last will and testament notify when one of them goes offline. for MQTT.

lwtnotfy.py

#!/usr/bin/python
#
# October 2013
# Subscribes to multiple topics (and will also block sub-topics, based on keywords) and if it see's the work
# 'offline' it sends a notification to the pushingbox topic. Mainly using this to track if a program goes offline
#
# by Matthew Bordignon / @bordignon
# 


import mosquitto
try:
    import json
except ImportError:
    import simplejson as json
#used for re-connecting broker and exiting
import signal
import sys
import socket
import time

#BEGIN SETTINGS 
broker = "mqtt.localdomain" #mqtt broker location
broker_port = 1883 #mqtt broker port
topics = [
        '/house/xbmc/+/status',
        '/lwt/#'
        ]

blocked_topics = [ 
        'lwt2notify', #this program no need to notify
        'tweet2mqtt', #the tweet2mqtt program as it runs as a cronjob
        'xvsdfi44-PC', #xbmc on my lenovo laptop
        'projector' #xbmc on my projector
        ]
willtopic = "/lwt/lwt2notify" #last will and testament (will_set) topic location
topic_notify = "/software/mqtt2pushingbox/message"
#END SETTINGS

mqttc = mosquitto.Mosquitto()
        
def cleanup(signum, frame):
    """
    Signal handler to disconnect and cleanup.
    """
    try:
        mqttc.publish(willtopic, "offline", retain=True)
        print("Disconnecting from broker")
        mqttc.disconnect()
    except:
        print("no broker?")
    print("Exiting on signal %d", signum)
    sys.exit(signum)

def on_connect(mosq, obj, rc):
    print("on-connect - rc: "+str(rc))
    for topic in topics: #subscibing to multiple topics
        print("Subscribing to %s", topic)
        mqttc.subscribe(topic, 0)
    mqttc.publish(willtopic, payload="online", qos=0, retain=True)
    print("--")

def on_message(mosq, obj, msg):
    print(msg.topic+" "+str(msg.qos)+" "+str(msg.payload))
    if msg.retain == 1:
        print("Skipping retained %s" % msg.topic)
        return

    if blocked_topics is not None:
        for t in blocked_topics:
            if t in msg.topic:
                print("Skipping blocked topic %s" % msg.topic)
                return
    if str(msg.payload) == 'offline':
        print('device is offline, sending notification')
        try:
            subject = 'Program offline atm'
            mess_text = msg.topic+" -- "+msg.payload

            payload = json.dumps(dict(
                                sub = subject.encode('utf-8'),
                                txt = mess_text.encode('utf-8')
                            ))
             
            print ""
            print payload
            print ""
            mqttc.publish(topic_notify, payload , retain=False) 
        except:
            raise
    
def on_publish(mosq, obj, mid):
    print("on-publish - mid: "+str(mid))

def on_subscribe(mosq, obj, mid, granted_qos):
    print("on-Subscribed: "+str(mid)+" "+str(granted_qos))

def on_log(mosq, obj, level, string):
    print(string)



def main():
    """
    The main loop in which we stay connected to the broker
    """
    #define the callbacks
    mqttc.on_message = on_message
    mqttc.on_connect = on_connect
    mqttc.on_publish = on_publish
    mqttc.on_subscribe = on_subscribe
    
    mqttc.will_set(willtopic, payload="offline", qos=0, retain=True)
    mqttc.reconnect_delay_set(delay=3, delay_max=30, exponential_backoff=True)
    
    try:
        mqttc.connect(broker, broker_port, 60)
    except Exception, e:
        print("MQTT connection failed: %s" % (str(e)))
        sys.exit(1)
    
    while True:
            try:
                mqttc.loop_forever()
            except socket.error:
                print("MQTT server disconnected; sleeping")
                time.sleep(5)
            except:
                raise
    
if __name__ == '__main__':
       
    signal.signal(signal.SIGINT, cleanup)
    signal.signal(signal.SIGTERM, cleanup)
    
    main()

Presumably this script could be replaced using mqttwarn? Except that you wouldn't be able to track when mqttwarn stopped of course...

@summerboy12 here you go, mqttwarn version. https://gist.github.com/matbor/9657897