# MQTT, messages from offline devices

**URL:** <https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036>\
**Category:** General\
**Created:** [26 February 2022 20:29 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036 "2022-02-26T20:29:46Z")\
**Posts on this page:** 20\
**Page:** 2

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**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [27 February 2022 21:52 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/21 "2022-02-27T21:52:28Z")

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Did you say you had checked in MQTT Explorer that there is nothing retained on the reply topic?

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**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [27 February 2022 22:06 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/22 "2022-02-27T22:06:30Z")

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Well, as the broker was power reset the other day, I can not say for sure.  
(Hey, I'm being honest)

I have since powered on the `Alarm_Clock` checked it's MQTT stuff. As said that _rogue message's_ `retain` flag was blank.

I set it to `false` and shall have to wait and see what happens now.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [28 February 2022 10:02 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/23 "2022-02-28T10:02:11Z")

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> [@Trying\_to\_learn](#):
>
> Well, as the broker was power reset the other day, I can not say for sure.

With default settings in Mosquitto powering off does not lose retained topics, so the question is valid:

Did you say you had checked in MQTT Explorer that there is nothing retained on the reply topic?

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [28 February 2022 23:18 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/24 "2022-02-28T23:18:40Z")

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As of now, I can't see the retained topic.

So all the retained messages are stored on the SD card?

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [1 March 2022 09:13 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/25 "2022-03-01T09:13:28Z")

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Provided you have mosquitto set to save retained data to the file system then it will save it on the file system (wherever it is configured to do that). The default mosquitto is to save it to the file system. Have a look in /etc/mosquitto/mosquitto.conf and you will probably find

```auto
persistence true
persistence_location /var/lib/mosquitto/

```

which I guess is fairly self explanatory.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [1 March 2022 20:24 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/26 "2022-03-01T20:24:17Z")

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Thanks.

However: since the power cycle I have stopped seeing the rogue messages.

Also to be considered I set the `retain` flag from being **BLANK** to `false`.

So I can't say definitively what is going on as since then all is ok.

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**Author:** ![krambriw](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/krambriw/32/5429_2.png) [@krambriw](https://discourse.nodered.org/u/krambriw)\
**Post date:** [2 March 2022 06:55 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/27 "2022-03-02T06:55:15Z")

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> [@Trying\_to\_learn](#):
>
> Also to be considered I set the `retain` flag from being **BLANK** to `false`

I guess you mixed up things there a bit, don't know why, but to my understanding the retain flag should never be BLANK, it is the the payload that should be BLANK if you wanna reset the retained value. The retain flag should be either false or true

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [2 March 2022 07:20 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/28 "2022-03-02T07:20:00Z")

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Yeah, fair enough.

I was going through all the `MQTT` nodes and saw that the node had it's `retain` flag blank.  
So I set it to `false`.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [2 March 2022 10:18 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/29 "2022-03-02T10:18:47Z")

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> [@krambriw](#):
>
> to my understanding the retain flag should never be BLANK

In fact when you add an MQTT Out node the retain field is blank, and in the help text it says it defaults to false, so presumably leaving it blank is ok.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [2 March 2022 10:40 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/30 "2022-03-02T10:40:07Z")

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Yeah, but from what I've seen, leaving it blank doesn't make if `false`.

I'll try to remember to go back and delete that setting and make it blank again and see if it happens again.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [2 March 2022 10:57 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/31 "2022-03-02T10:57:23Z")

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> [@Trying\_to\_learn](#):
>
> from what I've seen, leaving it blank doesn't make if `false`

I suspect that what you have seen was caused by something else, unless the message you sent overrode the retain setting (perhaps it was left over from a previous node).

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [2 March 2022 11:00 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/32 "2022-03-02T11:00:40Z")

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Drats....

After setting to `FALSE`, I can't change the setting back to being blank.

So I can't really test it. ☹

Maybe when I have time and I power up another machine I will try to remember to check the MQTT node settings.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [2 March 2022 11:01 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/33 "2022-03-02T11:01:32Z")

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> [@Colin](#):
>
> perhaps it was left over from a previous node

That is an interesting thought, I realise. An MQTT In node message has msg.retain set from the received topic. So if you feed the message from an In node (possibly via other nodes) to an MQTT Out node which has Retain blank then it will use msg.retain from the In node, which may not be false.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [2 March 2022 11:06 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/34 "2022-03-02T11:06:34Z")

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These are the _settings_ of which I was speaking.

 ![Screenshot from 2022-03-02 22-03-39](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/2/3/23ffce36ef0a98062da6031e222747170632cd9c.png)

When I opened it a while back they were blank.  
After changing them to `false` the messages have seemingly stopped.

But I don't want to be too quick in saying it has been solved just yet.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [2 March 2022 11:23 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/35 "2022-03-02T11:23:49Z")

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Good point, I had forgotten it is LWT messages you were referring to. I have just checked and with node-red 2.2.2 leaving Retain blank for LWT messages does default to not retained.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [2 March 2022 11:31 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/36 "2022-03-02T11:31:50Z")

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`Alarm_Clock` is only 2.1.6....

I have older versions and I may check them when I get time to power up those machines.

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<div class="post-metadata">

**Author:** ![PeteKnight](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/peteknight/32/18345_2.png) [@PeteKnight](https://discourse.nodered.org/u/PeteKnight)\
**Post date:** [3 March 2022 13:01 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/37 "2022-03-03T13:01:42Z")

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I think that your reference to birth and death certificates is a bit confusing. I guess you’re actually referring to the LWT functionality?

Think of LWT as being like the thing you see in the movies, where the good guy goes to his solicitor and says “If you don’t hear from me by this time next week, mail this dossier to the New York Times” 😀

The LWT instruction you issue from PubSubClient is exactly the same. It’s telling the broker that if it hasn’t heard from the client for a while then it should post message x to topic y.  
The amount of time before the broker regards the client as “missing in action, presumed dead” is determined by the timeout setting of the broker. Using Mosquitto, that seems to default to around 30 seconds.

The documentation and examples for LWT with PubSubClient are a bit vague and it took a bit of research to learn how to make the most of the functionality.

This is the MQTT connect command I use when a device connects/reconnects to the server…

```auto
if(MQTTclient.connect(mqtt_client_id.c_str(), mqtt_username, mqtt_password, (base_mqtt_topic + "/Status").c_str(),0, 1, "Dead"))
{
    MQTTclient.publish((base_mqtt_topic + "/Status").c_str(),"Alive",true);

```

As you’ll see, the `'connect()` command defines the path for the LWT message, and the message itself ("Dead"). If the connection is successful then the next line publishes an “Alive” message to the target topic.

The syntax for the `.connect` command is:

```cpp
  /* 
  MQTT Connection syntax:
  boolean connect (client_id, username, password, willTopic, willQoS, willRetain, willMessage)
  Connects the client with a Will message, username and password specified.
  Parameters
    client_id : the client ID to use when connecting to the server.
    username : the username to use. If NULL, no username or password is used (const char[])
    password : the password to use. If NULL, no password is used (const char[])
    willTopic : the topic to be used by the will message (const char[])
    willQoS : the quality of service to be used by the will message (int : 0,1 or 2)
    willRetain : whether the will should be published with the retain flag (int : 0 or 1)
    willMessage : the payload of the will message (const char[])
  Returns
    false - connection failed.
    true - connection succeeded
  */

```

I choose to set the retain flag to `true` for both the `.connect()` command and the subsequent `publish()` command, and this works very well for me.

Just to clarify the `base_mqtt_topic` in my commands, I organise my devices with a hierarchy like this:

```auto
Home
  Lounge
    Table_Lamp
    Main_Lights
    Fan_Controller
  Bedroom
    Bedside_Lamp    
    Main_Lights
    Fan_Controller

```

So the device that controls the bedside lamp in the bedroom will have a base MQTT topic of `Home/Bedroom/Bedside_Lamp` so the topic for that device's LWT is `Home/Bedroom/Bedside_Lamp/Status`. I do this because I use the same core code for all of my devices, and only have to change one variable to adjust the device's MQTT settings.

Having said all of that, I don't actually use the LWT data that much.  
Instead, I have each device publish heartbeat data every 5 seconds or so (the actual interval is randomised at start-up, to void all devices sending their heartbeats at the same time). Each device sends its RSSI value as part of this heartbeat, and this is what I monitor using a Timeout node to detect when a device goes offline.

The LWT status is handy though, when looking at devices in MQTT Explorer. It gives an instant feedback on whether the device is alive or dead, and therefore whether all of the other data can be trusted, without having to wait to see if the values are updated in the next heartbeat period.

Pete.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [3 March 2022 20:07 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/38 "2022-03-03T20:07:23Z")

</div>

Hi Peter,

It's good to read your reply as I do similar things also.

When starting I knew nothing about the _Birth_ and _LWT_ messages.

Where you _send_ messages from your devices every 5 seconds, I have a central machine pinging everyone every _n_ seconds and the ping replies are used to determine if they are dead/alive.

I'm using the _Birth_ and _LWT_ message more as an exercise in seeing them work after I found out what they are.

_We_ may be lucky and tomorrow I may have some more information on this as I have just superseded a machine with a new one, and though that should not make a difference: I'm hoping some of the _Gremlins_ have survived the change.

But what I also do is a bit more checking as well.  
All my _Certificates_ are sent on two topics: `SOM` and `EOM`.

But on my main machine I also have a bit more code - of which I just mentioned - and it is listening to the `SOM` topic.

If it sees one, it publishes a message on a different topic. This basically tells all connected devices to reply with their details. (Not on the `SOM/EOM` topic. The topic is `IFF`

What I've been seeing are a few `LWT` messages from dead machines every now and then.  
They are bunched in with other `SOM` and `EOM` (or more so: `EOM` and `SOM`) messages.

Reading replies: this is normal - supposedly. The broker broadcasts all _those_ messages when it deems it needed as it has just received one of their type.  
I can see some good and bad things about that.  
So putting that aside, ok.... Here's where it gets confusing for me.

As well as these _expected_ `LWT` messages (and even `BirthCertificates` scattered in there as well) I am getting the messages the machine sends when it is asked "Who are you?" (IFF)  
This as NOTHING to do with the `BS` and `LWT`. Rather they are sent on a different topic altogether.

Now, going slightly off topic:  
Why am I saying that tomorrow may be good.  
(things have got a a lot better in the past year or so)  
On a somewhat predictable basis: every morning at 05:00 my main WAP goes down.  
(Why: \*Who knows. But it does. Then comes back.)

That has a knock on effect that a few WiFi devices are flagged as _Dead_ by the broker and when the WAP comes back there is some negotiating happening.

So I get a few `LWT` and `BS` messages.  
And with the `BS` messages the machine sends out a few `IFF` messages and gets their replies.

Why/how are machines that are NOT CONNECTED and even POWERED OFF reply to this?

This morning - alas - other things happened and I didn't get the WAP going down as usual.  
Of course. I'm looking for the problem, so it won't present itself.

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<div class="post-metadata">

**Author:** ![Colin](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/colin/32/17040_2.png) [@Colin](https://discourse.nodered.org/u/Colin)\
**Post date:** [3 March 2022 20:49 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/39 "2022-03-03T20:49:11Z")

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> [@Trying\_to\_learn](#):
>
> All my _Certificates_ are sent on two topics: `SOM` and `EOM` .

They are not certificates.  
How are you sending them to two topics? It is only possible to specify one topic for each message.

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<div class="post-metadata">

**Author:** ![Trying\_to\_learn](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/trying_to_learn/32/28400_2.png) [@Trying\_to\_learn](https://discourse.nodered.org/u/Trying_to_learn)\
**Post date:** [3 March 2022 20:56 UTC](https://discourse.nodered.org/t/mqtt-messages-from-offline-devices/59036/40 "2022-03-03T20:56:07Z")

</div>

Sorry I think that is more a confusion of their _names_. It is a birth _CERTIFICATE_... The `LWT` is a death _CERTIFICATE_.

That is what I am meaning by that statement.

Topics:

 ![Screenshot from 2022-03-04 07-55-02](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/0/8/08560939e8fdea538d19719ec2f0070bdd0fe72d.png)

I can see two here.

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