mqtt_client_switch.lua

-- based upon work by rutierut and MikeV
-- control gpio pin from mq

wifiname = "MYWIFINET"
wifipass = "MYWIFIPASS"

broker = "BROKERIP"  -- IP or hostname of MQTT broker
mqttport = 1883          -- MQTT port (default 1883)
mqtttopic = "switch1"
userID = ""              -- username for authentication if required
userPWD  = ""            -- user password if needed for security
clientID = "ESP1"        -- Device ID
pin = 8                  -- GPIO15 = 8

gpio.mode(pin, gpio.OUTPUT)
connected = 0            -- mq connected
ready = 0                -- wifi ready
-- Wifi credentials
wifi.setmode(wifi.STATION)
wifi.sta.config(wifiname, wifipass)
wifi.sta.connect()
mq = mqtt.Client(clientID, 15)

function mqtt_start()
    if ready == 1 and connected ~= 1 then
      mq:connect( broker , mqttport, 0, function(conn)
        print("Connected to MQTT: " .. broker .. ":" .. mqttport .." as " .. clientID )
        connected = 1
      end)
    end
end

function mqtt_action(conn, topic, input)
        print(input)
         if input == "ON" then
           gpio.write(pin, gpio.HIGH)
         elseif input == "OFF" then
           gpio.write(pin, gpio.LOW)
         else
           print('ignored message')
         end
end

function mqtt_sub()
    if connected == 1 then
      mq:publish(mqtttopic, clientID .. " connected", 0, 0,
        function(conn) print("sent connected msg") end)
      mq:subscribe(mqtttopic, 0, function(conn)
            print('Subscribed to topic: ' .. mqtttopic)
        end) 
      print("setting up message callback")   
      mq:on("message", mqtt_action)
      tmr.stop(2)      
    end
end

function wifi_watch ()
    status = wifi.sta.status()
    if status == 5 and connected == 0 then
        print("connected to wifi.")
        ready = 1
    elseif status < 5 then
        ready = 0
        if status == 1 then
            if connected == 0 then
                print('connecting to wifi...')
            else 
                print("reconnecting to wifi...")
                wifi.sta.connect()
            end
        end
    end
end

function mqtt_heartbeat()
    if connected == 1 then
      mq:publish(mqtttopic, clientID .. " connected", 0, 0)
    end
end
tmr.alarm(0, 1000, 1, wifi_watch)
tmr.alarm(1, 1111, 1, mqtt_start)
tmr.alarm(2, 1222, 1, mqtt_sub)

That's a really nice skeleton, thanks. IMO it's only missing something like the following:

mq:on("offline", function(connection)
  print("MQTT: offline")
  mqttConnected = 0
end)

That would allow you to work around the heartbeat.