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Implementation
provisional simulation end. In this case no future rollback could occur that can undo the
provisional simulation end, which is therefore guaranteed. If the GVT calculation
confirms a simulation end then no GVT is send back to the LPs but instead the
Simulation Controller calls the method endOfSimulationByTransaction() of all LPs as
shown in Figure 15 of section 5.2.3.
5.3.3 State Saving and Rollbacks
Optimistic synchronisation algorithms execute all local events without guarantee that
additional events received later will not violate the causal order of events already
execute. In order to correct such causal violations they have to provide means to restore
a past state before the causal violation occurred so that the new event can be inserted
into the event chain and the events be executed again in the correct order. A common
technique to allow the restoration of past states is called State Saving or State
Checkpointing where an LP saves the state of the simulation into a list of simulation
states each time the state changes or in defined intervals.
The parallel simulator implemented employs a relatively simple state saving scheme.
Each time the simulation time is incremented the LP serialises the state of the
Simulation Engine and saves it together with the corresponding simulation time into a
state list. Each state record therefore describes the simulation state at the beginning of
that time, i.e. before any Transactions were moved. To keep the complexity of this
solution low the standard object serialisation functionality provided by Java is used to
serialise and deserialise the state to and from a Stream object that is then stored in the
state list. The state list keeps all states sorted by their time. The saving of the state is
implemented in the method saveCurrentState() of the LogicalProcess class.
The purpose of saving the simulation state is to allow LPs to rollback to past simulation
states if required. The functionality to rollback to a past simulation state is implemented
in the method rollbackState(). Using the example shown in Figure 22 the principle of
rolling back to a past simulation state is briefly explained.