Small script to check the actives hosts on the network and, according to the results, start/stop the torrents. NB: The high priority torrents remains untouched.

transmission.js

// Small script to check the actives hosts on the network and, according to the results, start/stop the torrents.
// NB: The high priority torrents remains untouched.
//
// cron task to exec it every 5min: */5    *       *       *       *       root    node /volume1/web/tasks/transmission/transmission.js > /dev/null

var Transmission = require('transmission'); // node-transmission - https://github.com/FLYBYME/node-transmission
var exec = require('child_process').exec;

// Log in to transmission
var transmission = new Transmission({
	username: 'username',
	password: 'password'
});

var
	hosts = ['192.168.0.10'], // List of the hosts to check
	hostsL = hosts.length,
	hostsI = 0,
	hostActive = false,
	torrents = []
;

hosts.forEach(function(host) {
	exec('ping -w 1 ' + host, function (error, stdout, stderr) {
		if (!error) hostActive = true;
		if (++hostsI !== hostsL) return;

		// List all torrents
		transmission.get(function(err, arg) {
			if(err) return console.error(err);

			// Find the normal/low prority torrents
			arg.torrents.forEach(function(torrent) {
				if (
					torrent.bandwidthPriority !== 1 && (    // Add only the low/normal priority torrents
						hostActive && torrent.status > 0 || // Add only the started torrents if one host is active or
						!hostActive && torrent.status === 0 // Add only the stopped torrents if no hosts are active
					)
				) torrents.push(torrent.id);
			});

			// Process the queue
			transmission[hostActive ? 'stop' : 'start'](torrents);
			console.log((hostActive ? 'Stopping ' : 'Starting ') + torrents.length + ' torrent(s).');
		});

	});

});