QoS Protocol Flows

This section specifies the sequences of messages which will flow between client and broker in order to implement the various Quality of Service levels. Refer to the QoS section for details of the supported Qualities of Service.

QoS 0 - "at most once"

Client	Message and direction	Broker
QoS = 1	PUBLISH ---------->	Action: publish message to subscribers

QoS 1 - "at least once"

Client	Message and direction	Broker
QoS = 1, DUP=0, Message ID = x	PUBLISH ---------->	Action: publish message to subscribers
Action: discard message	PUBACK <----------	Message ID = x

If the client does not receive the PUBACK within some application-specific timeout period, or if a failure is detected and the communications session has to be re-established, the client will resend the PUBLISH message with the DUP flag set.

Upon receipt of a DUPlicate message from the client, the broker will publish the message to the subscribers again, and send another PUBACK message.

QoS 2 - "exactly once"

Client	Message and direction	Broker
QoS = 2, DUP=0, Message ID = x	PUBLISH ---------->	Action: log message to persistent store
	PUBACK1 <----------	Message ID = x
Message ID = x	PUBACK2 ---------->	Action: publish message to subscribers
Action: discard message	PUBACK3 <----------	Message ID = x

Each part of the protocol flow will be retried with the DUP bit set in the event of a detected failure, or a timeout. The additional protocol flows ensure that the message is only delivered to subscribers once and once only.

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Last Modified: 26-Mar-99