@Override
  public String group(byte[] identifier, byte[] namespace) {
    String namespaceString;
    if (namespace == null || namespace.length == 0) {
      namespaceString = NamespaceDescriptor.DEFAULT_NAMESPACE_NAME_STR;
    } else {
      namespaceString = Bytes.toString(namespace);
    }
    return providerId + GROUP_NAME_DELIMITER + namespaceString;
  }
A WAL grouping strategy based on namespace. Notice: the wal-group mapping might change if we
support dynamic namespace updating later, and special attention needed if we support feature like
group-based replication.
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/NamespaceGroupingStrategy.java