title: Rolling restart with ansible handlers
body: <p>I want to run an ansible playbook that installs a service and restarts it if anything has changed since the last run (more or less the canonical use-case for ansible handlers).</p>
  
  <p>But I want a <em>different parallelism</em> for installing than for restarting: I want to install on all the hosts at a time but, if the "service-restart" handler gets invoked I want that to run on X hosts at a time.</p>
  
  <p>I know this is possible with different plays that have different <code>serial</code> values. But I can't see how I could make use of handlers if I go this route. And I can't afford to have a single playbook with a <code>serial</code> value like <code>2</code>, as most of the time nothing will change for that service.</p>
  
  <p>Can the handlers span multiple plays? Or is there any other way to do this without hacks?</p>