Publish

From MQTT client to AMQP network

The FE needs an attached link with sender role to the topic address. The QoS level for attaching link depends on the publishing QoS.

Actually two links are attached :

• one link shared between messages published with QoS levels 0 and 1.
• one link only for messages published with QoS 2, because recovering link could be needed.

The link for QoS levels 0 and 1 is attached with :

• rcv-settle-mode : first (0)
• snd-settle-mode : unsettled (0)

The link for QoS level 2 is attached with :

• rcv-settle-mode : second (1)
• snd-settle-mode : unsettled (0)

The MQTT client sends PUBLISH message to FE which maps the message to the following AMQP messages.

AMQP_PUBLISH : sent to the “topic” address

the "subject" system property is just empty for publishing messages (it's different from all the other messages which have a well defined "subject"). It's just because all the other messages are special AMQP messages, not just like a publishing one.

the MQTT QoS level is copied to the x-opt-mqtt-qos annotation. At same time the "durable" header field is set as FALSE if QoS level is 0, TRUE if QoS level is 1 or 2. When the x-opt-mqtt-qos annotations isn't present (i.e. AMQP_WILL published from a native AMQP client which doesn't add it), fallback to use only "durable" and if it's TRUE consider QoS level 1.

QoS level 0 (AT_MOST_ONCE)

For QoS level 0, no acknowledge is provided.

QoS level 1 (AT_LEAST_ONCE)

For QoS level 1, after disposition on the AMQP client, the FE sends the PUBACK to the MQTT client.

QoS level 2 (EXACTLY_ONCE)

For QoS level 2, the FE sends the AMQP_PUBLISH message as "unsettled" waiting for disposition (with "unsettled"); after receiving it, FE sends the PUBREC to the MQTT client and the first step in the acknowledge process ends. The MQTT client replies with PUBREL message; the FE sends the disposition (with "settled") waiting for the receiver disposition (with "settled") so that it can send the PUBCOMP to the MQTT client.

From the above images it's clear that the FE doesn't detach the link on the topic for each publish; it leaves the link attached.

A timeout could be used to detect links inactivity for detaching them.

From AMQP network to MQTT client

The FE has an attached link with receiver role to the unique client publish address $mqtt.to.[client-id].publish where all published messages are conveyed (see "Connection"). This link has QoS as AT_LEAST_ONCE so in all cases, the messages received from the AMQP network is "unsettled" (even for QoS level 0).

The FE receives published messages as AMQP_PUBLISH messages (see previous paragraph).

For handling QoS level 2, the FE attaches a link with sender role to the address $mqtt.[client-id].pubrel (for sending AMQP_PUBREL messages). It should have QoS as AT_LEAST_ONCE so with :

• rcv-settle-mode : first (0)
• snd-settle-mode : unsettled (0)

QoS level 0 (AT_MOST_ONCE)

For QoS level 0, message is published as "unsettled" but the FE can sends immediately the disposition (with "settled") before sending the PUBLISH to the remote MQTT client and without waiting for any acknowledge (of course, due to QoS level 0).

QoS level 1 (AT_LEAST_ONCE)

For QoS level 1, message is published as "unsettled". The FE sends the PUBLISH to the MQTT client waiting for PUBACK; after receving it, FE sends the disposition (with "settled") to acknowledge the message.

QoS level 2 (EXACTLY_ONCE)

For QoS level 2, message is published as "unsettled". The FE sends the PUBLISH to the MQTT client waiting for PUBREC; after receving it, FE doesn't settle the original AMQP_PUBLISH message yet. Instead, FE sends an AMQP_PUBREL message to the AMQP side. The first phase of QoS 2 exchange isn't finished yet because we need to be sure that before settling the original AMQP_PUBLISH message, the AMQP_PUBREL is available to the AMQP network. If something goes wrong, the AMQP_PUBLISH (not yet settled) is resent or the exchange restarts from sending the AMQP_PUBREL. So, the FE waits for disposition (with "settled") from the AMQP_PUBREL and then sends the disposition (with "settle") for the original AMQP_PUBLISH message. The first phase of QoS 2 exchange is ended. Whatever it will happen, the exchange will restart from the AMQP_PUBREL. In the second phase, the AMQP side publishes the AMQP_PUBREL to the FE, which sends PUBREL to the MQTT client and receives the PUBCOMP for then sending disposition (with "settled") to the AMQP side (for the AMQP_PUBREL).

AMQP_PUBREL : sent to the unique client address $mqtt.[client-id].pubrel