public void close() {
        shutDown.set(true);
        downloadExecutor.shutdown();
        try {
            if (!downloadExecutor.awaitTermination(30, TimeUnit.SECONDS))
                log.log(Level.WARNING, "FileReferenceDownloader failed to shutdown within 30 seconds");
        } catch (InterruptedException e) {
            Thread.interrupted(); // Ignore and continue shutdown.
        }
    }
Downloads file reference from config server and keeps track of files being downloaded

@author hmusum
filedistribution/src/main/java/com/yahoo/vespa/filedistribution/FileReferenceDownloader.java