Active database management systems have been developed for applications needing an automatic reaction in response to certain events. Events can be simple in nature or complex. Complex events rely on simpler ones and are usually specified with the help of operators of an event algebra. There are quite a few papers dealing with extensions of existing event algebras. However, a systematic and comprehensive analysis of the semantics of complex events is still lacking. As a consequence most proposals suffer from different kinds of peculiarities. Independent aspects are not treated independently, leading to shady mixtures of aspects in operators. Moreover, aspects are not always treated uniformly. Operators may have other semantics than expected. The paper addresses these problems by an extensive and in-depth analysis of the foundations of complex events. As a result of this analysis, a (formal) meta-model for event algebras is introduced that subdivides the semantics of complex events into elementary, independent dimensions. Each of these dimensions are discussed in detail. The resulting language specification fulfils the criteria for a good language design (like orthogonality, symmetry, homogeneity, lean set of language constructs) to a large extent.
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