@Override
  public Iterable<Entry<RegionInfo, Long>> filterBySubject(String namespace) {
    rlock.lock();
    try {
      return regionUsage.entrySet().stream()
        .filter(entry -> namespace.equals(entry.getKey().getTable().getNamespaceAsString()))
        .collect(Collectors.toList());
    } finally {
      rlock.unlock();
    }
  }
Fetches the namespace quota. Visible for mocking/testing.
hbase-server/src/main/java/org/apache/hadoop/hbase/quotas/NamespaceQuotaSnapshotStore.java