private Set<Long> findNewSessionsInFileSystem() {
        File[] sessions = tenantFileSystemDirs.sessionsPath().listFiles(sessionApplicationsFilter);
        Set<Long> newSessions = new HashSet<>();
        if (sessions != null) {
            for (File session : sessions) {
                try {
                    Duration consideredNew = Duration.ofSeconds(Math.min(sessionLifeTimeInSeconds(), 300));
                    if (Files.getLastModifiedTime(session.toPath()).toInstant()
                             .isAfter(clock.instant().minus(consideredNew)))
                        newSessions.add(Long.parseLong(session.getName()));
                } catch (IOException e) {
                    log.log(Level.FINE, "Unable to find last modified time for " + session.toPath());
                }
            }
        }
        return newSessions;
    }
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