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Re: [paho-dev] Message re-delivery in QOS1 and QOS2

1) When the connection is reestablished (as per the MQTT 3.1.1 standard)

2) There would be no redelivery.  If the TCP connection does not break, then messages will be delivered.  If messages are not delivered, or responses not received, then the connection will be broken by client or server, assuming that the keepalive timeout is not set to 0.


On 08/21/2016 01:46 PM, Nidhi Kushwaha wrote:
Thanks for the clarifications Ian.

I have two additional follow up questions -

When publishing the message at QOS1/QOS2( with stateful session).
1.If the QOS hand shakes are not complete then when the message re-delivery will be attempted by Paho?
2.Assuming a ideal scenario with no TCP connection break between publishing client and broker, message re delivery takes place in same session ?


On Tue, Aug 16, 2016 at 9:30 PM, <paho-dev-request@xxxxxxxxxxx> wrote:

Message: 1
Date: Tue, 16 Aug 2016 00:39:15 +0530
From: Nidhi Kushwaha <nidhii.kushwaha@xxxxxxxxx>
To: paho-dev@xxxxxxxxxxx
Subject: [paho-dev] How to use the cleanSession parameter and
        persistence mechanism for mqtt publisher(publishing messages at QOS 1)


I have recently started learning eclipse Paho(Java client).And I have
executed the MqttPublishSample from

I have few doubts regarding the cleanSession parameter and persistence

1.If a MQTT client is only publishing messages to topics with QOS 1, does
the cleanSession parameter need to set as false ?
Session maintenance is required?If yes why?

2.How persistence mechanism helps in achieving QOS 1 for a MQTT Publisher?

2.1 In what way MemoryPersistence mechanism helps in achieving QOS.

2.2 If MQTT client disconnects(connection lost) during publishing, what
will be the role of persistence mechanism w.r.t QOS 1.
For the published messages for which PUBACK message is not received?Paho
API will handle the QOS 1 requirement?

2.3 How a MQTT async publisher will know QOS1 is not achieved for
particular message.i.e PUBACK message is received.

3.In order to use persistence mechanism do I need to set the cleanSession
parameter as true? I want to know if there
is any co-relation between the two? or both are independent feature.

Note:- All the questions I have asked is from the view point of developing
a MQTT  *Publisher client.*



Message: 2
Date: Tue, 16 Aug 2016 11:25:29 +0100
From: Ian Craggs <>
To: paho-dev@xxxxxxxxxxx
Subject: Re: [paho-dev] How to use the cleanSession parameter and
        persistence mechanism for mqtt publisher(publishing messages at QOS 1)


1)  If the cleansession parameter is set to true, this means that in the
event of an accidental break of the TCP connection, the QoS 1 messages
which were being sent at the time of the break will not be reattempted.
This means that some QoS 1 messages could be lost.

2) The persistence mechanism stores the state if the application shuts
down.  When the application restarts, the client library will continue
processing the unsent messages.

2.1) This will help in the case of a TCP connection break, but the
application does not shut down (or die).

2.2) If cleansession is set to false, yes.

2.3) A callback will be made when the puback is received, or the token
will complete.  If no puback is received, then these notifications will
not occur.

3) No.  You need to set the cleansession parameter to false.  If it is
set to true, then persistence is not needed, as session state is
discarded on any TCP connection break.


End of paho-dev Digest, Vol 56, Issue 8

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Ian Craggs                          
icraggs@xxxxxxxxxx                 IBM United Kingdom
Paho Project Lead; Committer on Mosquitto

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