[OASIS Issue Tracker] (MQTT-565) Shared subscription delivery behaviour where QoS differs

[OASIS Issues Tracker <workgroup_mailer@lists.oasis-open.org>]

Richard Coppen created MQTT-565:
-----------------------------------

             Summary: Shared subscription delivery behaviour where QoS differs
                 Key: MQTT-565
                 URL: https://issues.oasis-open.org/browse/MQTT-565
             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
          Issue Type: Bug
          Components: core
    Affects Versions: 5
            Reporter: Richard Coppen


Comment fromÂTakatoshi on MQTT comments list

I have a question about Shared Subscritions.
 
 In 4.8.2 Shared Subscriptions
 https://urldefense.proofpoint.com/v2/url?u=https-3A__docs.oasis-2Dopen.org_mqtt_mqtt_v5.0_os_mqtt-2Dv5.0-2Dos.html-23-5FToc3901250&d=DwIFaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=HFOYf-FWdPMlPq2g5pc8M7HUYiSp2sC9P2hDoZdjdRo&m=n9Lr__8q3-_IUeX0hPXa8E8WDT03yjgYr1cGDV-rcdU&s=QasW28pSWOMwxYmNiXVKaK6uvdmrWOlPtHWqUZGVTig&e= 
 ,
 
 The spec said that as follows:
 
 ---
 
 ÂÂ Â Â Â If the Server is in the process of sending a QoS 2 message
 to its chosen subscribing Client and the connection to the Client
 breaks before delivery is complete, the Server MUST complete the
 delivery of the message to that Client when it reconnects
 [MQTT-4.8.2-4] as described in section 4.3.3. If the Client's Session
 terminates before the Client reconnects, the Server MUST NOT send the
 Application Message to any other subscribed Client [MQTT-4.8.2-5].
 
 ÂÂ Â Â Â If the Server is in the process of sending a QoS 1 message
 to its chosen subscribing Client and the connection to that Client
 breaks before the Server has received an acknowledgement from the
 Client, the Server MAY wait for the Client to reconnect and retransmit
 the message to that Client. If the Client'sSession terminates before
 the Client reconnects, the Server SHOULD send the Application Message
 to another Client that is subscribed to the same Shared Subscription.
 It MAY attempt to send the message to another Client as soon as it
 loses its connection to the first Client.
 
 ---
 
 What happened the following case ?
 
 Client A: Session Expiry Interval 10 seconds, SUBSCRIBE $share/sn1/t1 QoS 1
 Client B: Session Expiry Interval 10 seconds, SUBSCRIBEÂ $share/sn1/t1 QoS 2
 
 A broker use a round robin algorithm. The next target is Client A.
 
 1. Publisher send a message to t1 and its QoS is 2.
 2. The broker send PUBLISH the message to Client A.
 3. Client A disconnected before send PUBACK.
 4. The broker set 10 seconds timer to expirer (terminate) the Session
 for Client A.
 5. The timer is fired (before Client A reconnects).
 
 The broker SHOULD send the message to the Client B ?
 
 Consider another case.The next target is Client B.
 
 1. Publisher send a message to t1 and its QoS is 2.
 2. The broker send PUBLISH the message to Client B.
 3. Client B disconnected before send PUBREC.
 4. The broker set 10 seconds timer to expirer (terminate) the Session
 for Client B.
 5. The timer is fired (before Client B reconnects).
 
 The broker MUST NOT send the message to the Client A ?
 
 It is weird for me that the broker's behavior is different. It depends
 on the delivery target QoS.
 
 Am I missing something ?
 
 I personally think that if the delivery target is decided once,
 delivery target shouldn't be changed even if the Session is
 terminated.
 That is QoS2 behavior. If QoS1 is the same, it is simple.
 
 Why the behavior of QoS1 and QoS2 are diffferent ?
 
 Thanks,
 Takatoshi



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