Understanding MQTT Topics

Hello All!

Is message order maintained across wildcard topics? Suppose I have two devices publishing to:
living_room/temp and
living_room/humidity

Will a subscriber to
living_room/+
receive the messages in the original publishing order?

Or long story short – Is a wildcard topic containing “#” and “+” operators also an Ordered Topic by Default as described in the OASIS MQTT Spec?
https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901240

Hi. Thanks for the great info. Can you anyone tell me whether wildcard subscriptions are updated dynamically (ie. when a new topic is created) or whether it’s just a snapshot of existing topics at the time of the subscription? Thanks in advance

Hi Steve,

I have a question that I’m struggling to find a direct answer to and was hoping you could point me in the right direction. I have the mosquitto broker installed on my server and a handful of esp32s publishing to topics and everything is going well. I’d like to create a website(locked behind auth of course) that on connect grabs the latest values for device/+ topics using websockets to sync the statuses. I’m using the mqtt.js library and connecting is fine, but it will only receive messages on publish but not on connect. Is what I’m trying to do possible with the mqtt protocol/websockets or is this a fools errand?

I have my mostquitto broker set up to retain messages and when esp32s publish they’re passing the retain flag as well.

Is it possible to use the + and the # in the same subscription request?

For example:
/iot/devices/+/temperatures/#

to match all of :
/iot/devices/unknownfridge/temperatures/top/back
/iot/devices/unknownfridge/temperatures/top/right
/iot/devices/anotherfridge/temperatures/ bottom/back

I have a system set up like this and it’s not responding, though it reports that subscriptions are successful. If I try hardcoding the name instead of “+” it works.

Hi
how can we manage multiple topic subscription in MQTT without using #?
eg.published topics
device1/temperature
device1/humidity
device3/temperature
device3/humidity
subscribe without #
example device1/#,device3/#
because device1 and device3 level can have 100 topics
do we need to use any async calls for the same ?
so that if we are subscribing 100 topics then every topic should have one thread working for that topic itself

please provide solution as early as possible.

Regards,
Ramesh

Hi steve,
I find it odd that it is not possible to get a list of (subscribed) topics from a broker.
The broker has to know which topic to send to which client, so this list should exist somewhere but is not accessible ?
Or does the broker just sends off every message to every client and let the client decide if they want to accept it or not ? (which would be rather inefficient)

Hi Steve,
I am making an iot application where I have to connect to topics of multiple pharmacies with multiple refrigerators each containing 7 switches(show on and off status) and one temperature sensor and one humidity sensor.
My topics look like this:
For refrigerator 1 in pharmacy 1: Pharmacy1/Refrigerator1/Switch1 …. (7 topics for switchers), Pharmacy1/Refrigerator1/sensor1, Pharmacy1/Refrigerator1/sensor2
For refrigerator 2 in pharmacy 1: Pharmacy2/Refrigerator2/Switch1 ………. and so on.

1. How do I input the topics in my python? So that I can subscribe to all the topics. and then work on payload coming from each topic.
MY action for switches is different, action for humidity is different and for temperature is different.

I tried using wildcard for subscribing, but then, I cannot use the action separately for switches and sensors.

2. I want to count the on switches in each pharmacy. So I have put a callback function on the payload. But, my code works only for Refrigerator1 in pharmacy1. I want to count switches (which are on) for all the refrigerators in each pharmacy.

How do I find only switch topics/only sensors from the long list? As I want to make it scalable for a large number of pharmacies and refrigerators.

IF I use wildcard for subscribing: subscribe to topic #/Switch1 , #/Switch2 ….. , #/sensor1, #/sensor2 and then write :
if topic == Pharmacy1/+/Switch1, it gives errors that the topics are not in the list

Hi!
I have a structure like this:
device1/temperature
device1/humidity
device2/temperature
device2/humidity
device3/temperature
device3/humidity

Is there any way to subscribe to get all the temperature and humidity but only device1 and device3? (avoiding receive topics under devide2 or other possible devices)
It would be something like this:
device1|device3/#
Being the pipe a logical or operator

Hello Steve, I have a MQTT gateway for my controllers, and I set up its publish topic. When I subscribe to it, the messages that are shown (that I receive) have a lot of replacement characters, they look like �J�o����
+��)1�
Is the problem the protocol that my gateway is sending the data in, or is it something else?
Thank you in advance,
rgds

Hi Steve,

I have a subscription using a wildcard and want to make sure I have received all the available topics before my program moves on.
Is there a way to do this other than just using a timer to wait for a set time and hoping that everything has been received?

Great information here and extremely useful!

Cheers,

Rob

Hello Steve,
Greetings!
I am following your documentation, I am new born baby in this Mosquitto MQTT app installation and configuration.
Got struck on my Cluster and Scale configuration.
Steps I followed.
1. Edited “/etc/mosquitto/mosquitto.conf”
## maximum connection to mqtt allowed
max_connections 20000
2. /etc/security/limits
#
#
* soft nofile 20000
* hard nofile 20000
3. Based troubleshooting guidance I followed the links and created new file under
/etc/init/mosquitto.conf, added the lines
limit nofile 16384 16384 
With all these settings, I didn’t successes still I am getting the same error
cat /proc/3003/limits
Limit Soft Limit Hard Limit Units
Max cpu time unlimited unlimited seconds
Max file size unlimited unlimited bytes
Max data size unlimited unlimited bytes
Max stack size 8388608 unlimited bytes
Max core file size 0 unlimited bytes
Max resident set unlimited unlimited bytes
Max processes 14679 14679 processes
Max open files 1024 4096 files
Max locked memory 65536 65536 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 14679 14679 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited us

Thank you and appreciate, if you can guide me, what is the missed part here.

Hello Steve,
I publish data to 2 topics from 2 different clients and later I want to subscribe to these topics and save the data in 2 tables named gnss and acc in a database.
For example:
Client 1 will publish data to topics: my/device1/gnss and my/device1/acc
Client 2 will publish data to topics: my/device2/gnss and my/device2/acc
Now I subscribe to all these 4 topics by using multilevel wildcard: my/#
But how can I save the data of all clients into 2 tables gnss and acc irrespective of the client.
Code:
if msg.topic == “my/device1/gnss”:
sql = “””INSERT INTO gnss (topic, gnssdata) VALUES (%s,%s)”””
tuple = (msg.topic, msg.payload)
if msg.topic == “my/device1/acc”:
sql1 = “””INSERT INTO acc (topic, accdata) VALUES (%s,%s)”””
tuple = (msg.topic, msg.payload)
The above code will just save the data from Client1. How to do this for all the clients if I am having various clients which are publishing data to the broker.
I tried using: if msg.topic == “my/+/gnss”:
sql = “””INSERT INTO gnss (topic, gnssdata) VALUES (%s,%s)”””
tuple = (msg.topic, msg.payload)
But this doesn’t work.

Hello, this was a great intro to MQTT.
Can Brokers/Servers have licensing server associated to each /topic? Does it have this facility, lend easily to this method of billing or does it need to architecting?
Kind Regards
Ivan