caption: The Algorithm Garbage Collection - must ensure the safety still holds after discarding messages from log - generates checkpoint (a snapshot of the state) every once in a while - when a replica generates a checkpoint, it multicasts checkpoint message with seq number and digest of state; if a replica receives 2f+1 matching checkpoint messages, the checkpoint becomes stable and any messages associated with seq numbers less than that of the checkpoint are discarded View Changes - provides liveness - triggered by timeout to prevent backups from waiting forever - timer starts when backup receives a valid request; it stops when the replica is no longer waiting to execute the request - with commit phase invariant, view change guarantees total ordering of requests across views (by exchanging checkpoint information across views)
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