This work presents an Application Domain model for Adaptive Hypermedia Systems and an architecture for its support. For the description of the high-level structure of the application domain we propose an object-oriented model based on the class diagrams of the Unified Modeling Language, extended with (i) a graph-based formalism for capturing navigational properties of the hypermedia and (ii) a logic-based formalism for expressing further semantic properties of the domain. The model makes use of XML for the description of metadata about basic information fragments and “neutral” pages to be adapted. Moreover, we propose a three-dimensional approach to model different aspects of the adaptation model, based on different user's characteristics: an adaptive hypermedia is modeled with respect to such dimensions, and a view over it corresponds to each potential position of the user in the “adaptation space”. In particular, a rule-based method is used to determine the generation and deliver process that best fits technological constraints.
on which to perform adaptation (the what ), and our belief is that the most promising approach in modeling the application domain is data-centric (in fact many recent researches employ well-known database modeling techniques); (ii) a simple representation of the logic of the adaptation process, distinguishing between adaptation driven by user needs and adaptation driven by technological constraints (the how ); (iii) the support of a wide range of adaptation sources.
The logical structure and contents of an adaptive hypermedia are described along two different layers. The lower layer aim is to define the content of XML pages and the associated semantics (using an object-oriented model) and navigational features of the hypermedia (with a directed graph model). The upper layer describes the structure of the hypermedia as a set of views associated to groups of users (i.e. stereotype profiles) and some semantic relationships among profiles (using logical rules). Finally, the adaptation model is based on a multidimensional approach: each part of the hypermedia is described along three different adaptivity dimensions, each related to a different aspect of user’s characteristics (behavior, used technology and external environment). A view over the Application Domain corresponds to each possible position of the user in the adaptation space. The XML pages are independent from such position, and the final pages (e.g. HTML, WML, etc.) to be delivered are obtained through a transformation that is carried out in two distinct phases, the first one driven by the user’s profile and environmental conditions, and the second one driven by technological aspects. 2
In our approach to the modeling of adaptive hypermedia we chose to adopt the classical ObjectOriented paradigm, since it permits a complete high-level description of concepts. Furthermore, we chose to adopt XML as the basic formalism due to its flexibility and data-centric orientation. In fact, XML makes it possible to elegantly describe data access and dynamic data composition functions, allowing the use of pre-existing multimedia basic data (e.g. stored in relational databases and/or file systems) and the description of contents in a completely terminal-independent way.
We model the (heterogeneous) data sources by means of XML metadescriptions (Sec. 2.2). The basic information fragments are extracted from data sources and used to compose descriptions of pages which are “neutral” with respect to the user’s characteristics and preferences; such pages are called Presentation Descriptions (PD ). The PDs are organized in a directed graph for navigational aspects, and in an object-oriented structure for semantic purposes in the Description Layer (Sec. 2.3), while knowledge-related concepts (topics) are associated to PDs and profiles in the Logical Layer (Sec. 2.4). The transformation from the PDs to the delivered final pages is carried out on the basis of the position of the user in an adaptation space (Sec. 2.1). The process is performed in two phases: in the first phase the PD is instantiated with respect to the environmental and user dimension and a “technological independent” PD is generated. In the second phase (Sec. 2.5) the PD is instantiated with respect to the technology dimension. 2.1
In our proposal the application domain is modeled along three abstract orthogonal adaptivity dimensions: – User’s behavior (browsing activity, preferences, etc.); – External environment (time-spatial location, language, socio-political issues, etc.); – Technology (user’s terminal, client/server processing power, kind of network, etc).
The position of the user in the adaptation space can be denoted by a tuple of the form [B; E; T ]. The B value captures the user’s profile; the E and T values respectively identify environmental location and used technologies. The AHS monitors the different possible parameters that can affect the position of the user in the adaptation space, collecting a set of values, called User, Technological and External Variables. On the basis of such variables, the system identifies the position of the user.
The user’s behavior and external environment dimensions mainly drive the generation of pages content and links. Instead, the technology dimension mainly drives the adaptation of page layout and the page generation process. For example, an e-commerce web site could show a class of products that fits the user’s needs (deducted from his/her behavior), applying a time-dependent price (e.g. night or day), formatting data with respect to the user terminal and sizing data with respect to network bandwidth. 2.2
Information fragments are the atomic elements used to build hypermedia contents; fragments are extracted from data sources that, in the proposed model, are described by XML meta-descriptions. The use of metadata is a key aspect for the support of multidimensional adaptation; for example, an image could be represented using different detail levels, formats or points of view (shots), whereas a text could be organized as a hierarchy of fragments or written in different languages. Each fragment is associated to a different portion of the multidimensional adaptation space. By means of meta-descriptions, data fragments of the same kind can be treated in an integrated way, regardless of their actual sources; in the construction of pages the author refers to metadata, thus avoiding too low-level access to fragments.
A number of XML meta-descriptions have been designed making use of XML Schemas [
</result-structure> </xquery>
In such meta-description the key elements are the statement (eventually with some parameters, #key in the example) and a description of the resulting XML document. Specific parts of the documents extracted by means of the above-shown query, could be described as follows: <xdoc alias="book-author"> <instance alias="db" description="database authors" location-type="xquery" location="authorsquery(#key=databases).name" schema="..."/> <instance alias="comp" description="compression author" location-type="xquery" location="authorsquery(#key=compression).name" schema="..."/> </xdoc>
We emphasize that the meta-description of an XML document is abstracted with respect to its actual source, referred by means of location attributes; furthermore, many instances of the same document can be differentiated within the same meta-description (in the example above, authors of books having different subjects). Complex meta-descriptions allow direct referencing of single information fragments by means of aliases, dot-notation and parameters. For example, book-author.db in the previous example refers to the db instance of the book-author fragment, where db is in turn an alias for the query authorsquery(#key=database).name. 2.3
In the description layer the application domain is modelled as a directed graph, where nodes are the presentation descriptions and arcs represent links. Furthermore, we apply the object-oriented paradigm to capture the semantic relationships among the PDs: we define the PDs as objects which are instances of predefined classes.
Presentation descriptions are composed of four sections:
– The OOStructureInfo section includes information concerning the object-oriented
organization of the domain. The interface of each class of PDs is composed of the set of ingoing and
outgoing links. Furthermore, a type is associated to each link to express its semantics, and
to define the compatibility among outgoing and ingoing links of different classes (e.g. a PD
A can be linked to another PD B if the PD A has an outgoing link whose type is the same
of the type of an ingoing link of the PD B ). With regard to inheritance, a subclass inherits
the information fragments and the links of the parent classes. The OOStructureInfo section
is edited by means of a tool based on the Unified Modeling Language [
In the following, we show the XML Schema structure of the OOStructureInfo section. It includes the name of the PD class (entityName element), a set of parent classes (superEntityName elements) and the set of links that define the PD interface (link elements, each with an associated name and type). The logical layer aim is to model the domain adaptivity with respect to stereotype user profiles. The logical model is a set of Profile Views (PV ), where each PV is a view of the overall hyperspace domain associated to a user profile. The PV associated to the profile p includes all the PDs accessible by the users belonging to profile p, therefore it represents the hypermedia domain and the navigational space for that profile.
Following the two-layer definition of the application domain model, the model design is composed of two design phases, operating at two different abstraction levels. The first phase is related to the definition of the navigational directed graph: the author designs the presentation descriptions and their relations as hyperlinks. In the second phase the author identifies the user profiles and, on the basis of his/her domain knowledge, designs the PVs associated to each profile. The PV design can be carried out incrementally: for each PD, the author determines all the user profiles that can access it. At the end of the process, each PV is composed of all the PDs that have been associated to the corresponding user profile.
Each PV (and consequently each associated profile) corresponds to a set of topics that represents the knowledge contained in the PV (knowledge description). More precisely, this correspondence can be formally defined through a function ·(¢), that can be applied to a single PD, or to a user profile, and returns the corresponding set of topics: – ·(¢), applied to the presentation description P Di, returns the set of topics captured by P Di; – ·(¢), applied to the user profile p, returns the knowledge domain of p: ·(p) = [i·(P Di) j P Di 2 P Vp, where P Vp is the profile view associated to p.
The instantiation of a particular PD with respect to a given profile B and a given position along the external environment dimension E can be seen as the application of a function ± that transforms a given PD into a technology-independent XML page, called PdD, which will be given as input to the multichannel layer (Sec. 2.5):
± : (P D; B; E) ¡! PdD:
Furthermore, we improve the logical description of the hypermedia structure by means of a Semantic Precedence Operator, Ã, which is used to define constrains about profile changes. If applied to two knowledge domains, e.g. ·(p2) Ã ·(p1), the operator points out that a user cannot access topics related to the profile p2 until he/she has accessed some topics included in the knowledge domain of p1 (i.e. until he accesses the PDs that capture those topics). This constraint can be better specified by means of a semantic precedence matrix. This matrix has as many rows as the topics of ·(p2) and as many columns as the topics of ·(p1). Each element (i; j) of the matrix can assume a boolean value; true means that the user must visit a PD containing the topic j of the domain ·(p1) before his/her profile can change form p1 to p2 and the topic i of the domain ·(p2) can be accessed. So each row specifies which topic of ·(p1) a user belonging to the profile p1 must know to change his profile to p2 by entering a particular topic of ·(p2). For example, a semantic precedence matrix with all values equal to true means that, whatever is the entry point to the profile p2, the user must have visited all the topics of ·(p1). An entry point of a profile p2 is defined as a node (PD) that, when accessed through a hyperlink, allows the user to change his profile to p2. The semantic precedence operator can be used in general logic rules; e.g. ·(p1) Ã ·(p2) ^ (·(p3) _ ·(p4)) is a rule that expresses a more complex relationship among the profiles involved. 2.5
The technology dimension drives the adaptation of the page layout to the client device (PC, handheld computer, WAP device, etc.), and the page generation method. The technology adaptation is performed by means of a Multichannel Layer. In the following we will describe (i) the alternative page generation methods, (ii) the technological variables, and (iii) the multichannel layer. Page generation methods. The final pages displayed on the client device (written in HTML, WML, etc.) are dynamically generated by performing a transformation of the PdD using a XSL document/program. Several XSL stylesheets can be used to transform the PdDs, depending on the client device features. Moreover, three different page generation methods have been designed: a. The page generation takes place entirely on the server. It picks out the Information Fragments, applies the transformation using the appropriate XSL document, and then sends the page (HTML, WML, etc.) to the client. The main drawback of this method is that the client cannot access the XML content. As an example, if the client is an application, e.g. a workflow or a distributed computing application, it could need to access and process the XML data. b. Similar to the method (a), but the server sends to the client HTML (WML) pages that contain XML data islands. These data are not processed by the server XSL processor, and are not displayed on the client device, but can be accessed by client programs. c. The page generation is performed entirely on the client: the server sends to the client both the XML document and the XSL document that the client device will use to carry out the transformation. The technological variables. The technological variables are used by the multichannel layer to adapt the presentation and the generation process to the client device. On our system we use five groups of technological variables: 1. Variables related to the XML and XSL support on the client device; 2. Variables addressing the client device processing power (client-side XSL formatting may take place only if the device can manage such a complex and time-consuming operation); 3. Variables related to the client device display features (resolution, dimensions, etc.); 4. Variables concerning the kind of client data usage (e.g., it is useful to know whether or not clients need to access and process the “pure” XML data); 5. Variables related to the server processing workload.
The variables belonging to the first three groups are extracted from the device knowledge base (shown in the next Section), while data usage variables are associated to the client (e.g. they can be associated to the user profile), and group 5 variables are determined by the server. The Multichannel Layer. The main components of the multichannel layer are the Device Knowledge Base (DKB ) and the Presentation Rules Executor (PRE ).
The device knowledge base is a repository composed of a set of entries, each describing the features of a specific client device. For each client request, the client device is determined according to the data contained in the User Agent field of the request, and the corresponding device entry is selected. Each entry is composed of variable-value pairs, where variables correspond to the technology variables belonging to the first three groups.
The presentation rules executor determines the presentation layout and the page generation method based on a set of rules that check the values assumed by the technological variables. Rules are defined in an ad hoc XML syntax, and are modelled according to the well known eventcondition-action (ECA) paradigm. The following XML fragment shows a typical presentation rule. <rule id="1"> <conditions> <technological-variable group="1" xml-support="yes"/> <technological-variable group="2" processing-power="high"/> <technological-variable group="3" display-area="small" res-value="medium"/> <technological-variable group="4" xmldata-need="no"/> </conditions> <action> <xsl-formatting value="clientside" method="c" stylesheet="AdHocStylesheet.xsl"/> </action> </rule>
The rule states that if the client does not need to elaborate the XML data content (xmldata-need belonging to group 4), the client device fully supports XSL transformation (group 1 and 2 variables), and display features are appropriate for the data to visualize (group 3 variables), the PRE chooses the page generation method c (client-side XSL formatting), and the XSL stylesheet AdHocStylesheet.xsl.
Note that the above rule does not consider the server-side workload. Another presentation rule may state that XSL formatting has to take place on the server side (even if the client device supports XML and XSL) if the client does not need to access XML data, unless the server workload exceeds a specified workload threshold.
Events of the ECA paradigm are implicitly managed by the adaptive system and correspond to the user choice of a given PD. Conditions correspond to checks on the technological variable values. Actions are performed when a logical expression (a presentation rule), composed of atomic conditions, is evaluated. The above XML syntax can be used to compose complex presentation rules, based on all the possible combinations of technological variable values. Actions mainly consist of the choice of the page generation method and the choice of the most suitable XSL stylesheet that will drive the XSL formatting.
Figure 1 shows the multichannel layer architecture. This is a logical layer that uses components belonging to the three architectural tiers (see Section 3). The Request/Response Manager processes client requests (coming either from a wired or a from a wireless device), and after the generation of the PdDs passes both to the PRE. The PRE extracts the user-agent data from the requests and accesses the device knowledge base to evaluate the technological variables. Then, the PRE executes the presentation rules contained in the presentation rules repository, according to the ECA paradigm.
The PRE chooses the page generation method and the XSL stylesheet (picking it out form the Stylesheet Repository). In the case of client-side formatting (page generation method c), the PRE passes both the XML data (the PdD) and the selected stylesheet to the Request/Response manager, which in turn sends them to the client. In the case of server-side formatting (page generation methods a or b), the PRE activates the Data Formatting Module, which converts the XML data according to the XSL directives, and then passes the formatted data to the Request/Response Manager, which in turn sends it to the client. 3
We have designed and are currently implementing an architecture for the support of the proposed model, comprising an authoring suite and a run-time system.
The main tasks of the authoring suite (named Java Adaptive Hypermedia Suite – JAHS ) are the
following: (i) composition of the UML class diagrams; (ii) composition of the semantic precedence
logic rules; (iii) transformation of the class diagrams and semantic precedence rules into an XML
formalism; (iv) browsing of the data sources and composition of the XML metadescriptions; (v)
construction of the presentation descriptions; (vi) specification of the event-condition-action rules
for the instantiation of the PDs with respect to the technological dimension. For space reasons,
we do not further detail the authoring suite; the interested reader can refer to [
The run-time system has a three-tier architecture (Fig. 2), comprising the User, the Application and the Data tiers. The user tier receives final pages to be presented and eventually scripts or applets to be executed (e.g. for detecting local time, location, available bandwidth). The user’s terminal and the terminal software (operating system, browser, etc.) are communicated by the terminal User Agent (e.g. the browser).
The User Modeling Component (UMC ) maintains the most recent actions of the user and
evaluates them giving as a result the user’s profile. In this paper we do not address such issue,
demanding it to an external module; in our previous work we have proposed an approach in which
hypermedia links are mapped into graph edges weighted with the probability of following them,
and a probabilistic algorithm is used to estimate the user’s profile ([
The Adaptive Hypermedia Application Server (AHAS ) runs together with a Web Server. It executes the following steps: (i) it communicates to the UMC the most recent choices of the user; (ii) it extracts from the XML repository the PD to be instantiated; (iii) it extracts the basic data fragments from the data sources on the basis of the user position; (iv) it interacts with the modules of the multichannel layer in order to generate the final page.
The data tier comprises the Data Sources Level, the Repository Level and a Data Access Module. The data sources level is an abstraction of the different kinds of data sources; each data source Si is also accessed by a Wrapper software component, which generates the XML metadata describing the data fragments stored in Si. The Repository Level is a common repository for data provided by the Data Source Level or produced by the author. It stores (i) XML documents including the Presentation Descriptions, metadata, Schemas, XSL stylesheets, and the active rules shown in Sections 2 and 2.5; (ii) the UML class diagrams representing the logical structure of the hypermedia. Finally, the data access module implements an abstract interface for accessing the data sources and the repository levels. In this paper we presented a model for adaptive hypermedia systems using the object-oriented paradigm and XML. An adaptive hypermedia is modeled considering a three-dimensional adaptation space, including the user’s behavior, technology, and external environment dimensions. The adaptation process is performed evaluating the proper position of the user in the adaptation space, and transforming “neutral” XML pages according to that position.
We believe that the main contributions of this paper are: – a new data-centric model to describe adaptive hypermedia specifically concerned with a flexible and effective support of the adaptation process; the model integrates a graph-based description of navigational properties and an object-oriented semantic description of the hypermedia, and uses a logical formalism to model knowledge-related aspects; – a flexible and modular architecture for the run-time support of adaptive hypermedia systems, with particular regard to the adaptation with respect to technological aspects.