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When any Bookie goes down in the BookKeeper cluster, there is no way to recover the lost data from that Bookie server. For example, if we have 2 replicas for a ledger in BK cluster, and a node goes down from it, we will be running the cluster with single replica. Running clusters with single or no replicas will be a risk, as nodes may fail in general. To avoid such situations, we need a mechanism for recovering the data to new
bookies for meeting the enough replica criteria (quorum size) and it is called as Auto-Recovery in BookKeeper.

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