Caution

This page has been updated a long time ago. Information found here could be outdated and may lead to missconfiguration.
Some of the links and references may be broken or lead to non existing pages.
Please use this docs carefully. Most of the information here now is only for reference or example!

Reliable MQTT connection with NodeMCU (part 2)¶

Thanks to "Modestas" post on my previous article now I will show you another solution to this topic, which is much easier to understand and simpler.

First we will create a config file like this:

--[[
File Name: config.lua
]]
local module = {}
    module.NodeID=node.chipid()

    -- mqtt Related Config    
    module.mqttHost="172.16.0.***"
    module.mqttPort="1883"
    module.mqttUserName="**************"
    module.mqttpassword="**************"
    module.mqttLwtTopic="NodeMCU/lwt/"..module.NodeID

    module.mqttSubscibeTopics={["NodeMCU/"..module.NodeID.."/command"]=0,["NodeMCU/"..module.NodeID.."/relayCh/+"]=0}
    module.mqttPublishTopicStatus="NodeMCU/"..module.NodeID.."/status/"
    module.mqttUpdateStatusInterval=60000
    module.mqttUpdateStatusTimerId=2
return module

You can use table object to store these parameter/value pairs instead of this module. Maybe later I will write a post about 'how to do that'.

Create MQTT client and set LWT
```
m=mqtt.Client(config.NodeID, 10, config.mqttUserName, config.mqttpassword)
m:lwt(config.mqttLwtTopic, "Inactive", 0,1)
```
And do not forget to set isMqttAlive value to false. We will use this variable to determine if mqtt connection is alive or not. The initial value must be false, because at the first run the ESP isn't connected to the broker.
connectToMqtt() Function

function connectToMqtt()
    m:connect(config.mqttHost, config.mqttPort, 0, 0, function(client) 
        isMqttAlive = true 
        print("Successfully Conencted to MQTT broker: "..config.mqttHost.." on port: "..config.mqttPort)

-- Here you can do some useful things. Example subscribe to a topic, or set up what should happen if a message is received. (`mqtt.client:on()`)

    end,
    function(con,reason)
        print("Faild to connect to MQTT broker: "..config.mqttHost.." on port: "..config.mqttPort..", Reason: "..reason)
    isMqttAlive = false
    end)
end

This function will be used to connect to the MQTT broker, and the isMqttAlive variable is also will be set here: -If the ESP is successfully connected to the broker, isMqttAlive will be true.
-If something goes wrong, this variable will be false.

Set up mqtt.client:on("offline"....)

m:on("offline", function(con) 
    isMqttAlive = false 
    print("Disconnected from MQTT")
end)

If the ESP disconnects from the broker this will set isMqttAlive to false.

Final steps

After you call connectToMqtt() function you have to check the value of isMqttAlive before each message publication.

In my case I use a DHT22 sensor to monitor temperature and humidity, and I send update every 10 minutes. To do this a timer should be used. Example:

tmr.alarm(config.mqttUpdateStatusTimerId,config.mqttUpdateStatusInterval, tmr.ALARM_AUTO, function()
if isMqttAlive == false
    then
        print("Reconnect To MQTT:"..config.mqttHost.." on port: "..config.mqttPort)
        connectToMqtt()
    else
        print("MQTT is OK: "..config.mqttHost.." on port: "..config.mqttPort)
        mqttUpdateGeneralStatus(m)
    end
end)

As you can see, first, I check if isMqttAlive is ture or false. - If it is false I call connectToMqtt() function to (re)connect to the broker.
- If it is true the mqttUpdateGeneralStatus(m) function will be called, which queries the sensor and send the actual temperature and humidity values to the broker.

A Complete Example

local config=require("config")
local setup=require("setup")
local mqttSP=require("mqttSP")
m=mqtt.Client(config.NodeID, 10, config.mqttUserName, config.mqttpassword)
m:lwt(config.mqttLwtTopic, "Inactive", 0,1)


isMqttAlive=false

-- Conenct Function
function connectToMqtt()
    m:connect(config.mqttHost, config.mqttPort, 0, 0, function(client) 
        isMqttAlive = true 
        print("Successfully Conencted to MQTT broker: "..config.mqttHost.." on port: "..config.mqttPort)
        mqttSP.mqttSubsribe(m,config.mqttSubscibeTopics)
        mqttSP.mqttOnMessage(m)
        m:publish(config.mqttLwtTopic,"Active",0,1)
        m:on("offline", function(con) 
           isMqttAlive = false 
           print("Disconnected from MQTT")
        end)
    end,
    function(con,reason)
        print("Faild to connect to MQTT broker: "..config.mqttHost.." on port: "..config.mqttPort..", Reason: "..reason)
    end)
end

-- Connect To Broker
connectToMqtt()

-- Start Timer
tmr.alarm(config.mqttUpdateStatusTimerId,config.mqttUpdateStatusInterval, tmr.ALARM_AUTO, function()
if isMqttAlive == false
    then
        print("Reconnect To MQTT:"..config.mqttHost.." on port: "..config.mqttPort)
        connectToMqtt()
    else
        print("MQTT is OK: "..config.mqttHost.." on port: "..config.mqttPort)
        mqttSP.mqttUpdateGeneralStatus(m,setup.getPublicIp())
    end
end)

REFERENCES:

https://nodemcu.readthedocs.io/en/master/en/modules/mqtt/