digraph G {

  user[label="Resource\nOwner"];
  exchange[label="Client"];
  messenger[label="Messaging service"];
  challenger [label="Challenger HTTPD"];
  helper [label="Helper script"];
  DBMS [label="Postgres\nDBMS"];
  subgraph cluster_0 {
    challenger;
    helper;
    DBMS;
    label="Challenger OAuth 2.0 service";
  }
  user->exchange[label="0. request that\nrequires\nvalidated\naddress"];
  exchange->challenger[label="1. authorize\naddress\nvalidation"];
  user->challenger[label="2. Claimed\naddress"];
  challenger->DBMS;
  DBMS->challenger;
  challenger->helper[label="3. TAN code\n+ address"];
  helper->messenger[label="4. TAN code\n+ address"];
  messenger->user[label="5. TAN code"];
  user->challenger[label="6. TAN\ncode"];
  user->exchange[label="7. Token"];
  exchange->challenger[label="8. Get\nvalidated\naddress"];
}