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08/16/2019
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Hi Team,
Consider scenario where a Subscriber is using MQTT-JSON profile and expect the DataSetMetadata messages to be received via MetadataQueue.
The metadata is required to be set in the DataSetReader for it to decode the DataSet messages from a DataSetWriter.
If I am creating a PubSubConfiguration, unless I have metadata, I cannot set it in DataSetReader and hence cannot complete creation of PubSubConfiguration. But unless I complete PubSubConfiguration, I cannot start receiving the metadata messages on the MetadataQueue.
Does the specification expect that the Subscriber knows Metadata before hand, and then use it to create DataSetReader? Then what is the use of having MetadataQueue? How do we handle this scenario?
Thanks,
Deepali
05/30/2017
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The WG is developing a standard MQTT topic tree which would allow a Subscriber to discover metadata by using MQTT wildcards.
This would eliminate the ‘chicken and egg’ problem which currently exists.
08/16/2019
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Okay .. will the connection to MQTT metadata topic be independent of the rest of the PubSubConfiguration .. so that metadata can be received before creating PubSubConfiguration?
05/30/2017
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The metadata topics will have different information at different levels.
At the datasetwriter level there will be a topic that only reports the datasetmetadata so changes to the data in the messages can be detected.
At the writergroup level there will the complete configuration for the the writergroup and any contained dataset writers.
You should be able to construct a PubSubConfiguration from the pieces of configuration that are made available.
You do not need to know the topics in advance if you use MQTT wildcard subscriptions.
i.e. /+/+/+/metadata
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