Quality Of Service

Supported by

Java script

SGC Default Protocol and MQTT implements a QoS (Quality of Service) for message delivery, there are 3 different types:

Level 0: "At most once", where messages are delivered according to the best efforts of the underlying TCP/IP network. Message loss or duplication can occur. This level could be used, for example, with ambient sensor data where it does not matter if an individual reading is lost as the next one will be published soon after.

Level 1: "At least once", where messages are assured to arrive but duplicates may occur.

Level 2: "Exactly once", where message are assured to arrive exactly once. This level could be used, for example, with billing systems where duplicate or lost messages could lead to incorrect charges being applied.

Level 0

The message is delivered according to the best efforts of the underlying TCP/IP network. A response is not expected and no retry semantics are defined in the protocol. The message arrives at the server either once or not at all.

The table below shows the QoS level 0 protocol flow.

Client	Message and direction	Server
QoS = 0	PUBLISH `---------->`	Action: Publish a message to subscribers

Level 1

The receipt of a message by the server is acknowledged by a ACKNOWLEDGEMENT message. If there is an identified failure of either the communications link or the sending device or the acknowledgement message is not received after a specified period of time, the sender resends the message. The message arrives at the server at least once.

A message with QoS level 1 has a Message ID in the message.

The table below shows the QoS level 1 protocol flow.

Client	Message and direction	Server
QoS = 1 Message ID = x Action: Store message	PUBLISH `---------->`	Actions: Store message Publish a message to subscribers Delete message
Action: Discard message	ACKNOWLEDGEMENT `<----------`

If the client does not receive an ACKNOWLEDGMENT message (either within a time period defined in the application, or if a failure is detected and the communications session is restarted), the client may resend the PUBLISH message.

Level 2

Additional protocol flows above QoS level 1 ensure that duplicate messages are not delivered to the receiving application. This is the highest level of delivery, for use when duplicate messages are not acceptable. There is an increase in network traffic, but it is usually acceptable because of the importance of the message content.

A message with QoS level 2 has a Message ID in the message.

The table below shows the QoS level 2 protocol flow. There are two semantics available for how a PUBLISH flow should be handled by the recipient.

Client	Message and direction	Server
QoS = 2 Message ID = x Action: Store message	PUBLISH `---------->`	Action: Store message
	PUBREC `<----------`	Message ID = x
Message ID = x	PUBREL `---------->`	Actions: Publish a message to subscribers Delete message
Action: Discard message	ACKNOWLEDGEMENT `<----------`	Message ID = x

If a failure is detected, or after a defined time period, the protocol flow is retried from the last unacknowledged protocol message. The additional protocol flows to ensure that the message is delivered to subscribers once only.