# MQTT - not getting it - again. message structure

**URL:** <https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493>\
**Category:** General\
**Created:** [25 September 2021 03:49 UTC](https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493 "2021-09-25T03:49:11Z")\
**Posts on this page:** 5\
**Page:** 1

<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:** [25 September 2021 03:49 UTC](https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493/1 "2021-09-25T03:49:11Z")

</div>

It would seem my thinking I understand MQTT and messages is wrong.

Or I am missing something - again.

This is my _test_ flow.

```auto
[{"id":"b87b117d64f2fc0a","type":"mqtt out","z":"65c9b63cb09879a0","name":"","topic":"test","qos":"","retain":"","respTopic":"","contentType":"","userProps":"","correl":"","expiry":"","broker":"8941f4c3.0f151","x":340,"y":1870,"wires":[]},{"id":"8cccb4dff9644103","type":"mqtt in","z":"65c9b63cb09879a0","name":"","topic":"test","qos":"2","datatype":"auto","broker":"8941f4c3.0f151","nl":false,"rap":true,"rh":0,"x":470,"y":1870,"wires":[["1401dff22e7b167b"]]},{"id":"1401dff22e7b167b","type":"json","z":"65c9b63cb09879a0","name":"","property":"payload","action":"","pretty":false,"x":590,"y":1870,"wires":[["639e04cb31c14b7d","2adf3a80409bc323"]]},{"id":"639e04cb31c14b7d","type":"debug","z":"65c9b63cb09879a0","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":730,"y":1870,"wires":[]},{"id":"523ce62d14a37f87","type":"inject","z":"65c9b63cb09879a0","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"payload\":\"text\"}","payloadType":"json","x":190,"y":1870,"wires":[["b87b117d64f2fc0a"]]},{"id":"2adf3a80409bc323","type":"switch","z":"65c9b63cb09879a0","name":"","property":"payload.payload","propertyType":"msg","rules":[{"t":"eq","v":"text","vt":"str"}],"checkall":"true","repair":false,"outputs":1,"x":590,"y":1940,"wires":[["bbc460a221fd8fd6"]]},{"id":"bbc460a221fd8fd6","type":"debug","z":"65c9b63cb09879a0","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":740,"y":1940,"wires":[]},{"id":"8941f4c3.0f151","type":"mqtt-broker","name":"Local","broker":"127.0.0.1","port":"1883","clientid":"","usetls":false,"compatmode":false,"keepalive":"60","cleansession":true,"birthTopic":"","birthQos":"0","birthPayload":"","closeTopic":"","closeQos":"0","closePayload":"","willTopic":"","willQos":"0","willPayload":""}]

```

Local MQTT broker.

I create a `JSON` message - I hope - in the `inject` node.

Send that into the `MQTT` node then it is received.

I send it through a `JSON` node and look at the output in a `debug` node.

I need the `JSON` node to _de-construct_ the message from the `MQTT` wrapping.

Ideally I should get `text` in the `debug` node.

Then the `switch` node will pass the message is the `msg.payload` (don't get ahead of me) is `text`.

Or so it should.

But it only works if I make the `switch` node compare the `msg.payload.payload`.  
Shouldn't the `JSON` node strip all that stuff off?

SORRY!  
I know I've been through this before and I remember way back in the early days NicOleary helped me when I was going around in circles with the `JSON` (or more so: message structure) problem.

But for reasons unknown: I can't seem to get it working to simply send a message through.

---

<div class="post-metadata">

**Author:** ![zenofmud](https://sea2.discourse-cdn.com/flex026/user_avatar/discourse.nodered.org/zenofmud/32/316_2.png) [@zenofmud](https://discourse.nodered.org/u/zenofmud)\
**Post date:** [25 September 2021 06:55 UTC](https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493/2 "2021-09-25T06:55:50Z")

</div>

Andrew, lets walk thru what you have done. First the `inject` node

You need to remember that `msg` is an object and it initially contains only a single name/value pair. You can see this if you have an `inject` node and delete the 'payload' and 'topic' options and connect it to a `debug` node set to display the complete msg object. This is what you will see in the debug panel:

```auto
9/25/2021, 2:13:47 AMnode: 68761c9e6b27fb67
msg : Object
object
  _msgid: "89ad90bf1540ce68"

```

You can add a 'name/value' pair named 'payload' to the `msg` object and the value can take many forms. It can be a string, a number or an other object.

This is what it would look like containing a string:

```auto
9/25/2021, 2:20:34 AMnode: 68761c9e6b27fb67
msg : Object
object
  _msgid: "61ef2e5133c3a9d1"
  payload: "Hi Andrew"

```

so this `msg` object has two name/value pairs and the second name/value pair has a name 'payload' and a value 'Hi Andrew'

Now let's look at the inject you used  
 ![Screen Shot 2021-09-25 at 2.09.47 AM](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/2/1/21dfc3164620d7806e9fe0c756a11fb0d7369857.png)

In this case, you have added two name/value pairs to the `msg` object. The first name/value pair has a `name` called `payload` and a `value` that is an object. That object contains a name/value pair and you have chosen to use `payload` as the name and the string `text` as the value. So your complete omsg object looks like this:

```auto
msg : Object
object
_msgid: "f4d7a31e38eff874"
  payload: object
     payload: "text"
  topic: ""

```

So to get to the value 'text' you need to use `msg` (to identify the object), `.payload` (to identify the name/value pair that has a 'name' of 'payload') then within the 'payload' the value is an object, so you need to use the 'name' to access the value and you used the name 'payload' (ohhh confusion alert!!) so you need to use `.payload` to finally complete the access to the value 'text'.

So the full path is `msg.payload.payload`

You probably should have used a 'name' of 'mydata' to keep from getting confused and then the path would be `msg.payload.mydata`.

\_ \_

Secondly the `mqtt-out` node. If the data (msg.payload) being sent is an object, it is converted to a string before being sent. So in your case msg.payload is converted to a string and sent.

The `mqtt-in` receives the string and you send it thru a `json` node to convert it back to an object (remember that the `mqtt-in` node now has an option to convert it to an object so you don't need the following `json` node.

[If you set the `mqtt-in` node to convert the string to an object then pass it thru a `json` node, it will be converted back to a string]

So you send the msg to a `switch` node and you test `msg.payload.payload` to see if the value is 'text' and it is, so you pass the msg on to a `debug` node.

In the `debug` node you display the contents of `msg.payload` and the contents is an object so that is what is displayed. IF you want to display the value of the name/value pair that is in `msg.payload` then you have to specify the 'name' so your debug would have to be this:  
 ![Screen Shot 2021-09-25 at 2.50.53 AM](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/3X/a/1/a164a158e1e9e97a072b25df208a2b81dbb74300.png)  
Remember, the switch node does not change the contents of the `msg` object, it is just testing parts of it.

If you wanted the 'value' of the name/value pair `msg.payload.payload` to end up in `msg.payload` you could use a `change` node to set `msg.payload` to `msg.payload.payload`

Now you might understand why I suggested you set the original msg.payload to a name/value pair where the name was 'mydata'.

---

<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:** [25 September 2021 06:58 UTC](https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493/3 "2021-09-25T06:58:29Z")

</div>

You are sending to mqtt a payload containing `{payload: "text"}` so that is exactly what you get out of the JSON node. Where is the confusion?

You don't need the JSON node though. In the mqtt in node tell it to parse the JSON for you and that will achieve the same thing.

---

<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:** [25 September 2021 07:48 UTC](https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493/4 "2021-09-25T07:48:49Z")

</div>

I seem to be making things difficult for myself.

@zenofmud  
I think I am making my own life more difficult than it needs to be, but.....

I have been bitten by a big mistake I just realised and I'm not sure you actually meant to show me it.

The fact is that the `inject` is sending what ever I put in it already .... embedded (wrong word) as `msg.payload`, and not `msg.{}` so I am shooting myself in the foot.

The problem is compounded because in the real application of it I have made problems for myself which are only now coming to light.

If I had a _different_ structured message - the way I meant it to be in the **example** the message would actually be - or should have been -  
`msg.{"payload": "text"," something_else": "Foo"}`

So I've moved on from that stupid mistake.

Now, the **other** problem.

Back to the basics though:

`msg.payload = {"payload":"Hello"}`

Which is .......  
ARGH!

Because it is the `inject` node it is already in `paload`... I just said that. Yikes!  
Ok, that's the ah-ha! moment.

So if I _simply_ send "Hello" as `msg.payload` into the `MQTT` node.... at the other end, I don't need the `JSON` node.

To make my life a bit easier I think I will pick/make a whole new _topic_ in `MQTT` and send the message and parse it all alone.

I was trying to _piggyback_ it on an existing channel that expects `JSON` messages and when my effort was received it was causing problems (in the **REAL** version) and so I went down the this rabbit hole, and got lost/confused.

Sorry to both of you: @zenofmud and @Colin.  
It was a _Brain failure_ in a regrettably bad way.  
I'll leave it here.

---

<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/flex026/uploads/nodered/original/1X/d073cd938eafa2e558d7c2cd59003b3ef4963033.png) [@system](https://discourse.nodered.org/u/system)\
**Post date:** [9 October 2021 07:49 UTC](https://discourse.nodered.org/t/mqtt-not-getting-it-again-message-structure/51493/5 "2021-10-09T07:49:42Z")

</div>

This topic was automatically closed 14 days after the last reply. New replies are no longer allowed.