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p.cradwick · Posted: Tue Jul 07, 2009 6:23 pm Post subject: MQTT Pub/Sub problem

We have a Pub/Sub system using MQTT to communicate with mobile clients. This has been working ok for the last couple of years. Last week however client A, registered on the broker as topic A, received a message for client B - topic B. Client B moreover had received this message correctly 6hrs earlier! We are not using 'retained' publications; my understanding is that the broker discards the message once it has been received by all subscribers (B only).

When this happened, client A had only just started a new shift. He had connected to the broker successfully and received a genuine message for topic A. One second later he received the message for topic B.
Both these messages were logged as having QoS=18!! which is not a valid value (only 2 bits; we use QoS=2).

My question is this:
Is there any scenario whereby client A could receive a message published for client B 6hrs earlier?
I am assured that clients cannot subscribe to any topic other than their own.

I can't guarantee that this hasn't happened before, but on this occaision it was glaringly obvious as client A turned up to do clients B's work and found it already done on the last shift.

Peter