS SGML, a prototypical Open Hypermedia System currently being devel-oped at our department. So far, TriGS SGML has focused on the OHS issues interoperability, extensi-bility, and computation. The system is able to handle arbitrary document type definitions, at the same time allowing the dynamic creation of element types.The approach of using a general purpose active object-oriented database system in comparison to the link-manager approach has several merits. The former allows any application, for example, to sub-scribe to notifications on events. Due to the underlying database functionality locking and event noti-fication are “for free'' and independent of the declaration of objects. The latter approach, such as HyperForm [40], lacks database functionality. Thus, if some object should be aware of concurrency control and notification control, to mention just two database issues, the respective behaviour has to be implemented by the user because it is not inherently provided by the underlying system.。