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mqttmosquitto

mqtt timestamp in the topic name: anti-pattern?

Would the mqtt community consider placing message information in the topic name an anti-pattern?

I have a client that has a vast library written around rabbitmq, and I'm trying to tweak their client and server code to allow them to configure their services for mosquitto instead. One central requirement for them is TTL, the clients can sometimes sit for hours publishing data before the server comes back online and they do not want messages to show up that are beyond their TTL.

Their message envelope system is an elaborate json and 1) it would be painful to wrap or alter this json 2) I do not want to incur the expense of unmarshalling json to retrieve a timestamp.

The easiest thing to do is place the timestamp at the end of the topic and consume with wildcards: mytopic/mysubtopic/{timestamp} consumed by mytopic/mysubtopic/#

Are there any unintended consequences for this, and would this be considered an anti-pattern?

Solution

  • Whether this is an anti-pattern is a matter of opinion; the spec defines the topic as "The label attached to an Application Message..." so does not preclude your usage. I can think of a few potential "unintended consequences" to your approach (which may, or may not, apply to your specific situation):

    • Retain flag: As per your comment you will not be able to set the Retain flag to 1 (because all messages would be retained).
    • Latest Message only when comms re-established: A subscriber may only want the latest message when communications are re-established. This can be achieved by publishing messages with the retain flag set to 1 which results in your subscriber receiving the latest message (and only the latest message; subject to QOS/CleanSession) on each topic it subscribes to (docs). As per the above this will not work with your topic structure.
    • Order of delivery: the spec requires that "A Server MUST by default treat each Topic as an 'Ordered Topic'" but there is no such guarantee across topics. Note that ordered delivery is dependent upon settings (see the "Non normative comment" in the spec) so this may not be an issue.
    • Topic Alias: MQTT V5 introduces Topic Alias which can be used to reduce the amount of data transmitted. This will not provide a benefit with your structure.