Let's say I've several devices each having a temperature. All messages related to device temperature are published on topics device/1/temerature, device/2/temperature, etc. . I handle all messages published on this topic with Python paho-mqtt with a callback function which uses a wildcard expression client.message_on_callback_add("device/+/temperature", ...). Is there a way to get the value of the wildcard expression, here + directly (w.o. need for parsing of msg.topic)?
I believe you are looking to extract the client id from the topic. The callback functions will have clientid as part of the callback. Pleaes check the functions in the below URL.
https://pypi.org/project/paho-mqtt/#callbacks
If you are using the default callback function on the message received, you will be able to get the client as a separate parameter.
No, the callback includes the topic the message was published to.
It is up to you to extract what ever information you need from the topic.
In this example, you can get the subscribed topic when the message arrived. You can split topic to get wildcard characters from subscribed topic.
import paho.mqtt.client as mqtt
def on_connect(client, userdata, flags, rc):
client.subscribe("device/+/temperature")
client.message_callback_add("device/+/temperature", handler("device/+/temperature"))
def handler(subscribed_topic):
wildcard_expression = subscribed_topic
def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload), wildcard_expression)
return on_message
client = mqtt.Client()
client.on_connect = on_connect
client.connect(<mqtt broker url>, 1883, 60)
client.loop_forever()
Related
For example, there I am listening to the settings / # topic. The device sends a line to the settings / topicN topic, where N is the device number.
The moment I receive something from the device, I want to see the previous SMS from it.
How to see the moment of receiving data from the device exactly according to
Is this thread just the latest post in python?
import paho.mqtt.client as mqtt
# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, flags, rc):
print("Connected with result code "+str(rc))
# Subscribing in on_connect() means that if we lose the connection and
# reconnect then subscriptions will be renewed.
client.subscribe("settings/#")
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload))
imei = msg.topic.split('settings/')[1]
data = msg.payload.decode()
print(imei)
print(data)
publish(client, imei)
def publish(client,imei):
topic = 'test/'+ imei
client.publish(topic,'hello')
print('SEND')
client = mqtt.Client()
user = 'test'
passw = '1111'
client.username_pw_set(user,passw)
client.on_connect = on_connect
client.on_message = on_message
client.enable_bridge_mode()
You don't.
If you are not subscribed to a topic then there is no way to see any old messages sent to that topic before you subscribed to it. (With the exception of if the message was published with the retained bit set to true, then the last message published will be delivered by the broker at the point of subscription before any new messages)
If you want to keep the history, then you need to create a client that is always subscribed to the topics and stores them in some sort of database.
I am working with a device that publishes to the topic test/123, where 123 is the name of the device. I need to subscribe to that topic (and processes received messages); in addition I also need to send a word to the same topic (test/123). The device only looks at this topic.
How can I distinguish between incoming and outgoing by content? More precisely, how to send correctly. In the on_message method you need to do this or you need to create another method, but then how to receive incoming messages there. From the incoming messages I need to get the name of the device and then work with it.
import paho.mqtt.client as mqtt
import paho.mqtt.publish as publish
# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, flags, rc):
print("Connected with result code "+str(rc))
# Subscribing in on_connect() means that if we lose the connection and
# reconnect then subscriptions will be renewed.
client.subscribe("/test/#")
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload))
imei = msg.topic.split('test/')[1]
data = msg.payload.decode()
print(imei)
print(data)
publish(imei)
def publish(imei):
client = mqtt.Client()
user = 'test'
passw = '1111'
client.username_pw_set(user,passw)
client.connect("localhost",1883)
topic = '/test/'+ imei
client.publish(topic,'hello')
print('SEND')
client.disconnect()
client = mqtt.Client()
user = 'test'
passw = '1111'
client.username_pw_set(user,passw)
client.on_connect = on_connect
client.on_message = on_message
client.connect("localhost", 1883, 60)
client.loop_forever()
MQTT does not differentiate between clients in any way, that means if a client subscribes to a given topic it will receive ALL messages on that topic, including the ones it publishes it's self. So with your current design you will always get the message you publish in response to the first message back and this will trigger re-sending that message.
MQTT messages do NOT contain any information about who published the message unless you choose to add it to the payload, so you have no way to identify the incoming message as being the one you just published and this will cause a message loop storm.
The CORRECT solution is to not use the same topic for the 2 messages.
MQTT v5 has a flag that can be passed as part of establishing the connection
which prevents messages being returned to the client that published them. At this time it does not appear that the Paho Python library has a way to set this flag.
If you are using MQTT v3.1.1 and the mosquitto or RSMB MQTT broker then there is an undocumented option (this is not part of the MQTT spec) that can be set which will also prevent messages being returned. The following code will ONLY work with the 2 brokers I have mentioned.
import paho.mqtt.client as mqtt
# The callback for when the client receives a CONNACK response from the server.
def on_connect(client, userdata, flags, rc):
print("Connected with result code "+str(rc))
# Subscribing in on_connect() means that if we lose the connection and
# reconnect then subscriptions will be renewed.
client.subscribe("test/#")
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload))
imei = msg.topic.split('test/')[1]
data = msg.payload.decode()
print(imei)
print(data)
publish(client, imei)
def publish(client,imei):
topic = 'test/'+ imei
client.publish(topic,'hello')
print('SEND')
client = mqtt.Client()
user = 'test'
passw = '1111'
client.username_pw_set(user,passw)
client.on_connect = on_connect
client.on_message = on_message
client.enable_bridge_mode()
client.connect("localhost", 1883, 60)
client.loop_forever()
p.s. do not start topics with a leading / while legal according to the spec, it will break things like shared subscriptions and adds an extra null to the start of the topic tree.
My Paho MQTT client does the following:
Subscribe to mytopic/#
Do something
Publish to mytopic/#
Problem:
The published message in step 3 arrives at step 1. I'd like to avoid adding a sender-attribute to the payload.
Is there a proper way of ignoring self-published messages? Something like the following (pseudocode):
def on_message(self, client, userdata, message):
if client.id == message.sender_client_id: # Is there anything like the sender_client_id?
return
Any idea? Thanks!
As of the MQTT v5 spec you can tell the broker not to send your own messages back to you as part of the subscription message.
This removes the need to add the identifier so you can then choose to ignore it.
This does of course rely on both the broker and the MQTT client supporting MQTT v5
This logic should work:
Assign an id to every client
every client publish on mytopic/{id}
every client sub to mytopic/#
ignore messages where message.topic starts with mytopic/{id}
If you are using MQTT v5, you can pass the noLocal option to the paho client when subscribing. This option tells the broker not to send back your own messages.
from paho.mqtt.subscribeoptions import SubscribeOptions
...
options = SubscribeOptions(qos=1, noLocal=True)
client.subscribe('mytopic/#', options=options)
def on_message(self, client, userdata, message):
if client.id == message.sender_client_id: # Is there anything like the sender_client_id?
return
In your pseudocode, you are asking for the client's identity but this is exactly opposite to the MQTT specification. In MQTT, two different clients are unaware of each other's identity, they only communicate via the MQTT broker by subscribing to the topics.
I am sending a number of messages simultaneously from multiple clients from one python script and trying to receive them on another script. The problem I am getting is that the message is received but only from the first client that gets connected and it keeps on looping over it.
What I need to have is that I get messages from each client in the manner they are published.
import paho.mqtt.client as mqtt
import time
def on_connect(client, userdata, flags, rc):
if rc == 0:
print("Connected to broker")
global Connected
Connected = True
else:
print("Connection failed")
def on_message(client, userdata, message):
print ("Message received: " + str(message.payload) + " from " + str(client))
Connected = False
client = mqtt.Client()
client.on_connect= on_connect
client.on_message= on_message
client.connect(host)
client.loop_start()
while Connected != True:
time.sleep(0.1)
client.subscribe("test")
print("subscribed")
client.loop_stop()
You are misunderstanding what the client argument in the on_message callback is.
This value is a link to the local instance of the MQTT client that has subscribed to the topic.
MQTT messages do not carry any information about the client that published them, unless you explicitly encode it into the payload. Part of the point of a Pub/Sub protocol like MQTT is to totally decouple the information creator (publisher) from the information consumer (subscriber).
Also you should move the call to client.subscribe("test") to inside the on_connect() callback because as you have it you are trying to resubscribe to the same topic 10 times a second which will achieve absolutely nothing, except to generate unneeded load on the broker.
I would like one RaspberryPi-A to send a massage to RaspberryPi-B, and RaspberryPI-B should send a massage back automatically. I know this is a beginners question, but I'm one and I have really struggled all day trying to find an anwser.
This is my client's code
import paho.mqtt.client as mqtt
MQTT_SERVER = "localhost"
MQTT_PATH = "test_channel"
def on_connect(client, userdata, flags, rc):
print("Connected with result code "+str(rc))
client.subscribe(MQTT_PATH)
# The callback for when a PUBLISH message is received from the server.
def on_message(client, userdata, msg):
print(msg.topic+" "+str(msg.payload))
# more callbacks, etc
client = mqtt.Client()
client.on_connect = on_connect
client.on_message = on_message
client.connect(MQTT_SERVER, 1883, 60)
client.loop_forever()
And this is my publisher’s code
import paho.mqtt.publish as publish
MQTT_SERVER = "192.168.1.5"
MQTT_PATH = "test_channel"
publish.single(MQTT_PATH, "Hello World!", hostname=MQTT_SERVER)
How should i write the code differently so my client would return the massage for example: "Hello back"?
There are 2 kinds of messaging models:
Point to Point (one to one) - a message is sent from one application to another application via a queue. There can be more than 1 consuming (receiver) applications but only one of them will receive the message.
Publish / Subscribe - is where a message is published to a topic and multiple consumers (subscribers) will each receive a copy of the message. There can be 1 or more applications publishing messages to the same topic and 1 or more applications consuming (receiving) the messages.
MQTT is built on the Publish / Subscribe messaging model.
Your description sounds like you want Point to Point messaging. Yes, you can bend MQTT to act like Point to Point but be aware that if you have multiple applications publishing messages to the same topic, you may get confused.
MQTT_PATH = "test_channel"
publish.single(MQTT_PATH, "Hello World!", hostname=MQTT_SERVER)
There is no such thing as a "channel" in MQTT. Your code is publishing a message to the topic called: test_channel.
It is better to use a little hierarchy in your topic names.
i.e.
pivk95/food/burgers
pivk95/food/fries
pivk95/food/pizza
pivk95/food/burritos
pivk95/drink/shakes
pivk95/drink/soft_drink
Just remember that any number of applications can publish messages to a given topic and any number of applications can subscribe to a given topic and receive copies of the messages.