Shared subscription delivery behaviour where QoS differs

Comment from Takatoshi 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:

—

·        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].

·        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, SUBSCRIBE  $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