Evidence-based lean logic profiles for conceptual data modelling languages

09/09/2018 ∙ by Pablo Rubén Fillottrani, et al. ∙ University of Cape Town Universidad Nacional del Sur 0

Multiple logic-based reconstruction of conceptual data modelling languages such as EER, UML Class Diagrams, and ORM exists. They mainly cover various fragments of the languages and none are formalised such that the logic applies simultaneously for all three modelling language families as unifying mechanism. This hampers interchangeability, interoperability, and tooling support. In addition, due to the lack of a systematic design process of the logic used for the formalisation, hidden choices permeate the formalisations that have rendered them incompatible. We aim to address these problems, first, by structuring the logic design process in a methodological way. We generalise and extend the DSL design process to apply to logic language design more generally and, in particular, by incorporating an ontological analysis of language features in the process. Second, availing of this extended process, of evidence gathered of language feature usage, and of computational complexity insights from Description Logics (DL), we specify logic profiles taking into account the ontological commitments embedded in the languages. The profiles characterise the minimum logic structure needed to handle the semantics of conceptual models, enabling the development of interoperability tools. There is no known DL language that matches exactly the features of those profiles and the common core is small (in the tractable ALNI). Although hardly any inconsistencies can be derived with the profiles, it is promising for scalable runtime use of conceptual data models.



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1 Introduction

Many conceptual data modelling languages (CDMLs) have been proposed over the past 40 years by several research communities (e.g., relational databases, object-oriented software) and for a range of motivations, such as spatial entities in geographic information systems, ontology-driven or not, and aiming for simplicity and leanness vs expressiveness. Assessing the modelling features of CDMLs over time, it exhibits a general trend toward an increase in modelling features; e.g., UML has identifiers since v2.4.1 (Object Management Group, 2012), ORM 2 has more ring constraints than ORM (Halpin, 2001; Halpin and Morgan, 2008), and EER also supports entity type subsumption and disjointness cf. ER (Song and Chen, 2009; Thalheim, 2009). Opinion varies about this feature richness and its relation to model quality (Moody, 2005) and fidelity of capturing all the constraints specified by the customer, and asking modellers and domain experts which features they think they use, actually use, and need showed discrepancies between them (Alberts et al., 2008). From the quantitative viewpoint, it has been shown that advanced features are being used somewhere by someone, but they are used relatively very few times, as both the quantitative analysis of CDML feature usage in 168 ORM diagrams (Smaragdakis et al., 2009), and 101 EER, UML, and ORM diagrams has shown Keet and Fillottrani (2015). With such insight into feature usage, it is possible to define an appropriate

logic as underlying foundation of a CDML so as to not only clarify semantics but also use it for computational tasks. Logic-based reconstructions open up the opportunity for, among others, automated reasoning over a model to improve its quality (e.g.,

Berardi et al. (2005); Queralt et al. (2012)) and runtime usage of the models, such as conceptual and visual query formulation (Calvanese et al., 2010, 2016; Soylu et al., 2017) and optimisation of query compilation (Toman and Weddell, 2011).

Concerning the ‘appropriate’ logic, many logic-based reconstructions have been proposed over the years (discussed below), which can be grouped into either the Description Logics based approach that propose logics from viewpoint of computational complexity, rather than needs and usages by modeller, or the as-expressive-as-needed approach, such as in full first-order predicate logic. Neither of these proposals, however, has taken a methodological approach to language design and brush over several thorny details of CDMLs, such as which core types of elements to formalise with their own semantics (aggregation, association ends), whether to include -aries (when , not ), and various advanced constraints. This has resulted into an embarrassment of the riches of logic-based reconstructions, which also hampers the actual use of logic-based conceptual data models in information systems for aforementioned tasks and therewith risk sliding into disuse.

We aim to address these problems in this paper. First, we will adapt and extend Frank’s DSL design methodology Frank (2013) into one suitable for language design more broadly, including conceptual data modelling languages, and informed by language design decision points emanating from ontology. Such ontology-driven language design decisions include, among others, positionalism of relations, the conceptual/computational trade-off and 3-dimensionalist vs. 4-dimensionalist. To the best of our knowledge, this is the first inventarisation of parameters of ontological commitments of language design of information and knowledge representation languages. Second, we apply this to the design of logics for several conceptual data modelling languages that is informed by the language feature usage reported in Keet and Fillottrani (2015). These logic ‘profiles’ cover the most often appearing features used, containing those features that cover about 99% of the features used in conceptual data models. The outcome is a so-called ‘positionalist’ and a ‘standard view’ core profile, and three language family profiles formalised in a DL, most of which have a remarkable low computational complexity. An example of UML Class Diagram that can be fully reconstructed into the standard view core profile (more precisely: ) is included in Fig. 1. It has a logical underpinning thanks to the knowledge base construction rules and three algorithms we propose in this paper, and therewith also has grounded equivalents in EER and ORM notation.

Figure 1: Sample UML Class Diagram containing all possible constraints of the Standard Core Profile, , which emanated from the evidence-based profile specifications.

This paper builds upon several previous papers on this topic (Fillottrani and Keet, 2015, 2016; Fillottrani et al., 2015; Keet and Chirema, 2016; Keet and Fillottrani, 2015). The main novel contributions in this paper are: i) methodological language design, including ontological aspects involved in it; ii) a new positionalist core profile; and iii) the profiles have been defined with a formal syntax and semantics now as well. The remainder of the paper is structured as follows. The state of the art and related works are discussed in Section 2. Section 3 presents our first contribution, which is a first inventarisation and discussion of ontological issues that affect language design. Our second main contribution is the logic-based profiles, which are described in Section 4. We close with a discussion in Section 5 and conclusions in Section 6.

2 Related work

Many conceptual data modelling languages have been proposed over the past 40 years; e.g., UML (Object Management Group, 2012), EER (Chen, 1976; Song and Chen, 2009; Thalheim, 2009) and its flavours such as Barker ER and IE notation, ORM (Halpin, 1989; Halpin and Morgan, 2008) and its flavours such as CogNIAM and FCO-IM, MADS (Parent et al., 2006), and Telos (Mylopoulos et al., 1990). Some of those are minor variants in notation, whereas others have to a greater or lesser extent a different number of features. Some ‘families’ of languages can be identified, which basically still run along the lines from which subfield it was originally developed. Notably, ER and EER originate from the relational database community (Chen, 1976), UML Class Diagrams from object-oriented programming (Object Management Group, 2012), and ORM (Halpin and Morgan, 2008) bears similarities with semantic networks and can be used for conceptual modelling for both database and object-oriented programming, and more recently also business rules. Each ‘family’ has their own set of preferred tools and community of users.

Besides these three main groups, some CDMLs have been developed specifically for additional features (e.g., temporal extensions) or somewhat revised graphical notations of the elements, such as different colours and a so-called ‘craw’s feet’ icon vs ..n or ..* for ‘many’ multiplicity/cardinality. We will not address this here, but instead will focus on the underlying language features from a logic-based perspective to which the best graphical elements could be attached as ‘syntactic sugar’ (see, e.g., (Calvanese et al., 1999; Keet, 2013) for this approach), and language design.

2.1 Logic-based reconstructions of CDMLs

A lot of effort has gone into trying to formalise conceptual models for two principal motivations: 1) to be more precise to improve a model’s quality and 2) runtime usage of conceptual models. Most works have focussed on the first motivation. Notably, various DLs have been used for giving the graphical elements a formal semantics and for automated reasoning over them, such as (Artale et al., 2007; Berardi et al., 2005; Halpin, 1989; Hofstede and Proper, 1998; Kaneiwa and Satoh, 2006; Queralt and Teniente, 2008), although also other logics are being used, including OCL (Queralt et al., 2012), CLIF (Pan and Liu, 2010), Alloy (Braga et al., 2010), and Z (Jahangard Rafsanjani and Mirian-Hosseinabadi, 2011). There are also different approaches, which are more interested in the verification problem, notably using some variant of constraint programming (Cadoli et al., 2007; Cabot et al., 2008).

Zooming in on DLs, the language has been used for a partial unification of CDMLs (Calvanese et al., 1999), whereas other languages are used for particular modelling language formalisations, such as DL-Lite and for ER (Artale et al., 2007) and UML (Berardi et al., 2005), or OWL for ORM and UML (Wagih et al., 2013). These logic-based reconstructions are typically incomplete with respect to the CDML features they cover, such as omitting ER’s ‘keys’ (identifiers) (Calvanese et al., 1999) or -aries proper (Artale et al., 2007; Wagih et al., 2013), among many variants. Also, multiple formalisations in multiple logics for one conceptual modelling language have been published. ORM formalisations can be found in, among others, (Franconi et al., 2012; Halpin, 1989; Hofstede and Proper, 1998; Keet, 2009; Wagih et al., 2013), noting that full ORM and ORM2 (henceforth abbreviated as ‘ORM/2’) is undecidable due to arbitrary projections over

-aries and the acyclic role constraints (and probably antisymmetry). Even for the more widely-used ER and EER (henceforth abbreviated as ‘(E)ER’), multiple logic-based reconstructions exist from the modeller’s viewpoints

(Chen, 1976; Song and Chen, 2009; Thalheim, 2009) and from the logician’s vantage points with the family (Calvanese et al., 1998, 1999, 2001) and family (Calvanese et al., 2007) of languages.

The second principal reason for formal foundations of CDMLs, runtime usage, comprises a separate track of works, which looks as very lean fragments. The driver for language design here is computational complexity and scalability, and the model is relegated to so-called ‘background knowledge’ of the system, rather than the prime starting point for software development. Some of the typical runtime usages are: scalable test data generation for verification and validation (Nizol et al., 2014; Smaragdakis et al., 2009) and ontology-mediated query answering that involves, among others, user-oriented design and execution of queries (Bloesch and Halpin, 1997; Calvanese et al., 2010, 2016; Soylu et al., 2017), querying databases during the stage of query compilation (Toman and Weddell, 2011) and recent spatio-temporal stream queries that avail of ontology-based data access Eiter et al. (2017); Özçep et al. (2015) with a variant of (Calvanese et al., 2007).

In sum, many logics are used for many fragments of the common CDMLs, where the fragments have been chosen for complexity or availability reasons rather than for which features a modeller actually uses.

2.2 Language design

There are many aspects to the design of a representation language, with many choices and decisions. This is not a new challenge and there is the notion of requirements engineering for language design. For instance, a recent paper by de Kinderen and Ma (2015) proposes the vGREL approach for steps 1 and 2/3 of the overall language design pipeline proposed by Frank Frank (2013) and some of the 26 guidelines by Karsai et al. (2009) are applicable as well. We include here a summary of Frank’s waterfall model for domains-specific modelling languages Frank (2013), as shown in Figure 2, with the steps we focus on in this paper highlighted. We will step through these states in brief and with respect to applicability and related works on CDML design.

For step 1, the scope is conceptual modelling languages, and the goals are at least model interoperability and, at least to some extent if feasible, runtime use of conceptual models for so-called ‘intelligent systems’ or ontology or conceptual-model driven information systems.

Regarding steps 2 and 3, to the best of our knowledge, there is no requirements catalogue for CDMLs (step 2/3a), but several use cases (step 2/3b) have been reported in scientific literature; e.g., the termbanks for multiple languages requiring interoperability among ER and UML (Keet and Fillottrani, 2015) and runtime integration of information about food in the Roman Empire with ORM (Calvanese et al., 2016). Assigning priorities (step 2/3c) has been done for several languages, but mostly not explicitly. For instance, a priority may be the computational complexity of the problem of satisfiability checking, or scalability in the presence of large amounts of data (OWL 2 QL) or a scalable TBox (OWL 2 EL) (Motik et al., 2009), or to have as many language features as possible to cover all corner cases, such as the “narcissist” in medicine as the single example for local reflexivity in (Horrocks et al., 2006). Tables with alternative sets of conflicting requirements are available, such as the pros and cons of several logics for formalising conceptual models (Fillottrani and Keet, 2015) and

trade-offs for representing the KGEMT mereotopology in various logics (Keet et al., 2012). For the CDMLs, it has yet to be decided how to assign priorities. One could survey industry (Malavolta et al., 2013), but it has been shown in at least one survey that modellers do not know the features well enough to be a reliable source (Alberts et al., 2008). Thus, existing works fall short on providing answers to steps 2 and 3.

There are many papers describing a language specification (step 4), with the DLs the most popular by number of papers. Most of these papers do not have a metamodel, however. Regarding existing metamodels one may be able to reuse for the language specification: there are proposals especially in the conceptual modelling community, spanning theoretical accounts (e.g., (Halpin, 2004; Keet and Fillottrani, 2015)), academic proof-of-concept implementations (Atzeni et al., 2008; Boyd and McBrien, 2005; Venable and Grundy, 1995), and industry-level applications, such as in the Eclipse Modeling Framework111https://www.eclipse.org/modeling/emf/. The UML diagrams in the OWL and UML standards (Motik et al., 2009; Object Management Group, 2012) are essentially metamodels as well. To enable a comparison between CDMLs, a unified metamodel is required, which reduces the choice to (Atzeni et al., 2008; Boyd and McBrien, 2005; Keet and Fillottrani, 2015; Venable and Grundy, 1995). The most recent metamodel (Keet and Fillottrani, 2015) covers all the static structural components in unifying UML Class Diagrams, ER and EER, and ORM and ORM2 at the metamodel layer cf. the subsets in (Atzeni et al., 2008; Boyd and McBrien, 2005; Venable and Grundy, 1995), and has both a glossary of elements and the constraints among them. It was developed in UML Class Diagram notation for communication (Keet and Fillottrani, 2015) and formalised for precision (Fillottrani and Keet, 2014).

Step 5 in the language design pipeline—design of a graphical notation—is straightforward for the current scope, for it will be mapped onto the graphical notation of UML Class diagrams, EER, and ORM2. The architecture of the tool (step 6) is concurrently being worked on (Fillottrani and Keet, 2016), and only after that can one do step 7, which is thus outside of the current scope.

Figure 2: Language design, adapted from Frank (2013), where the focus of this paper is highlighted in bold and shared (green). The “234. Ontological analysis of language features” has been added to the 7-step process, which is elaborated on in Section 3.

While the 7-step waterfall process for domain-specific languages is generally applicable for logic-based CDML design as well, some ontological analysis during steps 2-4 should improve the outcome, to which we shall return in Section 3. The case for, and benefits of, using insights from ontology (analytic philosophy) to advance the modelling has been well-documented (Guarino, 2009; Guarino and Guizzardi, 2006), with many papers detailing improvements on precision of representing the information; e.g., deploying the UFO foundational ontology to improve the UML 2.0 metamodel (Guizzardi and Wagner, 2010) and examining the nature of relationships (Keet, 2009; Guizzardi and Wagner, 2008), and more general philosophical assessments about conceptual models, such as regarding concepts (Partridge et al., 2013) and 3D versus 4D conceptual models (West et al., 2010).

Thus, current resources fall short especially on a clear requirements specification and priority-setting for CDMLs and on ontology-driven language design. We will contribute to filling these gaps in the following two sections.

2.3 Quantitative assessments on language feature use

To the best of our knowledge, there are only two studies with a quantitative approach to CDML feature usage, which are on ORM diagrams used in industry and developed by one modeller (Smaragdakis et al., 2009) and on publicly available EER, UML, and ORM diagrams Keet and Fillottrani (2015), whose ORM data are similar to those reported in (Smaragdakis et al., 2009). The diagrams in the dataset of Keet and Fillottrani (2015) were analysed using a unified metamodel Keet and Fillottrani (2015), which facilitated cross-language comparisons and categorisation of entities in those languages into the harmonised terminology. A relevant selection of the terminology across the languages is included in Table 1. This metamodel’s top-type is Entity that has four immediate subclasses: Relationship with 11 subclasses, Role, Entity type with 9 subclasses, and Constraint that has 49 subclasses (i.e., across the three CDML families, there are 49 different types of constraint). In addition to the hierarchy, the entities have constraints declared among them to constrain their use; e.g., each relationship must have at least two roles and a disjoint object type constraint is only declared on class subsumptions.

This metamodel was used to classify the entities of the diagrams in a set of 101 UML, (E)ER, and ORM/2 models that were publicly available

222the models and analysis are available from http://www.meteck.org/swdsont.html, and not within the scope of this paper. The average size of the diagram (vocabulary+subsumption) is about 50 entities/diagram, totalling to 8036 entities, of which 5191 (i.e., 64%) are entities that were classified in an entity (language feature) that is included in all three language families and 1108 (13.8%) in ones that are unique to a language family (e.g., UML’s aggregation) Keet and Fillottrani (2015). The obtained usage frequency for each entity, together with the design choices described in Section 3, sustain the logic profiles given in Section 4.

metamodel term UML Class EER ORM/FBM DL
term Diagram
Relationship Association Relationship Fact type Role
Role Association/ member end Component of a relationship Role Role component
Entity type Classifier Object type
Object type Class Entity type Nonlexical object type/Entity type Concept
Attribute Attribute Attribute
Value type Lexical object type/Value type
Data type LiteralSpecification Data type Concrete domain
Table 1: Terminology of the languages considered (relevant selection).

3 Design choices for logic-based profiles

One could simply take the evidence of which CDML features are used most, and design a logic for it, or pick a logic one likes and make the best out of a logic-based reconstruction. This has resulted in an ‘embarrassment of the riches’ in the plethora of logic-based reconstructions, and even more so when the formalisations are examined in detail. None is the same. The reason for this is that there are several design choices that each can result in a different logic of different computational complexity, using different reconstruction algorithms, with varying tooling support. Such choices have not been state upfront, but they have to be piecemeal reconstructed by anyone interested in logic foundations for conceptual models, because such decisions are sparsely discussed in the literature. This brings us to the “4” of the “234. Ontological analysis of language features” extension to Frank (2013)’s waterfall procedure for language development (recall Fig. 2 in Section 2.2). The step 4, “language specification”, concerns affordances and features of the logic, such as 1) the ability to represent the conceptualisation more or less precisely with more or less constraints333this is distinct from subject domain coverage; to illustrate the difference: being able to represent, say, “has part =2 eyes” vs only “has part eyes” concerns precision, whereas omitting information about the eyes concerns coverage., and 2) whether the language contributes to support, or even shape, the conceptualisation and one’s data analysis for the conceptual data model for an information system, or embeds certain philosophical assumptions and positions. Regarding the latter, we identified several decision points, which may not yet be an exhaustive list, but it is the first and most comprehensive list to date. They are as follows, and explained and discussed for CDMLs afterward:

  1. Should the CDML be ‘truly conceptual’ irrespective of the design and implementation or also somewhat computational? That is, whether the language should be constrained to permit representation of only the what of the universe of discourse vs. not only what but also some how in the prospective system. The typical example is whether to include data types for attributes or not.

  2. Are the roles that objects play fundamental components of relationships, i.e., should roles be elements of the language?

  3. Will refinements of the kinds of general elements—that then have their own representation element—result in a different (better) conceptual model? For instance,

    1. to have not just Relationship but also an extra element for, say, parthood;

    2. to have not just Object type but also refinements thereof so as to indicate ontological distinctions between the kind of entities, such as between a rigid object type and the role it plays (e.g., Person vs Student, respectively);

    3. if only binary relationships are allowed, the modeller may assume there are only binary relations in the world and reifications of -aries vs the existence of -aries () proper;

    4. if only object types are allowed, the modeller may assume everything is a object in the world, though one may argue that ‘stuff’, such as Wine and Gold, is disjoint from object, and thus would have to be represented with a different element.

  4. Does one have a 4-dimensionalist view on the world (space-time worms) and thus a language catering for that or are there only 3-dimensional objects with, perhaps, a temporal extension?

  5. What must be named? The act of naming or labelling something amounts to identifying it and its relevance; conversely, if it is not named, perhaps it is redundant.

Little is known about what effects the different decisions may have. The data analysis of Keet and Fillottrani (2015) indicates that binaries vs -aries (Item 3c) and just plain relationship vs also with aggregation (Item 3a) does indeed make a difference at least for UML vs (E)ER and ORM/2, in that UML class diagrams have disproportionately fewer -aries and more aggregation associations than (E)ER and ORM/2. Regarding the former, it is known that -aries in UML class diagrams are hard to read due to the look-across notation (Shoval and Shiran, 1997) and it uses a different visual element (diamond vs line), compared to (E)ER and ORM/2 that use the same notation for both binaries and -aries; an investigation into the ‘syntactic sugar’ of the graphical elements is outside the current scope. The other options in Item 3 are philosophically interesting, but unambiguous industry requirements for them are sparse, except for ‘stuff’, because quantities of amounts of matter are essential to traceability of ingredients in, among others, the food production process and pills and vaccines production (e.g., Donnelly (2010); Solanki and Brewster (2016)). In this case, distinguishing between a countable object and a specific quantity of an amount of matter can more precisely relate, say, a specific, numbered, capsule of liquid medicine and the stock that the medicine came from, which, could aid investigations in case of contamination (e.g., the capsule was not cleaned properly vs a contaminated batch of liquid). To date, there is no CDML that includes this distinction other than one proposal for a UML stereotype by Guizzardi (2010) and a set of relations for stuffs that could refine UML’s aggregation association Keet (2016).

The only proposal for refinements of Object Type (Item 3b) is OntoUML Guizzardi (2005), which adds, among others, the stereotypes “role” and “phase”. More generally, the approach amounts to taking a foundational (top-level) ontology, such as UFO, GFO, or DOLCE, and to use some or all of those core categories to refine UML’s class or ER or ORM’s entity type. Choosing one foundational ontology may result in the situation where one’s diagrams or conceptual data modelling language ends up to be incompatible with one that adheres to another foundational ontology Khan and Keet (2013). At the time of writing, it is not clear where or how exactly such refinements provide benefits, other than the typical example of preventing bad hierarchies that OntoClean Guarino and Welty (2009) seeks to resolve. For instance, to not declare, say, Person as a subclass of Employee, because the former is a sortal and the latter is a role that the former plays for a limited duration. Instead, it either should be the other way around (Employee is a Person) or sideways (Employee is a role played by Person). Declaring each Object Type with their respective category and firing ontological rules, such as ‘a sortal cannot be subsumed by a role’, could then at detect automatically such quality issues in a diagram. From the viewpoint of logics and formalising it, something like this can be done with second-order logic, many-valued logics, or—easier on the logic, but ontologically unclean—have the elements in the diagrams be subsumed by the entities in a foundational ontology.

Conceptual models and features for implementation decisions (Item 1)

It is known theoretically at least that incorporating design and implementation decisions reduces potential for interoperability and integration with other systems. Two common examples are declaring data types of attributes (in UML and ORM/2) and entity type-specific attributes (in (E)ER and UML) vs value types that can be shared among entity types (ORM). ER and EER do not have data types, and its selection is pushed down to the design or implementation stage in the waterfall methodologies of database development; e.g., whether some attribute length should be recorded in integer or float is irrelevant in the data analysis stage. Value types, in contrast to attributes, can be reused by multiple entity types and can easily be reverted into entity types with minimal disruption to the diagram; e.g., a value type length means the same thing regardless whether it is used for the length of a Sofa or a Table for some furniture database. Of course, one can convert between attributes and value types Fillottrani and Keet (2015), but having a value type element in the language clearly enables extra analysis regarding common semantics in a model.

Practically, for the design of a logic, the inclusion of value types entails that one has two types of object types: one that can relate only to other object types and one that relates to a data type. That, is not simply only one unary predicate in FOL or ‘concept’ in DL for everything, but another, disjoint one, that must relate to a data type specifically. The inclusion of attributes, or not, affects the language insofar as one wants to create, or already has, a type of relation that relates an object type to a data type. For instance, OWL’s Data Property.

Ontological commitments for relationships (Item 2)

Let us first illustrate the possible decisions that Item 2 asks for. Irrespective of the representation decision, there is some relationship, say, teach that holds between Professor and Course. With the ‘just predicates’ decision, there is assumed to be no teach relationship, or at least not represented as such, but there would be at least one predicate, teaches or taught by in which Professor and Course participate in that specific order or in reverse, respectively. With the ‘there are roles too’ decision, then Professor would play the role [lecturer] in the relationship teach and Course would play the role [subject] in the relationship teach; hence, role is an element in the language and deemed to be part of the so-called ‘fundamental furniture’ of the universe. This distinction between predicates-only and roles-too has been investigated by philosophers, and are there called standard view and positionalist, respectively. It has been argued that the positionalist commitment with roles (argument places) is ontologically a better way characterising relations than the standard view that enforces an artificial ordering of the elements in the relation and requires inverses444and anti-positionalist is argued to be better than positionalist (Fine, 2000; Leo, 2008), but anti-positionalist is unpractical at this stage., and it is also deemed better with respect to natural language interaction and expressing more types of constraints (Fine, 2000; Keet, 2009; Keet and Chirema, 2016; Leo, 2008).

This has been discussed at some length in (Fillottrani and Keet, 2015) from a computational perspective, because the main issue is that the CDMLs are all positionalist, yet most logics use predicates with the standard view. To address this impasse, there are several options. One could commit to a logic-based reconstruction into a positionalist language, be this the family that is used for a partial reconstruction of ER Calvanese et al. (1998), UML class diagrams Berardi et al. (2005), and ORM Keet (2009), roles in the Z formalisation by Jahangard Rafsanjani and Mirian-Hosseinabadi (2011), or one that is yet to be designed. One also could deny positionalism in some way and force it into a standard view logic. For instance, one could change the [lecturer] and [subject] roles into a teaches and/or a taught by predicate and declare them as inverses if both are included in the vocabulary, or pick one of the two and represent the other implicitly through taught by or teaches, respectively. Sampling decisions made in related works showed that, e.g., Pan and Liu Pan and Liu (2010) use a hybrid of both roles and predicates for ORM and its reading labels also may be ‘promoted’ to relationships Wagih et al. (2013), the original ORM formalisation was without roles in the language Halpin (1989), and UML’s association ends are sometimes ignored as well (e.g., Braga et al. (2010); Queralt and Teniente (2008)), but not always Berardi et al. (2005).

Exploring the conversion strategies brings one to the computational complexity of the logic. Mostly, adding inverses does not change the worst-case computational complexity of a language; e.g., and are both PSpace-complete (Tobies, 2001), but a notable exception is the OWL 2 EL profile that does not have inverse object properties (Motik et al., 2009).

3D vs. 4D (Item 4)

The 3-dimensionalist approach takes objects to be 3-dimensional in space where the objects are wholly present at each point in time (i.e., they do not have temporal parts). Statements are true at the ‘present’, whilst being ignorant of the object in the past and future. If one wants to deal with time, it is added as an orthogonal dimension, as in, e.g., (Artale et al., 2007; Keet and Berman, 2017). The 4-dimensionalist approach, sometimes also called ‘fluents’, takes entities to exist in four dimensions, being space+time, they are not wholly present at each point in time, and do have temporal parts. Any statements can be about not only the present, but also the past and future (e.g. (Batsakis et al., 2017; West et al., 2010)). A typical example in favour of 4D is to represent as accurately as possible the notion of a holding or supra-organisation West et al. (2010), such as Alphabet and Nestlé: these companies exist for some time and keep their identity all the while they acquire and sell other (subsidiary) companies. In a 3D-only representation, one would have a record of which companies they own now, but not whether they are the same ones as last year (unless temporal information is added in some way; e.g., through database snapshots or time stamps).

The predominant choice of conceptual data modelling languages is 3D, although temporal extensions do exist especially for ER (e.g., (Artale et al., 2007; Keet and Berman, 2017)). We could not find any evidence that can explain why this is the case.

Naming things (Item 5)

The act of naming things entails the interaction between natural language and ontology, and their interaction. We do not seek to discuss that millennia-old philosophical debate here, but one that is applied within the current context. Naming elements happens differently across the three CDML families. For instance, in UML class diagrams, the association ends must be named but the association does not necessarily have to be named, which typically is the other way around in (E)ER except for recursive relations, whereas ORM diagrams commonly only have reading labels of a relationship rather than naming either the roles or the relationship themselves. ORM thus clearly distinguishes between the conceptual layer and the natural language layer, but such workings are not taken into account explicitly in any of the formalisations, nor has it been explicitly decided whether it should be (other than in a model for roles in Keet and Chirema (2016)). A systematic solution to this natural language ontology interaction has been investigated in the context of the Semantic Web, where the natural language dimension has its own extension on top of an OWL file Buitelaar and Cimiano (2014)

, although it still relies on naming classes and object properties. What the three CDML families do have in common is that object types are always named, hence, at least part of that separation of ontology and lexicon might be of use in praxis.

For designing a logic, this may not matter much, but it will have an effect on the algorithms to reconstruct the diagrams into a logical theory.

Assessment of popular languages and their commitments

We assessed the relatively frequently occurring logics for formalising conceptual data models on whether it would be possible to choose a ‘best fit’ language. This comparison is included in Table 2. The first section of the table summarises the main design decisions discussed in the preceding paragraphs, whereas the second part takes into consideration non-ontological aspects with an eye on practicalities. Such practicalities include among others, scalability, tooling support, and whether it would be easily interoperable with other systems. Regarding the latter, we include the Distributed Ontology, Model, and Specification Language DOL that was recently standardised by the Object Management Group, which provides a metalanguage to link models in various logics, including up to second order logic555http://www.omg.org/spec/DOL/ and http://dol-omg.org/..

The first section in the table suggests and FOL are good candidates for logic-based reconstructions of conceptual data models; the second section has more positive evaluations for and OWL 2 DL. Put differently: neither of them is ideal, so one may as well design one’s own language for the best fit.

Language features
– standard view + positionalist – standard view – standard view
– with datatypes – with datatypes – with datatypes + no datatypes
– no parthood primitive – no parthood primitive – no parthood primitive – no parthood primitive
– no -aries + with -aries – no -aries + with -aries
+ 3-dimensionalism + 3-dimensionalism + 3-dimensionalism + 3-dimensionalism
– OT refinement with subsumption – OT refinement with subsumption – OT refinement with subsumption – OT refinement with subsumption
– no NL-logic separation – no NL-logic separation partial NL-logic separation – no NL-logic separation
– very few features; large feature mismatch + little feature mismatch some feature mismatch, with overlapping sets + little feature mismatch
– logic-based reconstructions to complete + logic-based reconstructions exist – logic-based reconstructions to complete logic-based reconstructions exist
Computation and implementability
+ PTIME (TBox); AC (ABox) EXPTIME-complete N2EXPTIME-complete – undecidable
+ very scalable (TBox and ABox) somewhat scalable (TBox) somewhat scalable (TBox) – not scalable
+ several reasoners – no implementation + several reasoners – few reasoners
+ linking with ontologies doable – no interoperability + linking with ontologies doable – no interoperability with widely used infrastructures
+ ‘integration’ with DOL – no integration with DOL + ‘integration’ with DOL + ‘integration’ with DOL
+ modularity infrastructure – modularity infrastructure + modularity infrastructure – modularity infrastructure
Table 2: Popular logics for logic-based reconstructions of CDMLs assessed against a set of requirements; “–”: negative evaluation; “+”: positive; “NL-logic”: natural language interaction with the logic; “OT refinement”: whether the language permits second order or multi-value logics or can only do refinement of object types through subsumption.

4 Logic-based profiles for conceptual data modelling languages

We now proceed to define logics to characterise the minimalist semantics of each of the three families of CDMLs. This takes into account the ontological considerations discussed in the previous section as well as the evidence from Keet and Fillottrani (2015) and the requirement to have a coverage of around 99% of the used entities and constraint. Because of afore-mentioned ontological reasons in favour of roles as well as that all three CDML families are positionalist, a Positionalist Core is defined despite its current lack of implementation support (Section 4.1.1). Afterward, a standard view Standard Core and language-specific profiles are defined in Sections 4.1.2-4.1.5. An overview of the upcoming definitions and algorithms is shown in Fig. 3.

Figure 3: Sketch of the orchestration between the profiles and algorithms.

4.1 Profiles

Positionalism is the underlying commitment of the relational model and a database’s physical schema, as well as of the main CMDLs. It has been employed in Object-Role Modelling (ORM) and its precursor NIAM for the past 40 years (Halpin and Morgan, 2008), UML Class Diagram notation requires association ends as roles, and Entity-Relationship (ER) Models have relationship components (Fillottrani and Keet, 2015). On the other hand, First Order Logic and most of its fragments, notably standard DLs (Baader et al., 2008), do not exhibit roles (DL role components) among its vocabulary. In order to be able to do reasoning, conceptual schemas written in these CMDLs are generally translated into a DL by removing roles, and thus losing the connection that exists when the same role is played by different concepts. For example, the lives at role may be played by a person in a house rent relationship by a person in a house mortgage relationship. This role name is relevant for querying lets say the real estate owners in the database, so it is relevant for the model’s intended meaning but the translation splits them into two different relationships. Therefore, we consider relevant to design a positionalist core profile that preserves roles as first class citizens among the DL vocabulary. In case reasoning over advanced modelling features is needed, it is possible to switch to the standard core profile with the cost of losing this connection. This translation is given in Algorithm 1.

4.1.1 Positionalist Core Profile

In this section we define the logic that describes the positionalist core profile.

Definition 1

Given a conceptual model in any of the analysed CDMLs, we construct a knowledge base in by applying the rules:

  1. we take the set all of object types , binary relationships , datatypes and attributes in the model as the basic elements in the knowledge base.

  2. for each binary relationship formed by object types and , we add to the knowledge base the assertions and .

  3. for each attribute of datatype within an object type , including the transformation of ORM’s Value Type following the rule given in (Fillottrani and Keet, 2014), we add the assertion .

  4. subsumption between two object types and is represented by the assertion .

  5. for each object type cardinality in relationship with respect to its -th component , we add the assertions .

  6. we add for each mandatory constraints of a concept in a relationship the axiom or depending on the position played by in . This is a special case of the previous one, with .

  7. for each single identification in object type with respect to an attribute of datatype we add the axiom .

This construction is linear in the number of elements in the original conceptual model, so reasoning complexity on the theory is the same as on the conceptual model. We restrict it to binary relationships only, because general -ary relationships are rarely used in the whole set of analysed models. The (E)ER and ORM/2 models exhibit a somewhat higher incidence of -aries, so they are included in the respective profiles; see below. Also, we allow only one such constraint for each component, multiple cardinality constraints over the same component in a relationship are used very rarely.

can be represented by the following DL syntax. Starting from atomic elements, we can construct binary relations , arbitrary concepts and axioms according to the rules:

where and . For convenience of presentation, we generally use the numbers 1 and 2 to name the role places, but they can be any number or string and do not impose an order. Whenever necessary we note with the set of all role names in the vocabulary, with fixed argument places for attributes such that is the role played by the concept, and the role played by the datatype. These names must be locally unique in each relationship/attribute.

Although this syntax represents all knowledge bases, there are sets of formula following the syntactic rules that are not knowledge bases since they are not result of any translation of a valid conceptual model. For example, the knowledge base is not a knowledge base, because the syntactic production rules are only introduced to provide a proper semantic characterisation.

Now we introduce the semantic characterisation

Definition 2

An interpretation for a knowledge base in consists of a set of objects , a set of datatype values , and a function satisfying the constraints shown in Table 3. It is said that satisfies the assertion iff ; and it satisfies the assertion iff exists such that (mandatory 1) and for all it holds that (inverse functional).

Table 3: Semantics of .

In total, all the entities in the core profile sum up to 87.57% of the entities in all the analysed models, covering 91,88% of UML models, 73.29% of ORM models, and 94.64% of EE/EER models. Conversely, the following have been excluded from the core despite the feature overlap, due to their low incidence in the model set: Role (DL role component) and Relationship (DL role) Subsumption, and Completeness and Disjointness constraints. This means that it is not possible to express union and disjointness of concepts in a knowledge base obtained by formalising a conceptual model. Clearly they can be expressed by combinations of the constructors in , but this is not possible if we follow the previous construction rules. Since completeness and disjointness constraints are not present, reasoning in this core profile is quite simple.

This logic can be directly embedded into (attributes are treated as binary relationships, and identification constraint over attributes can represented as in Calvanese et al. (2001)) which gives EXPTIME worst case complexity for satisfiability and logical implication. A lower complexity would be expected due to the limitations in the expressivenes. For example, completeness and disjointness constraints are not present, and negation cannot be directly expressed. It is possible to code negation only with cardinality constraints (Baader et al., 2008, chapter 3), but then we need to reify each negated concept as a new idempotent role, which is not possible to get from the rules. Another form of getting contradiction in this context is by setting several cardinality constraints on the same relationship participation, which is also disallowed in the rules. In any case, the main reasoning problems on the conceptual model are only class subsumption and class equivalence on the given set of axioms.

In spite of all these limitations, no simpler positionalist DL has been introduced. In order to get lower complexity bounds we need to translate a TBox to a standard (non-positionalist) logic, like below.

   an atomic binary relationship; domain of ; range of
  if   then
     Rename to two ‘directional’ readings, and
     Make and a DL relation (role)
     Type the relations with
     Declare inverses with
     if  then
        if  is named then
            user-added label or auto generated label
        end if
        Make a DL relation (role)
        Type one , i.e.,
        Declare inverses with
     end if
  end if
Algorithm 1 Positionalist Core to Standard Core

4.1.2 Standard Core Profile

Considering formalisation choices such as the positionalism of the relationships (Keet, 2009; Leo, 2008) and whether to use inverses or qualified cardinality constraints, a standard core profile has been specified (Fillottrani and Keet, 2015). In case the original context is a positionalist language, a translation into a standard (role-less) language is required. Algorithm 1 (adapted from Fillottrani and Keet (2015)) does this work in linear time in the number of elements of vocabulary. The main step involves recursive binary relations that generally do have their named relationship components vs ‘plain’ binaries that have only the relationship named.

Definition 3

Given a conceptual model in any of the analysed CDMLs, we construct a knowledge based in by applying algorithm 1 to its knowledge base.

Again, the algorithm is linear in the number of binary relationships in the knowledge base, not affecting complexity results when reasoning.

Once this conversion step is done, the formalisation of the standard core profile is described as follows. It includes inverse relations to keep connected both relationships generated by reifying roles. Take atomic binary relations (), atomic concepts (), and simple attributes () as the basic elements of the core profile language , which allows us to construct binary relations and arbitrary concepts according to the following syntax:

Definition 4

A interpretation for a knowledge base in is given by , with the domain of interpretation for concepts, the domain of datatype values, and the interpretation function satisfying the conditions in Table 4. satisfies an axiom as in .

Table 4: Semantics of .

From the perspective of reasoning over , this is rather simple and little can be deduced: negation cannot be directly expressed here either, as discussed for . This leaves the main reasoning problem of class subsumption and class equivalence here as well. At most the DL (called in (Donini et al., 1991)) is expressive enough to represent this profile, since we only need , , inverse roles and cardinality constraints; has polynomial subsumption, but its data complexity is unknown. That said, using a similar encoding of conceptual models as given in Section 4.1.1, the language can be reduced further to - which is NLOGSPACE with some restrictions on the interaction between role inclusions and number restrictions, and the Unique name Assumption (UNA). Observe that adding class disjointness to this language would result in a high jump in the complexity, since then the reduction would be to - which is NP-hard (Artale et al., 2009).

4.1.3 UML Class diagram Profile

The profile for UML Class Diagrams strictly extends . It was presented extensively in (Fillottrani and Keet, 2015).

Definition 5

A knowledge base in from a given conceptual model in UML is obtained by adding to its knowledge base the following formulas and axioms:

  1. for each attribute cardinality in an attribute of datatype within an object type we add the assertion .

  2. for each binary relationship subsumption between relationships and we add the axiom .

The syntax is as in , with the additions highlighted in bold face for easy comparison:

With this profile, we cover of all the elements in the UML models of the test set. Absence of rarely used UML-specific modelling elements, such as the qualified association (relationship), completeness and disjointness among subclasses do indeed does limit the formal meaning of their models. On the positive side from a computational viewpoint, however, is that adding them to the language bumps up the complexity of reasoning over the models (to EXPTIME-hardness (Berardi et al., 2005)); or: the advantage of their rare use is that reasoning over such limited diagrams has just becomes much more efficient than previously assumed to be needed.

Definition 6

A interpretation for a knowledge base is a interpretation that also satisfies if and only if , with and .

Compared to , role hierarchies have to be added to the logic of the Core Profile, which yields the logic . To the best of our knowledge, this language has not been studied yet. If we adjust it a little by assuming unique names and some, from the conceptual modelling point of view, reasonable restrictions on the interaction between role inclusions and cardinality constraints, then the UML profile can be represented in the known , which is NLOGSPACE for subsumption and for data complexity (Artale et al., 2009). Also, if one wants to add attribute value constraints to this profile then reasoning over concrete domains is necessary. The interaction of inverse roles and concrete domains is known to be highly intractable, just adding them to gives NEXPTIME-hard concept satisfiability (Baader et al., 2005).

4.1.4 ER and EER Profile

The profile for ER and EER Diagrams also extends .

Definition 7

A knowledge base in from a given conceptual model in EER is obtained by adding to its its knowledge base the following formulas and axioms:

  1. we include atomic ternary relationships in the basic vocabulary.

  2. for each attribute cardinality in an attribute of datatype within an object type we add the assertion .

  3. for each weak identification of object type through relationship in which it participates as the -th component, the we add the assertion , such that and are all different.

  4. associative object type are formalised by the reification of the association as a new DL concept with two binary relationships.

  5. multiattribute identification is formalised as a new composite attribute with single identification.

This profile was presented extensively in (Fillottrani and Keet, 2015) and is here recast in shorthand DL notation. The syntax is as in , with the additions highlighted in bold face for easy comparison:

where and all and different.

Definition 8

An interpretation satisfies a knowledge base in is it is a interpretation, and satisfies iff for all it holds that if and then , , with and .

This profile covers relative frequent EER modelling entities such as composite and multivalued attributes, weak object types and weak identification, ternary relationships, associated objet types and multiattribute identification in addition to those of the standard core profile. This profile can capture of all the elements in the set of (E)ER models. Multivalued attributes can be represented with attribute cardinality constraints, and composite attributes with the inclusion of the union datatype derivation operator. Each object type (entity type) in (E)ER is assumed by default to have at least one identification constraint. In order to represent external identification (weak object types), we can use functionality constraints on roles as in (Calvanese et al., 2001) and its close relative Artale et al. (2017) or in (Toman and Weddell, 2009). Ternary relationships are explicitly added to the profile. If we want to preserve the identity of these relationships in the DL semantics, then we need to restrict to logics in the family. Otherwise, it is possible to convert ternaries into concepts by reification, as described in Algorithm 2, using three traditional DL roles and therefore allowing the translation into logics such as . Since associative object types do not impose new static constraints on the models, they are formalised by reification of the association as a new DL concept with two binary relationships. Finally, multiattribute identification can be represented as a new composite attribute with single identification.

This profile presents an interesting challenge regarding existing languages. The only DL language family that has arbitrary -aries and the advanced identification constraints needed for the weak entity types is the positionalist . However, also offers DL role components that are not strictly needed for (E)ER, so one could pursue a binary or -ary DL without DL role components but with identification constraints, the latter being needed of itself and for reification of a -ary into a binary (Algorithm 2). The family of languages may seem more suitable, then. Using Algorithm 2’s translation, and since we do not have covering constraints in the profile, we can represent the (E)ER Profile in the description logic (Artale et al., 2009) which has complexity NLOGSPACE for the satisfiability problem. This low complexity is in no small part thanks to its UNA, whereas most logics operate under no unique name assumption. A similar result is found in (Artale et al., 2007) for , but it excludes composite attributes and weak object types.

  v: domain of ; range of ; set of -components
  Reify into
  for all ,  do
      user-added label or auto generated label
     Make a DL role,
     Type as , where is the player ((E)ER entity type) in
     Add and
  end for
  Add external identifier
Algorithm 2 Equivalence-preserving n-ary into a binary conversion

4.1.5 ORM and ORM2 Profile

Unlike for the ER and EER profile, there is no good way to avoid the ORM roles (DL role components), as they are used for several constraints that have to be included. Therefore, to realise this profile, we must transform the ORM positionalist commitment into a standard view, as we did in Algorithm 1. This is motivated by the observation that typically fact type readings are provided, not user-named ORM role names, and only 9.5% of all ORM roles in the 33 ORM diagrams in our dataset had a user-defined name, with a median of 0. We process the fact type (relationship ) readings and ignore the role names following Algorithm 3. ’s relationship is typed, w.l.o.g. as binary and in -notation, as , with and variables for the ORM role names and and the participating object types. Let and be the fact type readings, then use to name DL role and to name DL role , and type as . This turns, e.g., a disjoint constraints between ORM roles of relationship and of into and .

   an atomic relationship
  if  is binary then
     Take fact type readings
     if there is only one fact type reading then
        Type with domain and range
        Declare and inverses
        Assign one reading to and the other to
        Type with domain and range accordingly
        Declare and inverses
     end if
      is -ary with
     Reify into , like in Algorithm 2, with for the binaries using the fact type readings as above
  end if
Algorithm 3 ORM/2 to standard view and common core.

The profile for ORM/2 Diagrams was presented in (Fillottrani and Keet, 2015), and a more detailed version including a text-based mapping as a restricted “” was developed in (Fillottrani et al., 2015) using   as underlying logic, yet that could cover only just over 96% of the elements in the set of ORM models, whereas this one reaches 98.69% coverage.

Definition 9

A knowledge base in from a given conceptual model in ORM2 is obtained by adding to its knowledge base the following formulas and axioms:

  1. each -ary relationship is reified as in Algorithm 3.

  2. each unary role is formalised as a boolean attribute.

  3. each subsumption between roles is represented by the formula .

  4. each subsumption between relationships is represented as a subsumption between the reified concepts.

  5. each disjoint constraint between roles and is formalised as two inclusion axioms for roles: and .

  6. each nested object type is represented by the reified concept of the relationship.

  7. each value constraint is represented by a new datatype that constraint.

  8. each disjunctive mandatory constraint for object type in roles is formalised as the inclusion axiom .

  9. each internal uniqueness constraint for roles over relationship objectified with object type is represented by the axiom

  10. each external uniqueness constraint between roles not belonging to the same relationship can be formalised with the axiom , where is the connected object type between all the , if it exists, or otherwise a new object type representing the reification of a new -ary relationship between the participating roles.

  11. each external identification is represented as the previous one, with the exception that we are now sure such exists. (i.e., the mandatoryness is added cf. simple uniqueness).

This slightly more comprehensive one is here recast in shorthand DL notation, with the additions highlighted in bold face for easy comparison:

Definition 10

A interpretation for a knowledge base is a interpretation with the constraints that , and . satisfies the assertion iff , and the assertion iff and for all objects it holds that .

We decided not to include any ring constraint in this profile. Although the irreflexivity constraint counts for almost half of all appearances of ring constraints, its participation is still too low to be relevant.

The semantics, compared to , is, like with the UML profile, extended in the interpretation for relationship subsumption. It also needs to be extended for the internal uniqueness, with the identification axioms for relationships. Concerning complexity of the ORM/2 Profile, this is not clear either. The EXPTIME-complete is the easiest fit, but contains more than is strictly needed: neither concept disjointness and union are needed (but only among roles), nor its fd for complex functional dependencies. The PTIME (Toman and Weddell, 2014) may be a better candidate if we admit a similar translation as the one given in Algorithm 2, but giving up arbitrary number restrictions and disjunctive mandatory on ORM roles.

4.2 Example application of the construction rules

Let us now return to the claim in the introduction about the sample UML Class Diagram in Fig. 1: that it has a logical underpinning in and therewith also has grounded equivalents in EER and ORM notation. The equivalents in EER and ORM are shown in Fig. 4.

The first step is to note that the reconstruction is obtained from + Algorithm 1 (by Definition 3). By the rules from Definition 1, we obtain the set of object types (fltr) {Person, Affiliation, …, Publisher} and of data types {Name, …, VAT reg no}. For the relationships, we need to use Algorithm 1, which we illustrate here for the association between Person and Affiliation: 1) bump up the association end names, has member and has, to DL roles; 2) type the relationships with:

and 3) declare inverses, . After doing this for each association in the diagram, we continue with step 3 of Definition 1, being the attributes. For instance, the Person’s Name we obtain the axiom

and likewise for the other attributes. Step 4 takes care of the subsumptions; among others is added to the knowledge base. Then cardinalities are processed in steps 5 and 6 (noting the algorithmic conversion from positionalist to standard view applies in this step), so that, for the membership association illustrated above, the following axioms are added to the knowledge base: (mandatory participation) whereas for, say, the scientist, it will be . Finally, any identifiers are processed, such as ISBN for Book, generating the addition of the to the knowledge base.

The process for the EER diagram is the same except that the name of the relationship can be used directly cf. bumping up the role names to relationship names. The reconstruction into ORM has two permutations cf. the UML one, which are covered by step 3 in Definition 1, being the conversion algorithm from ORM’s value types to attributes as described in Fillottrani and Keet (2014), and it passes through the second else statement of Algorithm 1 cf. the first if statement that we used for UML when going from positionalist to standard view.

Figure 4: The sample diagram of Fig. 1 rendered in EER and ORM2 notation; the common logic-based construction is discussed in the text.

Diagram construction rules, i.e., going in the direction from the logic-based profile to a graphical notation, can follow the same process in reverse. This can be achieved automatically, except where labels have to be generated. For instance, if one were to have a scenario on an interoperability tool of “UML diagram ORM diagram” and one wants to have the fact type readings, they will have to be added, which a user could write herself or it could be generated by one of the extant realisation engines for the controlled natural language666It would have rules that render, e.g., a into … has member … and a into … member of …, similar to OWL verbalisation Safwat and Davis (2017)

or SimpleNLG for natural language generation

Gatt and Reiter (2009).

5 Discussion

The methodological approach proposed is expected to be of use for similar research to inform better the language design process and elucidate ontological commitments that are otherwise mostly hidden. The five profiles form an orchestrated network of compatible logics, which serve as the logic-based reconstructions of fragments of the three main CDMLs that include their most used features. In the remainder of the section, we discuss language design, computational complexity, and look ahead at applicability.

Language design

To the best of our knowledge, there is no ‘cookbook process’ for logic or conceptual data modelling language design. Frank’s waterfall process Frank (2013) provided useful initial guidance for a methodological approach. In our experience in designing the profiles, we deemed our proposed extension with “Ontological analysis of language features” necessary for the conceptual modelling and knowledge representation languages setting cf. Frank’s DSLs. An alternative option we considered beforehand was Karsai et al. (2009)’s list of 26 guidelines, but they are too specific to DSLs to be amenable to CDML design, such as the DSL’s distinction between abstract and concrete syntax and their corresponding guidelines.

Zooming into that extra “Ontological analysis of language features” step, we had identified five decision points for language design with respect to ontology and several practical factors that are listed in Table 2 in Section 3. To the best of our knowledge, it is the first attempt to scope this component of language/logic design systematically and it may spur further research into it. Our contribution in that regard should be seen as a starting point for a broader systematic investigation into this hitherto neglected aspect. In making choices, we had to accommodate alternative design choices and the need to achieve high coverage. This was addressed by designing two alternative cores—positionalist and standard view (item 2 in Section 3)—and, importantly, three algorithms to achieve that level of compatibility. More precisely, Algorithm 1 provides the conversion option for item 2—roles or not—in a generic way, Algorithm 2 takes case of the binaries vs -aries (item 3a), and Algorithm 3 is a specific adaptation of Algorithm 2. All profiles have data types (item 1 in Section 3), for they are present in UML Class Diagrams and ORM/2, noting that it simply can be set to xsd:anyType and thus have no influence, which is the case for (E)ER. Further, if the intended semantics of the aggregation association was more specific in the UML standard, it would have merited inclusion in its profile (item 3b in Section 3), with then the onus on the DL community to find a way to add it as a primitive to a DL. If included, it would likely also be possible to design a conversion algorithm between the new primitive and a plain DL role with properties. Regarding adding more types of entity types to the language (item 3c), like sortal and phase: the one proposal (Braga et al., 2010; Guizzardi, 2005) is not in widespread use and therewith did not meet the evidence-based threshold for inclusion. It is also not clear how to represent in a decidable language such notions that are essentially based on OntoClean (Guarino and Welty, 2009) that requires modality and higher-order predicates, nor how an equivalence-preserving algorithm would look like, if possible at all.

Complexity considerations for the profiles

Traditionally, the DL research community has strived for identifying more and more expressive DLs for which reasoning is still decidable. The introduced profiles show that high expressivity is not necessary for representing most of the semantics of conceptual models, independently of the chosen modelling language. They thus are ‘lean’, evidence-based, profiles that, while not covering all corners of modelling issues, do have those features that are used most in practice. We summarise the complexity of each profile by immersion into a DL language in Table 5. The “Approximate DL” column is not an exact match for each profile, and often involves some extra assumptions that explains the different complexities. Low complexities are achievable by the standard profiles (i.e., those that give up on positionalism), due to the existence of a more accurate matching logic. Recall that is included in , , and . The biggest gap between the profiles and the matching DLs is given in showing more work on positionalist DLs is necessary, especially with respect to reasoning algorithms.

Main features Approx. DL Subsumption complexity

positionalist, binary relationships, identifiers, cardinality constraints, attribute typing, mandatory attribute and its functionality EXPTIME

standard view, binary relationships, inverses P

relationship subsumption, attribute cardinality NLOGSPACE

ternary relationships, attribute cardinality, external keys NLOGSPACE

entity type disjunction, relationships complement, relationship EXPTIME
subsumption, complex identifiers (‘multi attribute keys’) P
Table 5: Profile comparison on language and complexity; “Approx. DL”: the existing DL nearest to the profile defined.

An outstanding issue is whether object types in the diagrams are by default disjoint when not in a hierarchy, or not. Some research are convinced they are, and some are not; most formalisations and tools do not include it. Because of the lack of agreement, we have not included it. Note further that if this assumption were to be added, i.e., full negation in the profiles, it would affect the computational complexity of the profiles negatively.

It is also interesting to analyse at which point increasing expressiveness by adding new features to the language is worthwhile from the point of view of the modeller. If the feature is present, at least one modeller will use it, though mostly only occasionally. It is not clear if this is due to them being corner cases, a lack of experience on representing advanced constraints by modellers, tooling, or another reason. On the other hand UML’s aggregation as ‘extra’ feature cf. (E)ER’s and ORM/2’s plain relationships is being used disproportionally more often than part-whole relations in (E)ER and ORM/2. It remains to be investigated why exactly this is the case.

Toward applicability

The presented profiles may be applied as the back-end of CASE tools using the compatible profiles as unifying logics and orchestration of corresponding optimised reasoners for, say, Ontology-Based Data Access such that it focusses on the perceived language needs of the modellers (cf. the logic and technology, as in, e.g., Calvanese et al. (2017); Özçep et al. (2015)), whilst still keeping it tractable. The current conceptual modelling tools that have a logic back-end are still sparse (Braga et al., 2010; Farré et al., 2013; Fillottrani et al., 2012; Braun et al., 2017), and allow a modeller to model in only one language, rather than being allowed to switch between language families.

Using the common core for model interoperability by mapping each graphical element into a construct in is an option. However, one also would want to be precise and therefore use more language features than those in the common core, and when linking models, ‘mismatch’ links would still need to be managed, and wrong ones discarded. To solve this, an interoperability approach with equivalence, transformation, and approximation rules that is guided by the metamodel is possible (Fillottrani and Keet, 2014; Khan et al., 2016). There, one can have two models with an intermodel assertion; e.g., between a UML association and an ORM fact type. The entities are first classified/mapped into entities of the metamodel, any relevant rules are executed, and out comes the result, being either a valid or an invalid link. The ‘any relevant rules are executed’ is coordinated by the metamodel; e.g., the metamodel states that each Relationship has to have two or more Roles, which, in turn, have to have attached to it either an Object Type or Value Type, so those mapping and transformation rules are called as well during the checking of the link. The MIST EER tool Dimitrieski et al. (2015) has a similar goal, though currently it supports only EER and its translation to SQL and therewith is complementary to our work presented here.

The formal foundation presented here would enable such an interface were either multiple graphical rendering in different modelling language families could be generated, or link models represented in different languages in a system integration scenario.

6 Conclusions

A systematic logic design process was proposed that generalising and extending the DSL design process to be more broadly applicable by incorporating an ontological analysis of language features in the process. This first compilation of ontological commitments embedded in a logic design process includes, among others, the ontology of relations, the conceptual vs design features trade-off, and 3-dimensionalist vs. 4-dimensionalist commitments.

Based on this extended process with explicit ontological distinctions and the evidence of the prevalence of the features in the models, different characteristic profiles were specified into a suitable Description Logic, which also brought with it insights into their computational complexity. The common core profile is of relatively low computational complexity, being in the tractable . Without the negation, hardly any inconsistencies can be derived with the profiles, with as flip side that it is promising for scalable runtime use of conceptual data models.

We are looking into several avenues for future work, including ongoing tool development and more precise complexity results for the profiles so that it would allow special, conceptual data model-optimised, reasoners.


This work was partially supported by the National Research Foundation of South Africa and the Argentinian Ministry of Science and Technology. Any opinion, findings and conclusions or recommendations expressed in this material are those of the author and therefore the NRF does not accept any liability in regard thereto.


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