Is it necessary to create a topic filter?

[njourdane]

Here is the code sample from the readme:

async with Client("test.mosquitto.org") as client:
    async with client.filtered_messages("floors/+/humidity") as messages:
        await client.subscribe("floors/#")
        async for message in messages:
            print(message.payload.decode())

I don't understand why both client.filtered_messages("floors/+/humidity") and client.subscribe("floors/#") are required here, since mqtt topics are already filtered by the mqtt broker. As far as I understand, we could just subscribe to client.subscribe("floors/+/humidity").

So I would like to understand if the client.filtered_messages is really necessary and why/when to use it.

[frederikaalund]

Good question! :)

Indeed, you can just use subscribe("floors/+/humidity") combined with unfiltered_messages. This way, you get all messages that match the floors/+/humidity topic.

For some use cases, you want to subscribe to a general topic such as floors/# but handle the messages differently based on the actual topic of each received message. For this purpose, you use filtered_messages("floors/basement/#"), filtered_messages("floors/2nd-floor/#"), and then a single unfiltered_messages for the rest (everything from floors/# that isn't explicitly floors/basement or floors/2nd-floor). Does it make sense?

[njourdane]

For some use cases, you want to subscribe to a general topic such as floors/# but handle the messages differently based on the actual topic of each received message.

Hmm, I don't really understand because we could subscribe to both floors/basement/# and floors/2nd-floor/# independently and to tasks according to our needs. But I'm new to mqtt and I might miss some use cases.

I don't need filters anyway, but if I remove the line with the client.filtered_messages, I don't know how to handle incoming messages... Could you provide a few lines of code with a topic subscription without a filter?

[frederikaalund]

Hmm, I don't really understand because we could subscribe to both floors/basement/# and floors/2nd-floor/# independently and to tasks according to our needs

True, and you get all of said messages interleaved. We use filtered_messages to deinterleave the messages. Of course, if your use case doesn't require deinterleaving, then just go ahead and use unfiltered_messages. :+1:

I don't need filters anyway [...]

Great, just use unfiltered_messages. :) Something like:

async with Client("test.mosquitto.org") as client:
    async with client.unfiltered_messages() as messages:  # Receive all messages
        await client.subscribe("#")  # Tell the server to send me all messages
        async for message in messages:
            print(message.payload.decode())

[njourdane]

Thank you! Maybe you could add this explanation in the readme?

[frederikaalund]

Glad to help. :)

You're welcome to make a PR that improves the documentation. :+1: That's a nice way to contribute to this project and help others avoid the same situation.

[empicano]

I'll close this off, as this is covered in the documentation now.