Paho Python MQTT Client Objects

client-objectsThe main component of the Paho Python MQTT client library is the client class.

The class provides all the necessary functions to connect to an MQTT broker, publish messages, subscribe to topics and receive messages.

To use you will need to create a new client object from the client class.

The client constructor takes 4 optional parameters. The default values are shown below:

Client(client_id=””, clean_session=True, userdata=None, protocol=MQTTv311, transport=”tcp”)




To create a new client object you first need to import the Paho MQTT client, and then create the object as shown below:

import paho.mqtt.client as mqtt
client1 = mqtt.Client()

The Client name

Although the client name is optional, it is only optional if clean sessions. are True (default).

However even if you don’t provide a client name one is created for you by the client software.

The screen shot below is taken from the MQTT broker console and shows the result of the client connecting first without specifying a client id and then secondly supplying a client id.

In both case the broker sees a client id as the client will auto generate a random one.

mqtt-client-id
The client name is used by the MQTT broker to identify the client.

This is necessary when using persistent connections as the broker needs to store messages for the client when the client isn’t connected.

Duplicate Client ids

If a client connects with a client id that is in use, and also currently connected then the existing connection is closed.

Because the MQTT client may attempt to reconnect following a disconnect this can result in a loop of disconnect and connect.

The screen shots below show what happens when I try and connect a client (client2) to the broker using the same id as an existing client (client1).

mqtt-duplicate-client-id

Here is the view from the broker:

mqtt-broker-duplicate-ids

In the above you can clearly see that when client 2 connects client 1 gets disconnected. Client 1 then attempts a reconnect which in turn disconnects client 2.

Therefore be careful when assigning client IDs.

Creating Unique Client ids

To create a unique client id you can use one of these popular schemes.

  • Client prefix + Serial number of device
  • Client prefix + Random Number
  • Client prefix + Time
  • Client prefix + time+random number

Example Prefix + random number – Myclient-902314

Note: Use of a client prefix is recommended as brokers can filter based on a client-id prefix.

Clean Session Flag

This flag tells the broker to either

  • Remember subscriptions and Store messages that the client has missed because it was offline value =False or
  • Not to Remember subscriptions and not to Store messages that the client has missed because it was offline – value =True

By default it is set to True

See persistent connections by example to see how persistent connections and clean sessions work.

Clean Sessions and Client Id

With persistent connections the connection details are stored against the client id and so when using persistent connections (clean session False) you cannot use a random client Id.

Other Parameters

The user data and protocol parameters are discussed here and I wont discuss them here as the defaults are normally used.

Auxiliary Functions or Settings

There are several Client settings that may need to be changed before a connection is created.

These settings are changed using auxiliary functions. Here is a list of the functions that are available:

  • max_inflight_messages_set()Affects Message throughput
  • max_queued_messages_set()Affects Message throughput
  • message_retry_set(retry)Affects Message throughput
  • tls_set()Used for SSL security
  • tls_insecure_set(value)Used for SSL security
  • username_pw_set()  – Used for username and passwords
  • will_set()Used for setting last will and testament

The most common used ones are username_pw_set() ,tls_set(),and will_set().

See the Documentation for details

Websockets

websocket support is also built into the Paho MQTT client.

To Use Websockets with Python. Create the client object using the transport=websockets argument.

client1= paho.Client("control1",transport='websockets')

See Introduction to MQTT over WebSockets

Simple Client Object Modifications I Make

Important- See note below

I usually add additional flags to the Client object and use them for detecting successful connections and subscribes.

This I do before I create the client object. So often in my test scripts you will see an initialise client object function that looks like this.


def Initialise_client_object():
  #flags set
  mqtt.Client.bad_connection_flag=False
  mqtt.Client.connected_flag=False
  mqtt.Client.disconnected_flag=False
  mqtt.Client.suback_flag=False

Note: I don’t use the above any more as you cannot add lists using this method as they can then be changed by any object instance.

Instead I either create the flags as part of the client instance or I sub class the client object.

So I use either

import paho.mqtt.client as mqtt
def Initialise_clients(cname):
    #callback assignment
    client= mqtt.Client(cname,False) #don't use clean session
    if mqttclient_log: #enable mqqt client logging
        client.on_log=on_log
    client.on_connect= on_connect        #attach function to callback
    client.on_message=on_message        #attach function to callback
    client.on_subscribe=on_subscribe
	#flags set
    client.topic_ack=[]
    client.run_flag=False
    client.running_loop=False
    client.subscribe_flag=False
    client.bad_connection_flag=False
    client.connected_flag=False
    client.disconnect_flag=False
    return client

or creating a sub class

import paho.mqtt.client as mqtt

class MQTTClient(mqtt.Client):
   
   def __init__(self,cname,**kwargs):
      super(MQTTClient, self).__init__(cname,**kwargs)
      self.last_pub_time=time.time()
      self.topic_ack=[]
      self.run_flag=True
      self.subscribe_flag=False
      self.bad_connection_flag=False
      self.connected_flag=True
      self.disconnect_flag=False
      self.disconnect_time=0.0
      self.pub_msg_count=0
      self.devices=[]

Common Questions and Answers

Q- Can two clients have the same client ID?

A- No. If a client connects with a client id that is also currently connected then the existing connection is closed.

Client Connections

Once you have a client object you can connect to the broker —> Client Connections

Course Links

  1. Introduction to the Paho Python MQTT Client
  2. Introduction to the Client Class
  3. Connecting to a Broker
  4. Publishing Using The Paho Python MQTT Client
  5. Subscribing using The Paho Python Client
  6.  Receiving Messages with the Paho MQTT Python Client
  7. Understanding The Loop
  8. Understanding Callbacks
  9. Handling Multiple Client Connections
Please rate? And use Comments to let me know more

8 comments

  1. Why renew subscription is not working for message_callback_add . clean_session = False .How can i add qos=1 for message_callback_add. Help me to sort out this.

  2. HI Steve,
    Your Content is Fantastic ,and i thank you for giving this much of worthy content to the world.
    **Ques 1:** I run a local mosquitto broker and client .In client side i set clean_season = False and subscribed with qos=1. if client automatically reconnects use of loop_forever() , the subscriptions are renewed automatically and i got message even after client reconnection. But when the Client automatic reconnection due to Duplicate client the subscriptions are not renewed. Should i move client.subscribe into on_connect function or how should i solve this problem. (I am using loop_ forever() for automatic reconnection. )
    **Ques 2:** i read your mulitiple client handling posts now what are the parameter should i config to my broker if i 100k users are scaling to use my mosquitto broker.Suggest me best parameters

    1. Because you are using the duplicate client id the client clears the subscribe from the other client. Change client ids and it will solve your problem
      rgds
      steve

  3. Good write up that simplifies the python use of the PAHO library. I was having the issue where my client kept connecting then disconnecting and I could not figure out what was going on then your write up explained that you cannot have two client with the same client IDs. That was my issue so problem solved. Thanks.

    The other issue I am having is trying to support in a single configuration the setup of the connection details for connecting using 1883 (MQTT), 8883 (MQTT-TLS) and 443(WSS) to different MQTT hosts. Have all working if a restart my app for testing each, however when I have been connected to a host using a config and I try to change. I get an error trying to call tls_set() a second time. I cannot figure out how to reset once I disconnect from a MQTT host by using different configuration. Example: I was using 8883 with TLS certificates, then disconnect and want to reconnect to another host using credentials on 443 (WSS). I get the message that TLS was already setup when I call tls_set() again.

    I don’t want to have the user to close and reopen the app to change connections. Any help would be appreciated.

    1. You need to create a new client object as the old details are still there if you try to reuse the old one you can kep the same names.
      Rgds
      Steve

  4. Thanks ever so much for all this Steve.
    I clearly have much to learn regarding Python but where I did get a bit lost was in your section “The Client name” and working out the distinction between “client name” and “client id”. Seems to me at the moment that they are same thing but I have a lot to grasp on this subject as yet although I have managed to get a temperature sensor up & running

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