What to do with unused node?

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The ros::Publisher class provides a method that you could use:

ros::Rate poll_rate(100);
while(chatter_pub.getNumSubscribers() == 0)
    poll_rate.sleep();

This block of code will wait until there's a subscriber connected to continue. I'm not sure if this is what you need, but it is one option.

I'm not sure how your system of nodes is configured, but another option you could use would be to provide a "start" and "stop" service that you could call to tell the node when to compute. This is much easier than using the parameter server to do so.