Optimistic Approach
Positive messages refer to messages that correspond to simulator events. The effects of a positive message is unrolled by sending an anti-message (or negative message) to the process concerned to enable all the effects of erroneous computation to be canceled. A process on receiving an anti-message that corresponds to a positive message that it has already processed, that process is rolled back to undo the effect of processing the soon-to-be destroyed positive message. This procedure is repeated recursively until the correct simulation state is reached.
This is known as aggressive cancellation — whenever a process rolls back
to time T, anti-messages are immediately sent for any previously sent
positive messages with a timestamp larger than T.