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Subject: [OASIS Issue Tracker] (MQTT-269) MQTT-SN Feature: Topic Registration

    [ https://issues.oasis-open.org/browse/MQTT-269?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=63219#comment-63219 ] 

Peter Niblett commented on MQTT-269:

The current (July 16th) proposal is commendably simple, but you only get any benefit from it if the client sends multiple consecutive messages to the same topic across the same connection.  This is ok of course in the case where a client only ever uses one topic, but there are cases where a client might want to use more than one topic.  For example

i) A sensor device that can report more than one kind of event might well be coded to use separate topics for different event types, so that different consumers can subscribe to receive different types of event.

ii) An field area network device that connects multiple sensors to a remote MQTT server. The gateway is coded to use separate topics so that different consumers can subscribe to receive data from different sensors.

In both cases, I think it's reasonable for the client to want to publish to these topics on a common MQTT connection, rather than to create separate MQTT connections for each topic. 

There are several ways in which the proposal could be extended to accommodate these cases, for example

1. Have a new "metadata" property on publish which gives an alias for the full topic name.  This alias is scoped to the connection within which it is being used. You would use short aliases, for example 1,2,3...   If the Publish includes a non-zero length topic name (with or without the alias property being present) the message is published on that topic. If the Publish includes both the alias value and a non-zero lengthl topic name then it also establishes the alias->topic mapping for the current connection (replacing it if it already exists).  If the Publish includes the alias value and a zero length Topic name and an alias->topic mapping has been established then that topic name is used.  If the Publish has an alias value and a zero length Topic name but the alias->topic mapping hasn't been established, then an error is returned


2.  Have a metadata property, or special topic format, that specifies "topic -n". Zero length topic string means the last topic used (topic -1) as in the current proposal, but topic -2 would mean the preceding distinct topic, etc.

> MQTT-SN Feature: Topic Registration
> -----------------------------------
>                 Key: MQTT-269
>                 URL: https://issues.oasis-open.org/browse/MQTT-269
>             Project: OASIS Message Queuing Telemetry Transport (MQTT) TC
>          Issue Type: Improvement
>          Components: futures
>    Affects Versions: 5
>            Reporter: Ian Craggs
>            Assignee: Andrew Banks
> MQTT-SN allows topic names to be registered with the server (and client) so that a topic id only has to be sent on subsequent packets.  This can reduce the data flowing on the wire and is consistent with the goals of MQTT.  It would however complicate the packet flows.

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