The signing of a contract — a tree-structured application modelled with petri net building blocks

Abstract

The desire to replace conventional forms of cooperation within organizational structures by computer services raises the necessity to model cooperation and to describe it formally.

The complexity inherent to cooperating systems calls for a carefully structured design facilitating their analysis as well as their implementation and test.

In this paper we describe an example from the area of human cooperation — namely the negotiation phase of a car purchase financed by a bank loan. In doing so we exemplify fundamental ideas of modelling and specifying cooperating systems.

In a first informal analysis, the essential cooperation functions are identified. In subsequent steps, these functions are assigned to elements of a model structure which is generally applicable to tree-structured applications. Using a formally well founded building block approach, we derive from the problem structure thus obtained a highly modular net structure. The building blocks as well as their interrelations are specified by means of product nets, a class of formally defined high level Petri nets.

The model structure presented in this paper is closely related to the structure being proposed in ISO's Commitment, Concurrency and Recovery (CCR) Service, which assures atomic global transactions by correlating a set of bilateral cooperations.