I am using Python based paho mqtt client to publish data to mosquitto mqtt broker.
Let's assume a scenario, when client wanted to publish message and broker got disconnected.
So python based client object buffers that message in _out_message (Ordered Dictionary), and keep retrying to send messages.
I wanted to know,
For how long mqtt client will buffer such message?
Is there any time limit or retry limit after which client will drop the message.
I wanted to dump/log such messages.
According to Eclipse Paho Python documentation, you can set the maximum number of outgoing messages with Quality of Service greater than 0 (QoS > 0) that can be pending in the outgoing message queue with the method:
max_queued_messages_set(self, queue_size)
It seems that using the default value (0) all the messages are kept until the MQTT client is able to send them. So, in the end, I suppose that messages are kept until the Python process reach the memory limit imposed by the operating system.
You can force the MQTT client to discard messages using the method reinitialise.
reinitialise(client_id="", clean_session=True, userdata=None)
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In a web application with MQTT in python (using paho-mqtt lib) I would like to know if there is a way to get the broker status in real time, because the only way that i find is to store the variable "rc" into the method on_connect but it's more like a client/connection state.
EDIT 1 : after reading mosquitto broker documentation, i found that you can subscribe to '$SYS/broker/connection/#' which is supposed to give you back 1 if the connection is up and 0 if it goes down. However when i do :
subscribe.callback(self.message_callback, '$SYS/broker/connection/#', port = port, hostname=broker, auth=authentication, protocol=client.MQTTv31, tls=TLS)
impossible to get payload and topic this message although i'm doing exactly the same command to get messages from my sensors except that the topic is '#' and it's working perfectly.
Does anyone knows why ?
There is no way to poll the state of the connection to the broker from the client.
The on_disconnect callback should be called when the connection to the broker is dropped.
This should be kicked off when the keep alive times out, but also as the result of a failure to publish (if you try to publish data before the timeout expires).
Also the rc from a call to the publish command will indicate if the connection has dropped.
Currently developing something like "smart home" and I have few different devices in my home. All of them connected to OpenHab via MQTT. I'm using Paho MQTT library (Python) for my purposes.
Generally, MQTT has "keepalive" property. This property describes how much time my client will be connected (AFAIK it sends the ping to the server) to MQTT server when there are no updates on the subscribed topic.
But here I have a huge problem. Needed topic could be updated once per hour or even once per few days/months. Let's say that this is indoor alarm.
How can I avoid that keepalive timeout or ignore that field? Could it be unlimited?
You have miss understood what the keepalive value represents.
MQTT clients can stay connected indefinitely even if they do not publish or receive any messages. But the broker needs to keep track of which clients are still connected so it knows when to send the Last Will and Testament (LWT) message for the client. In order to do this it uses the keepalive time.
Every time a message is sent or received by the client, the broker resets a timer, if this timer exceeds 1.5 times the value of the keepalive time then the broker marks the client as disconnected and processes the LWT. To prevent clients with very low messages rates from being disconnected, such a client can send a PINGREQ packet at any time (most likely on timeout of the keepalive value) to the server/broker. The server receives the PINGREQ, answers with a PINGRESP packet and it will reset the keepalive timer to zero and leave the client in the connected state.
See Keep Alive section of the MQTT standard: (http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc385349238)
The Client can send PINGREQ at any time, irrespective of the Keep Alive value, and use the PINGRESP to determine that the network and the Server are working. If the Keep Alive value is non-zero and the Server does not receive a Control Packet from the Client within one and a half times the Keep Alive time period, it MUST disconnect the Network Connection to the Client as if the network had failed
When sending the initial MQTT CONNECT message from a client, you can supply an optional "keep-alive" value. This value is a time interval, measured in seconds, during which the broker expects a client to send a message, such as a PUBLISH message. If no message is sent from the client to the broker during the interval, the broker automatically closes the connection. Note that the keep-alive value you specify is multiplied by 1.5, so setting a 10-minute keep-alive actually results in a 15 minute interval.
Have a look at the Keep Alive section of the MQTT specification:
A Keep Alive value of 0 has the effect of turning off the Keep Alive mechanism. If Keep Alive is 0 the Client is not obliged to send MQTT Control Packets on any particular schedule. v5 spec source
Therefore, set the keep alive to 0, and then the client doesn't have to send a keep alive signal as often. The server should respect that this connection with a client (e.g. from last year) should still be connected, but it won't be guaranteed (The client might be disconnected when the server is shut down).
My publisher and broker are working on different systems. I am using QOS=2 for delivery of messages.I am using python paho mqtt broker. It is further extension to
MQTT - Is there a way to check if the client is still connected
1) When I publish a message to connected broker, it acknowledge me by calling on_publish() callback. but when I disconnected my broker running on different machine from the network, the publisher stores the publish message on local machine and again when I connect my broker to network it publishes all previous messages to broker. I think these messages stored as inflight messages (not confirmed), if these messages are inflight messages, then where these messages are stored,is there any limit of these inflight messages as I have not include anything in my code regarding inflight messages.
2)In first case I disconnect my broker from the network, now I shutdown my broker and again connect to network, my program call on_disconnect() callback says unexpected disconnection, then publishes all unpublished messages to broker.
a) I am confused why on_disconnect() callback calls only when I shut down by broker not on when I disconnect my broker from my local network.
b) on_disconnect() method calls only when my broker again connect to network.
Is there any way if my broker disconnect suddenly, then immediately it will be informed to publisher?
3) I am using mqtt in real time gps tracking, I want to store messages to local db when publisher not connected to broker , but I can't find any way how publisher comes to know immediately when it disconnected from broker.
4) Is using QOS=2 is best way to ensure message delivery to broker or using local db for storage purpose when disconnected to broker and then sync automatically with local db to publish all messages
I am trying to send a message using publish.single and then receive it and act upon the data received. Hence, I can't proceed unless I receive something from the server, so is there a way to write a statement that will wait for a message from MQTT before proceeding?
Example code:
publish.single("$topic", data, ip_address)
#can't do anything here
receive(data_from_broker) #or anythin that looks like it!
#continue with the program here
The short answer is you don't.
At least not in the way you describe. MQTT is an asynchronous protocol, there is no sense of sending a message and waiting for a response, a publishing client has no way to know if the is 0, 1 or n subscribing clients listening on the topic the message is published.
You will need to build something called a state machine to keep track of where in the program flow you are when messages are received.
e.g.
Application published message and sets flag in the state machine to say the message was sent
Remote client receives message and publishes a response
New message is received by the first client, it checks the state machine to determine that the message should be treated as a response to the original message.
To subscribe to a topic you will have to move on from using publish.single to the full MQTT client so you can setup the onMessage callback to handle the incoming messages.
Suppose that there is one redis-client which is subscribing to channel c1 and another redis-client publishes "data" to channel c1.
At this point, does the data published is pending in a redis-server by when the client subscribing to "c1" get the data(by calling pubsub.listen() or pubsub.get_message()) or go directly to the client subscribing to the channel c1 through a redis-server?
In other words, When a redis-client call pubsub.getMessage() or pubsub.listen(), does redis-client send a request to redis-server to get data or just get data from the local socket buffer?
When I read some documents it is saying that pubsub.get_message() use select module for a socket internally.
It seems to mean that the data subscribed already is in the client local buffer, not server.
Could you give me any suggestion?
When a client does PUBLISH, this command immediately pushes the message into all sockets that are interested in it. The message is not stored in redis. If a client wasn't subscribed at the time of publishing, he will never see the message.
I'm not familiar with python driver for redis, but judging from the names, I'd guess that listen sends a SUBSCRIBE command and get_message takes a message from the local buffer.
Redis pubsub is fire and forget.
In other words, when a publish command is sent, the message is received by online subscribers, and who wasn't subscribed/listening for messages when the publish command was sent, won't never get these messages.