public String execute() {
        final StringBuilder resultBuf = new StringBuilder();
        scoreBoosterList.forEach(b -> {
            try {
                final long count = b.process();
                resultBuf.append(b.getClass().getSimpleName()).append(" : ").append(count).append('\n');
            } catch (final Exception e) {
                logger.warn("Failed to update scores.", e);
                resultBuf.append(e.getMessage()).append('\n');