MQTT has the concept of a "session." See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901230 It contains subscription information, halfway PUBLISH (QoS1 and QoS2 only), and PUBREL packets. async_mqtt is automatically configured when the client sends a CONNECT packet and the broker receives it. All you need to do is set up the CONNECT packet and send it.

MQTT v3.1.1

The session is controlled by the clean_session flag of the CONNECT packet.

Note
 The broker could expire the session due to implementation limitations.

CONNECT with clean_session 0

the broker has the previous session

  • The broker returns CONNACK with session_present set to 1.

  • The broker sends halfway PUBLISH and PUBREL packets after the CONNACK is sent.

  • The client sends halfway PUBLISH and PUBREL packets after the CONNACK is received.

Note
 The MQTT v5.0 specification states that the client can send packets before the CONNACK is received, but since the CONNACK could indicate an error, waiting for the CONNACK is a reasonable design choice.

  • Both the client and the broker store the session during the connection and after disconnection.

the broker doesn’t have the previous session

  • The broker returns CONNACK with session_present set to 0.

  • The client clears halfway PUBLISH and PUBREL packets after the CONNACK is received.

Note
 The halfway packets are for the previous session. If the previous session doesn’t exist, the client-side halfway packets should be cleared.

  • Both the client and the broker store the session during the connection and after disconnection.

CONNECT with clean_session 1

  • Both the client’s and the broker’s sessions are cleared.

  • The broker returns CONNACK with session_present set to 0.

  • Both the client and the broker do not store sessions during the connection and after disconnection.

MQTT v5.0

The session is controlled by the clean_start flag and the Session Expiry Interval property of the CONNECT packet.

Note
 The broker could expire the session due to implementation limitations.

clean_start

CONNECT with clean_start 0

the broker has the previous session

  • The broker returns CONNACK with session_present set to 1.

  • The broker sends halfway PUBLISH and PUBREL packets after the CONNACK is sent.

  • The client sends halfway PUBLISH and PUBREL packets after the CONNACK is received.

Note
 The MQTT v5.0 spec states that the client can send packets before the CONNACK is received, but since the CONNACK could indicate an error, waiting for the CONNACK is a reasonable design choice.
the broker doesn’t have the previous session

  • The broker returns CONNACK with session_present set to 0.

  • The client clears halfway PUBLISH and PUBREL packets after the CONNACK is received.

Note
 The halfway packets are for the previous session. If the previous session doesn’t exist, the client-side halfway packets should be cleared.

CONNECT with clean_start 1

  • Both the client’s and the broker’s sessions are cleared.

  • The broker returns CONNACK with session_present set to 0.

Session Expiry Interval

CONNECT with Session Expiry Interval is greater than 0

  • Both the client and the broker store the session during the connection and after disconnection until the Session Expiry Interval seconds have passed.

CONNECT without Session Expiry Interval is greater than 0

  • Both the client and the broker do not store the session during the connection and after disconnection.