In PBFT view change what happens to committed operations after a valid snapshot

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PBFT says that if the timer of backup i expires in view v then it starts a view change for v+1 by multicasting <view-change, v+1, n, C, P, i> where n is the sequence number of the last stable checkpoint s and P is a set containing a set Pm for each request m that prepared at i with a sequence number higher than n. 

Now, the checkpoints are taken periodically so, there can be prepared messages at i with a sequence number higher than n which are already committed. We don't want these to be included in Pm as they are already committed. So, how does PBFT handles that?

Jul 27, 2018 in Blockchain by Shashank
• 10,400 points

1 answer to this question.

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I think that those messages are executed again. When a view-change happens, all the nodes would be in the same checkpoint. When the "new" primary for the view v + 1 receives 2⨍ valid view-change messages, multicasts a new-view message. 

In the message that is sent, it indicates: V: set of received and valid view-change messages. P: set of pre-prepared unprocessed messages. These messages are calculate as follows: From the last stable checkpoint, you get the sequence number of the last executed request. This value will correspond to the min-s. Take the largest sequence number of all prepared messages you have received. 

This value will correspond to the max-s. As a result, it generates as many pre-prepared messages as needed. Each node saves the log of the messages that are not inside a chekcpoint, so they don't need to be procesed again.

answered Jul 27, 2018 by aryya
• 7,450 points

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