public boolean downloadFromSource(FileReferenceDownload fileReferenceDownload, Spec source) {
        if (fileReferenceExists(fileReferenceDownload.fileReference(), downloadDirectory)) return false;

        fileReferenceDownloader.startDownloadFromSource(fileReferenceDownload, source);
        return true;
    }
Start a download from the specified source, don't wait for result
 @return true if download was started, false if file reference already exists
filedistribution/src/main/java/com/yahoo/vespa/filedistribution/FileDownloader.java