CODEWITHSUNDEEP
mqtt
odich daniel
odich daniel

Reputation: 87

MQTT paho client publish message and delete immediately the receiver consumes it

I have been trying to work with paho mqtt client to publish and receive messages with mosquitto as the broker and works fine. My use case although involves the sender publishing a message to the broker and disconnects, at this point, the receiver whether connected or disconnected should consume this message and delete it immediately. I have played with all the properties e.g QOS, retained messages, clean sessions, etc but none is yielding the result I want. Please help.

Upvotes: 0

Views: 1976

Answers (1)

hardillb
hardillb

Reputation: 59826

Assuming a Publish and Subscription at QOS2 the message will only ever be delivered to the subscriber once, there is nothing to delete from anywhere.

If you are trying to ensure that the message is only ever consumed by one specific client then I think you have a misunderstanding about what MQTT is.

MQTT is a PUB/SUB protocol, and as such is designed to totally decouple the subscriber from the publisher. The publisher doesn't know how many subscribers there are, just that it has published a message to a given topic.

0 to N (where N can be any number) of clients can subscribe to the topic. Using QOS, persistent subscriptions and the clean session flag, a client can indicate to the broker that it would like to receive any messages published since was last connected, but this will not influence any other clients that may have also subscribed to that topic.

Starting at MQTT protocol v5 (most brokers and clients currently still only support v3 as of Sept 2018) includes something called Shared Subscriptions* that can be used to round-robin deliver messages on a give topic to a group of clients so only 1 of the set will receive this message, but this does not prevent clients not part of the group from also receiving the message.

The last message with the retained flag set published to a topic will be delivered to all clients at the point they subscribe to the topic. This message can be cleared by publishing a new message with a null payload and the retained flag set. A client could publish a message like this as soon as it receives the retained message but there would still be a timing window where other clients may subscribe and receive the retained message.

*some v3 brokers have implemented propriety versions of this.

Upvotes: 0

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