private Optional<ApplicationVersions> activeApplicationVersions(long sessionId) {
        try {
            return Optional.ofNullable(getRemoteSession(sessionId)).map(this::ensureApplicationLoaded);
        } catch (IllegalArgumentException e) {
            // Do nothing if we have no currently active session
            return Optional.empty();
        }
    }
A session for which we don't have a watcher, i.e. hitherto unknown to us.

@param sessionId session id for the new session
configserver/src/main/java/com/yahoo/vespa/config/server/session/SessionRepository.java