The Coordination Development Environment

1 The Concept Coordination contracts [1,2] are a modelling primitive, based on methodologi-cal and mathematical principles [8,3], that facilitates the evolution of software systems. The use of coordination contracts encourages the separation of computation from coordination aspects, and the analysis of which are the " stable " and " unstable " entities of the system regarding evolution. Coordination contracts encapsulate the coordination aspects, i.e., the way components interact, and as such may capture the business rules [7] or the protocols [6] that govern interactions within the application and between the application and its environment. System evolution consists in adding and removing contracts (i.e., changing the business rules) between given components (the participants of the contract). As a result of an addition, the interactions specified by the contract are su-perposed on the functionalities provided by the participants without having to modify the computations that implement them. In fact, the components are completely unaware they are being coordinated. The contracts specify a set of rules, each with a triggering condition (e.g., a call to a method of one participant), and a rule body stating what to do in that case. Contracts are also unaware of the existence of other contracts. This facilitates enormously incremental system evolution, because explicit dependencies between the different parts of the system are kept to a minimum, and new contracts can be defined and added to the system at any time (even run-time), thus coping with changes that were not predicted at system design time. Consider the banking domain, in which ATX Software has several years of experience. Usually, there is an object class account with an attribute balance and a method withdrawal with parameter amount. In a typical implementation one can assign the guard balance≥amount restricting this method to occur in