title: How to limit actors per user
body: <p>I am trying to understand how I could model this with akka.</p>
  
  <p>My system processes messages for users, and the messages have to be processed serially for each user i.e. I cannot have 2+ actors processing a message at the same time for the same user.</p>
  
  <p>So if I get 100 messages, and they are all for different users, I can spawn as many actors as I need to handle them.</p>
  
  <p>But if I get 100 messages and they are for only 10 users, I have to ensure there is only 1 actor processing a given user at the same time.</p>
  
  <p>How could I model this in Akka?  How could I filter messages or manage that only 1 actor per user?</p>
  
  <p>note:  Each message will have a UserId with it.</p>