public OfflineMemberDetailsImpl fetchOfflineMembers() {
    ProxyBucketRegion[] proxyBuckets = partitionedRegion.getRegionAdvisor().getProxyBucketArray();
    Set<PersistentMemberID>[] offlineMembers = new Set[proxyBuckets.length];
    for (int i = 0; i < proxyBuckets.length; i++) {
      ProxyBucketRegion proxyBucket = proxyBuckets[i];
      if (partitionedRegion.getDataPolicy().withPersistence()) {
        Set<PersistentMemberID> persistedMembers =
            proxyBucket.getPersistenceAdvisor().getMissingMembers();
        if (persistedMembers == null) {
          persistedMembers = Collections.emptySet();
        }
        offlineMembers[i] = persistedMembers;
      } else {
        offlineMembers[i] = Collections.emptySet();
      }
    }
    return new OfflineMemberDetailsImpl(offlineMembers);
  }
Retrieve the set of members which are currently offline for all buckets.
geode-core/src/main/java/org/apache/geode/internal/cache/PRHARedundancyProvider.java