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Exercise 5 - Question 6

Verfasst: 7. Jun 2016 23:25
von Omar Erminy
During the exercise session corresponding exercise 5 there was no agreement on point C of question 6.

The question was regarding if a domino effect will also be triggered if there happen to be a checkpoint after P2 received message Mi. (refer to slide 22 of the lecture on checkpointing). Will there be a domino effect or not?

I would appreciate if this could be cleared out.



Re: Exercise 5 - Question 6

Verfasst: 10. Jun 2016 11:53
von sumandb

If there is a checkpoint at P2 after receiving the message mi (i.e. the receive event of message mi is check pointed at P2 say at Cb') and there is a crash at P1 after sending the message, there will be still domino effect as the send event is not logged at P1 leading to an orphan message at P2 (which will be logged at the checkpoint Cb'). This will cause P2 to roll back to Cb as P1 has rolled back to Ca and P3 will roll back to Cc. Now, P1 has an orphan message from P3 and hence P1 will roll back to it's previous checkpoint which in turn will lead to all processes rolling back to the first checkpoint eventually.

The domino effect would be avoided if there was a checkpoint immediately after send event at P1 i.e Ca' and after receive event at P2 i.e Cb'. If now the process P1 crashes (i.e. after the checkpoint which has the send event logged) , then P1 rolls back to Ca' and P2 rolls back to Cb' and all the processes reach a consistent state. (Note: No change at P3 and mj message send and receive event are logged).

Hope this makes it clear.