title: Live resources in Akka Stream flow description
body: <p>There is this note in the <a href="http://doc.akka.io/docs/akka-stream-and-http-experimental/1.0-RC3/stream-design.html#What_shall_users_of_streaming_libraries_expect_">akka-stream docs</a> stating as follows:</p>
  
  <blockquote>
    <p>… a reusable flow description cannot be bound to “live” resources, any connection to or allocation of such resources must be deferred until materialization time. Examples of “live” resources are already existing TCP connections, a multicast Publisher, etc.; … </p>
  </blockquote>
  
  <p>I have several questions concerning the note:</p>
  
  <ul>
  <li>Apart from the these two examples, what other resource counts as a live? 
  
  <ul>
  <li>Anything that cannot be safely (deep)copied? Like a <code>Thread</code>? </li>
  <li>Should I also avoid sharing anything that's not thread-safe? </li>
  </ul></li>
  <li>What about an <code>ActorRef</code> existing in the <code>ActorSystem</code> used by the <code>ActorFlowMaterializer</code>? </li>
  <li>How to defer allocation until materialization time? Is it safe for example to allocate it in the constructor of a <code>PushPullStage</code> but not in the create function of a <code>FlowGraph</code>?</li>
  </ul>