paho mqtt client broker connection

Question

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.

As an extra to the mqtt spec, you may want to look at the source code, github.com/eclipse/paho.mqtt.c/blob/master/src/…, line 142, it stores the messages to a local list when publishing. — Jackie

Jeff Lowrey Jeff Lowrey · Accepted Answer · 2016-09-06T14:45:31

Mqtt messages that are not delivered (the client hasn't received a PUBREL packet) are stored in it's local session storage area. . Similarly if the server has not received a PUBCOMP response for it's PUBREL message, it has to treat the message as unacknowledged and store relevant session state in it's local session storage area.

The size of the cache on both sides (the server and the client) are directly determined by the configuration of the client or server. This is mentioned in the first Non normative comment at the start of section 4.1 in the OASIS standard.

There is a good description (if occasionally confusing) description of this process for QoS 2 messages at the OASIS standard

paho mqtt client broker connection

2 Answers