A client (subscriber) can tag his subscription topics so that when it receives a PUBLISH he can know which subscriptions were matched by the triggering publish.
MQTT 5 specification (relevant to server implementation)
Publish properties 3.3.2.3.8
The Subscription Identifier can have the value of 1 to 268,435,455.
⚠️ It is a Protocol Error if the Subscription Identifier has a value of 0.
Multiple Subscription Identifiers will be included if the publication is the result of a match to more than one subscription, in this case their order is not significant.
Publish action 3.3.4
The PUBLISH packet includes the Subscription Identifier carried in the SUBSCRIBE packet, if there was one.
📓 If the Client specified a Subscription Identifier for any of the overlapping subscriptions the Server MUST send those Subscription Identifiers in the message which is published as the result of the subscriptions [MQTT-3.3.4-3].
📓 If the Server sends a single copy of the message it MUST include in the PUBLISH packet the Subscription Identifiers for all matching subscriptions which have a Subscription Identifiers, their order is not significant [MQTT-3.3.4-4].
📓 If the Server sends multiple PUBLISH packets it MUST send, in each of them, the Subscription Identifier of the matching subscription if it has a Subscription Identifier [MQTT-3.3.4-5].
It is possible that the Client made several subscriptions which match a publication and that it used the same identifier for more than one of them. In this case the PUBLISH packet will carry multiple identical Subscription Identifiers.
✋ If the subscription was shared, then only the Subscription Identifiers that were present in the SUBSCRIBE packet from the Client which is receiving the message are returned in the PUBLISH packet.
Subscribe properties 3.8.2.1.2
The Subscription Identifier can have the value of 1 to 268,435,455.
⚠️ It is a Protocol Error if the Subscription Identifier has a value of 0.
⚠️ It is a Protocol Error to include the Subscription Identifier more than once.
The Subscription Identifier is associated with any subscription created or modified as the result of this SUBSCRIBE packet.
If there is a Subscription Identifier, it is stored with the subscription. If this property is not specified, then the absence of a Subscription Identifier is stored with the subscription.
Subscribe action 3.8.4
The Subscription Identifiers are part of the Session State in the Server and are returned to the Client receiving a matching PUBLISH packet. They are removed from the Server’s Session State when the Server receives an UNSUBSCRIBE packet, when the Server receives a SUBSCRIBE packet from the Client for the same Topic Filter but with a different Subscription Identifier or with no Subscription Identifier, or when the Server sends Session Present 0 in a CONNACK packet.
The Server need not use the same set of Subscription Identifiers in the retransmitted PUBLISH packet. The Client can remake a Subscription by sending a SUBSCRIBE packet containing a Topic Filter that is identical to the Topic Filter of an existing Subscription in the current Session. If the Client remade a subscription after the initial transmission of a PUBLISH packet and used a different Subscription Identifier, then the Server is allowed to use the identifiers from the first transmission in any retransmission. Alternatively, the Server is allowed to use the new identifiers during a retransmission. The Server is not allowed to revert to the old identifier after it has sent a PUBLISH packet containing the new one.
Task list
[x] validation of subscription identifier in SUBSCRIBE. #803
[x] store Subscription identifier in SubscriptionDirectory, for non-shared and shared subscriptions. #804
if the subscription identifier is not present in subscribe then remove it.
during the match of topic filter return also the corresponding susbcription identifers for the various subscriptions.
[x] persist and load subscription identifier into the state of the session. #805
[x] add the matched subscription identifiers as properties to the corresponding PUBLISH and return the feature in connect ACK. #807
General description
A client (subscriber) can tag his subscription topics so that when it receives a PUBLISH he can know which subscriptions were matched by the triggering publish.
MQTT 5 specification (relevant to server implementation)
Publish properties 3.3.2.3.8
Publish action 3.3.4
[MQTT-3.3.4-3].[MQTT-3.3.4-4].[MQTT-3.3.4-5].Subscribe properties 3.8.2.1.2
Subscribe action 3.8.4
Task list
SUBSCRIBE. #803PUBLISHand return the feature in connect ACK. #807