  In this paper we analyse the requirements of performing parallel
transaction-oriented simulations within loosely coupled systems like ad hoc
grids. We focus especially on the space-parallel approach to parallel
simulation and on discrete event synchronisation algorithms that are suitable
for transaction-oriented simulation and the target environment of ad hoc grids.
To demonstrate our findings, a Java-based parallel simulator for the
transaction-oriented language GPSS/H is implemented on the basis of the most
promising shock-resistant Time Warp (SRTW) synchronisation algorithm and using
the grid framework ProActive. The analysis of our parallel simulator, based on
experiments using the Grid5000 platform, shows that the SRTW algorithm can
successfully reduce the number of rolled back transaction moves but it also
reveals circumstances in which the SRTW algorithm can be outperformed by the
normal Time Warp algorithm. Finally, possible improvements to the SRTW
algorithm are proposed in order to avoid such problems.
