The concurrency is an important issue to tackle when modelling real-time applications which are intrinsically concurrent. According to the definitions of real-time given in (CNRS 1988) and (Stankovic 88), real-time implies on one hand that the response are waited at a precise moment (neither before nor after) and on other hand, that such systems are coupled with the real world, therefore highly concurrent. This document overviews the support for concurrency modelling provided by UML. It describes first the main concept of active object and second a (non exhaustive) list of issues pertaining to concurrency mechanism interactions. The integration of concurrency issues within object-oriented environments generated a large amount of research.

In the case of object-oriented languages, most concurrency issues are related to the concept of active object (Atkinson 91), (Guerraoui 95) et (Tripathi, Oosten et al. 99). Concrete examples of concurrent object oriented languages are Act++ , Hybrid (Nierstratz 1987), ABCL (Yonezawa, Shibayama et al. 87), Argus (Liskov 1988), PRAL RT (Fouquier and Terrier 95), RTGOL (Sourrouille and Lecoeuche 95), TOM , etc.

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