tailieunhanh - Composite Event Specification in Active Databases: Model & Implementation

Active database systems require facilities to specify triggers that fire when specified events occur. We propose a language for specifying composite events as eveti expressions, formed using event operators and events (primitive or composite). An event expression maps an event history to anothe-r event history that contains only the events at which the event expression is “satisfied” and at which the trigger should 6re. We present several examples illustrating how quite complex event specifications are possible using event expressions. In addition to the basic event operators, we also provide facilities that make it easier to specify composite events. “Pipes” allow users to isolate sub-histories of interest. “Correlation variables” allow users to ensure. | Composite Event Specification in Active Databases Model Implementation N. H. Gehani H. V. Jagadish 0. Shmueli AT T Bell Laboratories Murray Hill New Jersey 07974 ABSTRACT Active database systems require facilities to specify triggers that fire when specified events occur. We propose a language for specifying composite events as event expressions formed using event operators and events primitive or composite . An event expression maps an event history to another event history that contains only the events at which the event expression is satisfied and at which the ữigger should fire. We present several examples illustrating how quite complex event specifications are possible using event expressions. In addition to the basic event operators we also provide facilities that make it easier to specify composite events. Pipes allow users to isolate sub-histories of interest. Correlation variables allow users to ensure that different parts of an event expression are satisfied by the same event thereby facilitating the coordmation of sub-events within a composite event We show how to efficiently implement event expressions using finite automata. Each event causes an automaton to change state. When an automaton reaches an accepting state a composite event of interest is recognized and the corresponding trigger fired. Events have attributes. For primitive events these could be parameters of the activity that caused the event selected parts of the database state or functions computed therefrom. For composite events attributes are derived from the attributes of the constituent primitive events. These attributes can be used in checking conditions and in any actions triggered. Event expressions can specify values or sets or ranges of values for particular attributes and can even require that some attributes be equal. The composite event specified by the expression does not occur unless the specified condition on attributes is satisfied. Permission to copy without fee all or pan .