code: public synchronized void start() {
      Preconditions.checkState(mProcess == null, "Master is already running");
      LOG.info("Starting master with port {}", mProperties.get(PropertyKey.MASTER_RPC_PORT));
      mProcess = new ExternalProcess(mProperties, LimitedLifeMasterProcess.class,
          new File(mLogsDir, "master.out"));
      try {
        mProcess.start();
      } catch (IOException e) {
        throw new RuntimeException(e);
      }
    }
docs: Launches the master process.