@Override
  protected void rollbackState(final MasterProcedureEnv env, final CreateNamespaceState state)
    throws IOException {
    if (state == CreateNamespaceState.CREATE_NAMESPACE_PREPARE) {
      // nothing to rollback, pre-create is just state checks.
      // TODO: coprocessor rollback semantic is still undefined.
      releaseSyncLatch();
      return;
    }
    // The procedure doesn't have a rollback. The execution will succeed, at some point.
    throw new UnsupportedOperationException("unhandled state=" + state);
  }
The procedure to create a new namespace.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/procedure/CreateNamespaceProcedure.java