Regeneration

Regeneration [Pu 86] is a similar replication scheme to Available Copies in that a client only requires one replica to service a read request but a write must be performed by all copies. When an update occurs, if fewer than the required number of replicas are available then additional copies are regenerated on other operating nodes. In doing so the system must check that there is sufficient space available on the target node for the new replica (in terms of the volatile and stable storage that it may use) and also that a copy does not already reside there. A write failure occurs if it is not possible to update the correct number of replicas, and a read failure occurs if no replica is available.

A recovering node and replica cannot simply rejoin the system. For each replicated resource the system must check to see whether the maximum number of replicas already exist. If so the recovering replica is deleted. If the maximum number does not exist the system must check one of the available replicas to determine whether the state of the recovering replica is consistent. If the recovering replica is inconsistent (i.e. an update has occurred since this replica failed) then it must be deleted because a new copy has already been created to take its place but is currently unavailable.