* the event source has already been closed or canceled.
   */
  fun cancel()

  fun interface Factory {
    /**
     * Creates a new event source and immediately returns it. Creating an event source initiates an
     * asynchronous process to connect the socket. Once that succeeds or fails, `listener` will be
     * notified. The caller must cancel the returned event source when it is no longer in use.
     */
    fun newEventSource(

    /** A set of names of threads that are currently waiting. */
    protected Set<String> waitingThreadNames = Collections.synchronizedSet(new HashSet<>());

    /**
     * Initializes the SystemHelper.
     * This method sets up system properties, caches, and other initial configurations.
     */
    @PostConstruct
    public void init() {
        if (logger.isDebugEnabled()) {

        } catch (final UnknownHostException e) {
            throw new FessSystemException("Failed to resolve the hostname: " + host, e);
        }
    }

    /**
     * Initializes the search engine client and configures indices.
     * Called automatically after dependency injection is complete.
     */
    @PostConstruct
    public void open() {
        if (logger.isDebugEnabled()) {