private synchronized JRTClientConfigRequest pollForNewConfig(long timeoutMillis) {
        JRTClientConfigRequest response = pollQueue(timeoutMillis);
        // There might be more than one response on the queue, so empty queue by polling with
        // 0 timeout until queue is empty (returned value is null)
        JRTClientConfigRequest temp = response;
        while (temp != null) {
            temp = pollQueue(0);
            if (temp != null)
                response = temp;
        }

        return response;
    }
A queue containing responses (requests that got a callback from JRT) that has not yet been handled.
config/src/main/java/com/yahoo/config/subscription/impl/JRTConfigSubscription.java