@Override
  protected List<WAL> getWALs0() {
    return cached.values().stream().flatMap(p -> p.getWALs().stream()).collect(Collectors.toList());
  }
A group-provider mapping, make sure one-one rather than many-one mapping
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/RegionGroupingProvider.java