void cancelDeferred(String tag) {
            lock.lock();
            try {
                EventContext eventContext = tagToEventContext.get(tag);
                if (eventContext != null) {
                    remove(eventContext);
                    size--;
                }
            } finally {
                lock.unlock();
            }
        }
A condition variable for waking up the event handler thread.
server-common/src/main/java/org/apache/kafka/queue/KafkaEventQueue.java