/**
  * Start the checker thread with the given masterId. If the thread is already running, kill it
  * first and then start anew. Returns immediately.
  */
 public void beginChecking(MasterId masterId) {
   stopChecking();
   if (masterId.getMasterUri() == null) {
     failureCallback.handleFailure("empty concert URI");
     return;
   }
   URI uri;
   try {
     uri = new URI(masterId.getMasterUri());
   } catch (URISyntaxException e) {
     failureCallback.handleFailure("invalid concert URI");
     return;
   }
   checkerThread = new CheckerThread(masterId, uri);
   checkerThread.start();
 }