Quality of Service

MQTT Quality of Service

There is a good explanation of the basic concept of Quality of Service in the MQTT protocol at the HiveMQ page. The main are

• Only the client sets the QoS level, that's the case for publishing as well as for receiving subscribed messages. In DK's components the QoS levels are defined as:

  type QoS_Level is (At_Most_Once, At_Least_Once, Exactly_Once);

  • At_Most_Once is level 0
  • At_Least_Once is level 1
  • Exactly_Once is level 2

• The packet identifier is a unique number per client. The packet identifier is needed for the acknowledge protocol between the client and the broker. As the QoS At_Most_Once doesn't use any acknowledgement it doesn't need a packet identifier. Accordingly Dmitry defines a type Packet_Identification as a variant record:

  type Packet_Identifier is new Unsigned_16;
  
  type Packet_Identification (QoS : QoS_Level := At_Most_Once) is
  record
  case QoS is
     when At_Most_Once =>
        null;
     when At_Least_Once | Exactly_Once =>
        ID : Packet_Identifier;
  end case;
  end record;