I am interested to know too
qos 2 is needed sometimes

Yes, implementing support for QOS 2 will increase the code size of the umqtt.simple module. Currently the publish method only handles QOS 0 and 1 and the logic for receiving PUBLISH packet from the server in the wait_msg method also only handles QOS 0 and 1.

For QOS 1 the method has to listen for a PUBACK packet from the server after it sends a PUBLISH packet.

For QOS 2 the client, after sending the PUBLISH packet, has to listen for a PUBREC packet from the server, then send a PUBREL packet and then listen for a PUBCOMP packet.

The simplistic way to implement this would be to handle all this in a synchronous, blocking fashion, i.e. for publishing client -> server, in the method publish replace the assert in line 142 with somethings like this (pseudo code):

while  True:
    opcode = wait_msg()
    if opcode == PUBREC:
        paket = read_packet()
        if paket.paket_id == packet__id:
            break
send_pubrel()
while  True:
    opcode = wait_msg()
    if opcode == PUBCOMP:
        paket = read_packet()
        if paket.paket_id == packet__id:
            break

And for receiving QOS 2 from server -> client, do something like this in the wait_msg method at line 197:

send_pubrec()
while  True:
    opcode = wait_msg()
    if opcode == PUBREL:
        paket = read_packet()
        if paket.paket_id == packet__id:
            send_pubcomp()
            break

The problem with this is:

• If the server doesn't send the PUBREC or PUBCOMP resp. the PUBREL package, the client will block forever (same it does currently with QOS 1, if no PUBACK is received)-.
• If the server sends any other packets with QOS 1 or 2 in between, which the spec allows, the client will get confused and probably end up in a dead lock.

The only way I see around these problems is handling reception of all packet types in wait_msg and passing them to different callback methods. But that would a) make a record of sent and received packet ids necessary (which increases memory requirements) and b) go against the stated goal of the module to avoid "callback hell".

@SpotlightKid Thank you so much!!
this was so helpful indeed, my gratitude.

@SpotlightKid Thank You ! that's pretty awesome

Is this still relevant? Are there other MQTT implementations that people know of that support QoS 2 ?