Lars Bækgaard

Event‐based conceptual modeling

Purpose – The purpose of the paper is to obtain insight into, and provide practical advice for, event‐based conceptual modeling.Design/methodology/approach – The paper analyzes a set of event concepts and uses the results to formulate a conceptual event model that is used to identify guidelines for creation of dynamic process models and static information models.Findings – The paper characterizes events as short‐duration processes that have participants, consequences, and properties, and that may be modeled in terms of information structures. The conceptual event model is used to characterize a variety of event concepts and it is used to illustrate how events can be used to integrate dynamic modeling of processes and static modeling of information structures.Originality/value – The results are unique in the sense that no other general event concept has been used to unify a similar broad variety of seemingly incompatible event concepts. The general event concept can be used to improve dynamic and static mo...

Real-time event control in active databases

Abstract Most existing languages for the specification of active rules are based on the ECA (event-condition-action) paradigm. Usually, rules specified by such languages are triggered by simple events like the update of a tuple in a relational database. We present a specification language that can be used to specify real-time triggering conditions in terms of complex event patterns. Our specification language can be used to formulate complex, triggering conditions for active rules in terms of event patterns that involve sequences, alternations, iterations, and parallel compositions. Also, the language can be used to specify soft, real-time constraints with respect to the minimum/maximum time required/allowed between pairs of events. Our language is designed to be flexible and general in the sense that it can be combined with most ECA rule languages. We formally define the syntax and semantics of the language and we illustrate its use by means of examples.

Incremental computation of set difference views

Views can be computed by means of recomputation or they can be computed incrementally. Incremental view computation outperforms view recomputation in many situations. This is particularly true in distributed database environments such as data warehousing environments. The authors present and analyze two algorithms for the incremental computation of relational set difference views. Set differences occur naturally in the definition of the set-division operator and in rewritten nested queries. The I/O efficiency of the incremental algorithms is compared to each other and to efficient recomputation algorithms. The cost analysis strongly indicates that incremental computation of set difference views outperforms recomputation of set difference views in many situations.

Transaction-Based Specification of Database Evolution

We present a two-layer language for the specification of database evolution in terms of transaction-based, dynamic integrity constraints. The first language layer is based on first-order logic and it is used to express dynamic constraints in terms of queries on the transaction history of a database. The second layer uses a customizable combination of text and graphics and its semantics are defined in terms of the first-order language. Our language is orthogonal to state-based constraint languages and it can be used as a supplement to these. Also, our language can be used in combination with all object-based or entity-based data models. We use examples to illustrate the use of the specification language.

On the Capabilities of Digimaterial Artifacts - Structures, Symbols, Actions.

The purpose of the paper is to propose and discuss three types of capabilities of digimaterial artifacts like laptop computers, cameras, cars, robots etc. Digimaterial artifacts are material artifacts that combine digital and non-digital elements by bearing one or more digital artifacts. Digital artifacts are linguistic expressions like, say, binary sequences of 0s and 1s. Software and databases are examples of digital artifacts. Paper pieces with digital inscriptions and cars with data and software are examples of digimaterial artifacts. Digimaterial artifacts can bear, and potentially manipulate, digital artifacts. We describe and discuss digimaterial structures and the capabilities that are enabled by these structures. And we describe and discuss the plastic nature of such structures and capabilities. We expect that our work can be used to understand digimaterial capabilities and to analyse and design digimaterial structures that possess a relevant set of

On non-trivial Nash stable partitions in additive hedonic games with symmetric 0/1-utilities

We consider the problem of computing non-trivial Nash equilibria in additive hedonic games with symmetric 0/1-utilities. Such a game can be represented by an undirected unweighted graph G(V,E) where a non-trivial Nash equilibrium corresponds to a partition of V into at least two sets such that each node has at least as many neighbours in its own set compared to any other set in the partition. We show that computing such an equilibrium is NP-complete. On the other hand, we show that such an equilibrium is computable in polynomial time if (1) G is triangle free, (2) G contains no 4-cycles sharing and edge, and (3) G is not a star. If G is not a star with girth at least 5 we show how to compute a non-trivial equilibrium in O(n)-time.