title: Multiple threads RxJava with .subscribe vs .concatmap
body: <p>I'm attempting to send an array of requests to an endpoint with <code>Observable.fromIterable(array).subscribe()</code> with the intent for each item to hit the backend in sequence. </p>
  
  <p>Currently, the Observable is not waiting for the async task of posting to the endpoint to complete before firing off the next request. I have two potential ideas but not sure how to implement either.</p>
  
  <ol>
  <li><p>Move <code>.subscribe().onNext()</code> to the callback to ensure each element is called after the async task is completed. </p></li>
  <li><p>Use <code>.concatmap</code> instead. I've seen this posted as a recommended method for people with similar actions as myself, some array of items with async tasks, but I'm not sure why it would work or how it works differently than what I have above.</p></li>
  </ol>