app eared in: Pro c. of the 3rd International Conference on Information and Knowledge Management (CIKM) '94, Maryland, MD, 1994.1 Storing HyTime Do cuments in an Ob ject-Oriented Database Klemens Bohm, Karl Ab erer GMD-IPSI Dolivostrae 15, 64293 Darmstadt Germany fkb o ehm, ab [email protected] Using examples Abstract from the area of This article is about so hypertext hypertext modelling. yp ermedia-do cument storage system has to meet An op en h should ease un- ts that are not satis ed by existing systems: it requiremen derstanding. 1 has to supp ort non-generic hyp ermedia do cumenttyp es , i.e. do cumenttyp es enriched with application -sp eci c se- mantics. It has to provide hyp ermedia-do cument access Figure 1: Sample Structure in an Argumentation Space metho ds. Finally, it has to allow the exchange of hyp erme- dia do cuments with other systems. On a technical level, an ob ject-oriented database-management system, on a logical sophisticated. Usually systems for hyp ertext-do cumenthan- level, a well established ISO standard, namely HyTime, is dling consist of three layers, a storage layer,an application used to satisfy the requirements mentioned ab ove. By means layer and a presentation layer [DeS86 ]: hyp erengines are the of the example of do cuments incorp orating hyp ertext struc- middle layer. With some hyp erengines, the storage layer is tures we discuss the impact of taking such an approachon made up with databases [ScS90 , MaS92 ]. With other ones, representation and pro cessing within the database system. this is not the case [SSS93 ]. 1 Intro duction In hyp ertext-do cuments of di erenttyp es the hyp erob- jects have sp ecial semantics in addition to the canonical fea- In the recent past the proliferation of the hyp ertext para- tures. In [SHT89] four spaces corresp onding to the di er- digm for representation of information has b een facilitated ent design activities within an authoring pro cess are identi- by technological advances. The main di erence b etween ed: the content space,the planning space, the argumen- conventional do cuments and hyp ertext do cuments is that tation space and the rhetorical space. These spaces are in the rst case the do cument structure as p erceived bythe part of SEPIA, a co op erativehyp ermedia authoring envi- reader is linear. In the second case, there is a graph struc- ronmentdevelop ed at our institute [Str+92]. An argumen- ture. To apply the hyp ertext paradigm hyperengines have tation space inter alia contains facts (`datum') and asser- b een develop ed. Hyp erengines administer the hyp ertext- tions (`claim'). There are not only di erentnodetyp es, but 2 sp eci c structures such as no des, links and anchors which also di erentlinktyp es: a so-link is a directed binary link will b e referred to as hyperobjects. They contain the realiza- from a datum to a claim. Figure 1 contains an example of a tion of generic op erations. An example for a no de op eration so-link. Other link typ es in the argumentation space are the is the calculation of the transitive closure, i.e. the identi ca- to-link and the contradicts-lin k. A contradicts- contributes tion of all no des in a do cument that can b e reached by link link links two no des with discrepantcontent. In the other traversal from that no de. Hyp erengines di er in the internal spaces the hyp erob ject's semantics likewise is sp ecial. In the representation of the hyp erob jects. Furthermore, op erations sequel we will refer to an argumentation-space structure, i.e. re ecting the hyp erob jects' semantics maybemoreorless ahyp ertext structure whose comp onents are claims, so-links 1 etc. as an argumentation-spacedocument. In our terminology,withhyp ermedia do cuments the content of the no des maybemultimedia data as opp osed to hyp ertext do cuments. In existing systems hyp erob jects' do cument-typ e-sp eci c 2 The edges of such a kind of graph structure are called links. Links semantics tends to b e hardco ded in the presentation layer. do not necessarily link no des in their entirety, but also structures Hence, exchanging hyp erdo cuments of non-generic typ es, within no des as, say, several words or sentences. The link ends are e.g. argumentation-space do cuments b etween hyp erengines called anchors [Con87]. or applying a hyp erengine in di erent contexts is not yet conceivable. We for our part envisage a hyp erengine sup- p orting hyp erob jects with partly sp ecial semantics based on a database system. Wewant to comply to a format for hy- p erdo cuments satisfying the following basic requirements: non-genericity, orientation towards hyp ermedia do cument storage and pro cessing, acknowledgement asan(interna- tional) standard. Hence, wehavechosen SGML/HyTime. document The problems we approach and solutions we give are fairly orthogonal to the particular format chosen. title abstract intro hytime ... With SGML (`Standard Generalized Markup Language') ... [ISO86, Her94] do cumenttyp es can b e de ned. In essence, authorlist affiliation title paragraph on paragraph on cument-type de nitions (DTDs) are attributed gram- SGML do ... hyperengines argumentation t structure. However, nothing mars sp ecifying the do cumen ... spaces author1 tics of do cument comp onents, which is said ab out the seman author2 ... ... ... are called elements in the SGML context. The HyTime Standard (`Hyp ermedia/Time-based Do cument Structuring Language') [ISO92, NKN91] basically is a list of SGML Figure 2: Tree Structure Corresp onding to this Do cument element-typ e de nitions for, say, links or presentation sched- ules. These elementtyp es are referred to as architectural forms. Their semantics is xed by the standard. - In this <!ELEMENT asdoc (node|so|contra|contrib)*> article we fo cus on the basic link features the HyTime stan- -- `asdoc' short for `argumentation-space dard provides. This facilitates a comparison of our concepts document', contra' is GI of 'contradicts-link', to conventional approaches to hyp erdo cument storage. We 'contrib' is GI of 'contributes_to-link' are, however, not aware of any related work on this topic (def.s omitted) -- dealing with di erenthyp ertext-do cumenttyp es. <!ELEMENT node CDATA> <!ATTLIST node id ID #REQUIRED In this article we describ e the database application frame- type (position|claim|datum) work for HyTime-do cument storage we are currently work- #REQUIRED> ing on. We concentrate on the following asp ects. <!ELEMENT so EMPTY> <!ATTLIST so claim IDREF #REQUIRED 1. In [ISO92, Ko e+93 ] it has b een mentioned that Hy- datum IDREF #REQUIRED> Time pro cessing can b e accelerated by means of an ... internal representation. Our ob jectiveistodevelop hyp ermedia-sp eci c index structures to sp eed up ac- cess op erations. We will explain that having more than Figure 3: Fragment of an SGML Do cumentTyp e De nition one internal representation of HyTime-architectural forms' for Argumentation-Space Do cuments instances to cho ose from maybeadvantageous. 2. We do not restrict ourselves to a set of xed hyp ertext the list of a no de's children the content of the element.In- structures: dynamic mo di cations of SGML/HyTime ternal no des are nonterminal elements. All elements have do cuments shall b e doable. With our approachboth atyp e, e.g. section or paragraph. With SGML the logi- the collection of do cuments and the set of do cument cal structure of do cuments of a certain typ e can b e de ned. typ es can b e mo di ed at runtime. In essence the content model of an element type is a reg- 3. Op erations are part of the database application : In ular expression sp ecifying how the contentofanelement other words, the database has the semantic control of that typ e may lo ok like. An element-typ e name is also over do cument comp onents. called generic identi er (GI). The element-typ e de nitions in a DTD may b e completed by the de nition of attributes. The platform on which realization will b e based is an ob ject-oriented database-management system (OODBMS) - Figure 3 contains a p ossible DTD for argumentation- the OODBMS VODAK develop ed at our institute [Kla+93 , space do cuments. Lines starting with `<!ELEMENT'intro duce KAN93]. By using a DBMS database features such as con- an elementtyp e together with its content mo del: instances currency control or querying capabiliti es are available. Even of asdoc contain a list whose elements are either instances though we limit ourselves to the description of the HyTime of elementtyp e node, so etc. CDATA is a terminal element hyp erlink features other facets of hyp ermedia do cuments re- typ e more or less comparable to the data typ e STRING. ected in HyTime, like spatial and temp oral relationship s, `<!ATTLIST' indicates the b eginning of attribute de nitions. can b e approached in the same way. For instance, elements of typ e node have an attribute of typ e ID and an attribute type of typ e (positionjclaimjdatum). The structure of this article is the following: the next Attributes of typ e ID are unique identi ers of the element section is an overview of the HyTime conception together they b elong to. An attribute of typ e IDREF is an ID refer- with examples from the hyp ertext area. In Section 4, our ence, one of typ e IDREFS a list of ID references.