private void addFailedSubscribed(Node node, NotifyListener listener) {
        Set<NotifyListener> listeners = failedSubscribed.get(node);
        if (listeners == null) {
            failedSubscribed.putIfAbsent(node, new ConcurrentHashSet<NotifyListener>());
            listeners = failedSubscribed.get(node);
        }
        listeners.add(listener);
    }
@author Robert HG (254963746@qq.com) on 5/17/15.
lts-core/src/main/java/com/github/ltsopensource/core/registry/FailbackRegistry.java