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To solve this problem it is necessary to compute a set of replicas to be removed from the assigned replicas when a reassignment changes.

The set set drop of replicas to be removed will be computed as follows:

1. At the point a new reassignment is created we compute the value top = len(original_assignment) and store this in ZK. We also

   1. We check top >= min.insync.replicas, logging a warning and ignore the reassignment if the check fails.

   2. Otherwise we store this in the /admin/reassignments/$topic/$partition znode.
2. Then, initially, and each time the reassignment changes, (i.e. in KafkaController.onPartitionReassignment()):
   1. Let assigned be the currently assigned replicas, ie. assigned = ControllerContext.partitionRepliacAssignment(tp)
   2. Define a sorted list of out-of-sync replicas: osr = assigned -- isr
   3. Create a list of current replicas existing = List(leader) ++ isr ++ osr

   4. Remove duplicates from existing (keeping the first occurrence in the list)


   5. Then, the set of replicas which can be dropped is: drop = assigned -- existing[0:top-1]

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