@Override
  public void removeCriticalMember(DistributedMember member) {
    if (logger.isDebugEnabled()) {
      logger.debug("DR: removing member {} from critical member list", member);
    }
    synchronized (memoryThresholdReachedMembers) {
      memoryThresholdReachedMembers.remove(member);
      if (memoryThresholdReachedMembers.isEmpty()) {
        setMemoryThresholdReached(false);
      }
    }
  }
@return a random persistent replicate, null if there is none
geode-core/src/main/java/org/apache/geode/internal/cache/DistributedRegion.java