CONCURRENT BEHAVIOR - A CONSTRUCT TO SPECIFY THE EXTERNAL BEHAVIOR OFOBJECTS IN OBJECT DATABASES

We present a linguistic construct to define concurrency control for th e objects of an object database. This construct, called concurrent beh avior, allows to define a concurrency control specification for each o bject type in the database; in a sense, it can be seen as a type exten sion. The concurrent behavior is composed by two parts: the first one, called commutativity specification, is a set of conditional rules, by which the programmer specifies when two methods do not conflict each other. The second part, the constraint specification, is a set of guar ded regular expressions, called constraints, by which the programmer d efines the allowed sequences of method calls. At each time during an a ctual execution, a subset of constraints may be active so limiting the external behavior of the object. A constraint becomes active when its guard is verified, where a guard is composed of the occurrence of som e method call m along with the verification of a boolean expression on the object state and the actual parameters of m. A constraint dies wh en a string of the language corresponding to the regular expression ha s been recognized. While the commutativity specification is devoted to specify the way in which the external behavior of an object is influe nced by the existence of concurrent transactions in the system, the co nstraint specification defines the behavior of the object, independent ly from the transactions. Since the two parts of the concurrent behavi or are syntactically distinct and, moreover, each of them consists of a set of independent rules, modularity in specifying the objects is en hanced, with respect to a unique specification. We outline an implemen tation of the construct, which is based on a look-ahead policy: at eac h method execution, we foresee the admissible successive behaviors of the object, instead of checking the admission of each request at the t ime it is actually made.