private void ensureSessionPathDoesNotExist(long sessionId) {
        Path sessionPath = getSessionPath(sessionId);
        if (curator.exists(sessionPath)) {
            throw new IllegalArgumentException("Path " + sessionPath.getAbsolute() + " already exists in ZooKeeper");
        }
    }
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