title: Send a data from server to the specific android device
body: <p>I'm new to Android so I'm looking for the best approach to send <code>text/data</code> from a <code>server</code> (webservice) to one specific <code>android device</code> at a time (each time to another user).
  The steps are:</p>
  
  <ol>
  <li>Webservice receives a json from the application, a logged in user
  ones in a time (pretty simple);</li>
  <li>The Server processes received data and sends it to the one specific user\device. Each time the server receives the data it
  follows the message to another user. </li>
  <li>Currently the user is an android app owner. The next step - the user can use another platform app and I register on the Server what
  kind of device sent/should receive the data.</li>
  <li>If user was offline the message should be removed form the waiting list and has to be sent to another user that is online.</li>
  </ol>
  
  <p>The main problem for me is to define how do I send a data to the specific user - by <code>ID\username</code>, device ID, other?
  I tried to practice GCM but it seems not exactly what I'm looking for because I'm planning to use the same server to send data not only to android devices but to iphone/windows as well. 
  Thank you.</p>