public void add(ApplicationInfo applicationInfo) {
        ApplicationId id = applicationInfo.getApplicationId();
        ApplicationInfo oldApplicationInfo = applicationsById.put(id, applicationInfo);

        final String logPrefix;
        if (oldApplicationInfo == null) {
            logPrefix = isComplete ? "New application " : "Bootstrapped application ";
        } else {
            logPrefix = isComplete ? "Reactivated application " : "Rebootstrapped application ";
        }
        logger.log(Level.FINE, () -> logPrefix + id.toFullString());

        updateHostnameVsIdMaps(applicationInfo, id);

        listeners.forEach(listener -> listener.applicationActivated(applicationInfo));
    }
Note: Returns an empty set for unknown application.
service-monitor/src/main/java/com/yahoo/vespa/service/duper/DuperModel.java