QualityOfServiceUseCases

Some use cases to understand how quality of service levels should affect behaviour of MQTT clients and brokers.

QOS 0 – zero or more deliveries

Normal Flow

  • client sends a QoS 0 message

QOS 1 – one or more deliveries

Normal Flow

  • client sends a QoS 1 message (PUBLISH, SUBSCRIBE, UNSUBSCRIBE) and stores in Persistent Outbox
  • client receives {PUB,SUB,UNSUB}ACK and removes message from Outbox

Death before Acknowledgement

  • client sends a QoS 1 message (PUBLISH, SUBSCRIBE, UNSUBSCRIBE) and stores in Persistent Outbox
  • client dies before {PUB,SUB,UNSUB}ACK received
  • client restarts with same client id and recovers Persistent Outbox
  • client resends messages in Outbox
  • client receives {PUB,SUB,UNSUB}ACK and removes message from Outbox

Transfer Interrupted

  • client sends a QoS 1 message (PUBLISH, SUBSCRIBE, UNSUBSCRIBE) and stores in Persistent Outbox
  • broker dies before message is transferred
  • client reconnects with same client id
  • client resends messages in Outbox
  • client receives {PUB,SUB,UNSUB}ACK and removes message from Outbox

QOS 2 – exactly one delivery

Normal Flow

  • client sends a QoS 2 PUBLISH message and stores in Persistent Outbox
  • broker replies with PUBREC
  • client sends PUBREL
  • broker responds with PUBCOMP
  • client receives PUBCOMP and deletes message from Persistent Outbox

QOS 0 – zero or more deliveries

Normal Flow

  • client A connects and subscribes to topic T
  • client B connects and publishes to topic T with QoS 0
  • broker delivers to all connected subscribers, namely client A

Client Subscribes, then Disconnects

  • client A connects and subscribes to topic T
  • client A disconnects
  • client B connects and publishes to topic T with QoS 0
  • broker gets list of interested connected subscribers for topic T, namely client A
  • noop: there are no connected subscribers to topic T

QOS 1 – one or more deliveries

Normal Flow

  • client A connects and subscribes to topic T, requesting QoS 1
  • client B connects and publishes to topic T with QoS 1, placing message in Persistent Outbox
  • broker acknowledges receipt with a PUBACK to client B
  • client B removes message from Persistent Outbox
  • broker delivers message to client A, placing message in Persistent Outbox
  • client A acknowledges by sending a PUBACK to the broker
  • broker removes message from its Persistent Outbox

Published QoS > Requested QoS

  • client A connects and subscribes to topic T, requesting QoS 0
  • client B connects and publishes to topic T with QoS 1, placing message in Persistent Outbox
  • broker acknowledges receipt with a PUBACK to client B
  • client B removes message from Persistent Outbox
  • broker delivers message as QoS 0 to client A (see QoS 0 use cases above)

Client Subscribes, then Disconnects, then Reconnects

  • client A connects and subscribes to topic T, requesting QoS 1
  • client A disconnects
  • client B connects and publishes to topic T with QoS 1, placing message in Persistent Outbox
  • broker acknowledges message with a PUBACK sent to client B
  • client B removes message from Persistent Outbox
  • client A is not connected: broker marks message as ‘pending’ for client A
  • client A connects
  • broker delivers ‘pending’ message to client A and places it in Persistent Outbox
  • client A acknowledges message with a PUBACK
  • broker removes message from Persistent Outbox

QOS 2 – exactly one delivery

Normal Flow

  • client A connects and subscribes to topic T requesting a QoS of 2
  • broker receives message from client B with QoS 2 destined for topic T, places it in Persistent inbox
  • broker replies to client B with PUBREC
  • client B replies with PUBREL
  • broker receives PUBREL and removes the message from the Inbox
  • broker acknowledges client B with a PUBCOMP
  • broker retrieves list of subscribers to topic T, namely client A
  • broker delivers message to client A and adds to Outbox
  • client A replies with a PUBREC
  • broker replies with a PUBREL
  • client A replies with a PUBCOMP
  • broker receives PUBCOMP and removes messae from Outbox

Subscriber Connects, Disconnects and Reconnects

  • client A connects and subscribes to topic T requesting a QoS of 2
  • broker receives message from client B with QoS 2 destined for topic T
  • broker replies to client B with PUBREC
  • client B replies with PUBREL
  • broker receives PUBREL and may release the message
  • broker acknowledges client B with a PUBCOMP
  • client A disconnects
  • broker retrieves list of subscribers to topic T, namely client A
  • broker marks message as ‘pending’ for client A
  • client A reconnects
  • broker delivers ‘pending’ message to client A
  • client A replies with a PUBREC
  • broker replies with a PUBREL
  • client A replies with a PUBCOMP
  • broker receives PUBCOMP and removes messae from Outbox

Source

MQTT Quality of Service use cases

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