Lua code for the ESP8266 to subscribe to a topic and output everything published to the topic. There is a bug where the ESP8266 will go offline after a few minutes if a lot of messages are being published to the topic.

ESP8266_subscribe_test.txt

-- Configuration to connect to the MQTT broker.
BROKER = "...com"        -- Ip/hostname of MQTT broker
BRPORT = 12054           -- MQTT broker port
BRUSER = "..."           -- If MQTT authenitcation is used then define the user
BRPWD  = "..."           -- The above user password
CLIENTID = "ESP8266-" ..  node.chipid() -- The MQTT ID

-- MQTT topics to subscribe
topic = "/maxFrequency" -- Add/remove topics to the array

-- connect to the broker
function mqtt_connect()
    print "Connecting to MQTT broker. Please wait..."
    m = mqtt.Client( CLIENTID, 120, BRUSER, BRPWD)
    m:connect( BROKER , BRPORT, 0, function(conn)
         print("Connected to MQTT:" .. BROKER .. ":" .. BRPORT .." as " .. CLIENTID )
         mqtt_sub() --run the subscription function
    end)
end

mqtt_connect()

function mqtt_sub()
    m:subscribe(topic, 0, function(conn)
        print("Subscribing topic: " .. topic)
    end)
    run_main_prog()
end

--main program to run after the subscriptions are done
function run_main_prog()
     print("Main program")
     -- Callback to receive the subscribed topic messages. 
     m:on("message", function(conn, topic, data)
        print(topic .. ":" )
        if (data ~= nil ) then
          print ( data )
        end
      end )

      m:on("offline", function(con) 
        print ("offline")
        print ("reconnecting...") 
        print(node.heap())
        tmr.alarm(1, 10000, 0, function()
           mqtt_connect()
        end)
      end)
end