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

MQTT v3.1.1

session is controlled by clean_session flag of CONNECT packet.

Note
 the broker could expire the session by implement limitation.

CONNECT with clean_session 0

the broker has the previous session

  • The broker returns CONNACK with session_present 1.

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

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

Note
 MQTT v5.0 spec said that the client can send packets before CONNACK is received but CONNACK could be error, so waiting CONNACK is reasonable design choice.

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

the broker doesn’t have the previous session

  • The broker returns CONNACK with session_present 0.

  • The client clears halfway PUBLISH and PUBREL after 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 session during the connection and after disconnection.

CONNECT with clean_session 1

  • Both the client and the broker session are cleared.

  • The broker returns CONNACK with session_present 0.

  • Both the client and the broker don’t store session during the connection and after disconnection.

MQTT v5.0

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

Note
 the broker could expire the session by implement limitation.

clean_start

CONNECT with clean_start 0

the broker has the previous session

  • The broker returns CONNACK with session_present 1.

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

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

Note
 MQTT v5.0 spec said that the client can send packets before CONNACK is received but CONNACK could be error, so waiting CONNACK is reasonable design choice.
the broker doesn’t have the previous session

  • The broker returns CONNACK with session_present 0.

  • The client clears halfway PUBLISH and PUBREL after 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 and the broker session are cleared.

  • The broker returns CONNACK with session_present 0.

Session Expiry Interval

CONNECT with Session Expiry Interval is greater than 0

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

CONNECT without Session Expiry Interval is greater than 0

  • Both the client and the broker don’t store session during the connection and after disconnection.