Time-Based Constraints in the Object Constraint Language

The Object Constraint Language (OCL) is a specification language which could be used for describing constraints on object-oriented models and other modelling artifacts. OCL is a part of the Unified Modelling Language (UML) which is the de facto standard for objectoriented analysis and design. OCL is designed to be used by software engineers and modellers and requires only modest mathematical training. This is achieved by keeping it simple and by employing a textual syntax rather than mathematical symbols. The kind of constraints which can be described using OCL include invariants on classes, preconditions and postconditions of operations. OCL uses @pre to refer to the value of a property immediately before the execution of an operation. However, OCL does not support the description of time-based constraints which say how values can change between earlier and later states. This paper describes an approach for incorporating time-based constraints within OCL in such a way so as not to compromise its simplicity. This approach is essentially based on using @pre in invariants as well as in postconditions of operations. To distinguish between invariants and time-based constraints we introduce the stereotype <>. We also introduce an operator eventually for expressing liveness constraints, and an operator initially for describing initial constraints. We illustrate the approach by describing constraints such as constant attributes of an object, constant associations, and values increasing or decreasing over time.