Why can't use wildcard in PUBLISH ?

[xu zhangxuan]

Hi all:

In my case, I want do something as following:

subs:

 client-1: A/B/C/D

 client-2: A/2/C/D

 client-3: A/B/3/D

first, I want push message to all clients subscribed to A/B (client-2 and client-3 in this case)

So I want to use A/B/+/+ as publish topic.

and then, I want push message to all clients subscribed to A/C (client-1 and client-2 in this case)

So I want to use A/+/C/+ as publish topic.

But using wildcard in publish message is not allowed. Any suggestions ?

---

Regards.

Zhangxuan Xu
plu...@gmail.com

[Roger Light]

Hi Xu,

The simple answer is that it doesn't work like that. None of your
clients are subscribed to A/B or A/C.

What you suggest sounds like a reasonable idea, so the question is why
it doesn't it work like that? In the current model, subscribers decide
what topics they listen to. The power to decide that lies with them
alone. In your scenario, the publisher now also controls what topics
subscribers are listening to, which is an important change and not
desirable from the subscriber point of view.

In your example, I would have subscriptions as follows:

client-1: A/C
client-2: A/B and A/C
client-3: A/B

I realise that you have only described a trivial example though.

Cheers,

Roger

[xu zhangxuan]

Hi Roger:

Thanks for your interpretation. As you say, In MQTT subscribers decide what topics they listen to.

But what I want is publisher decide who would receive the message. So it's totally different. Maybe I 

need make my own message breaker's subscribe-rule different from which defined in MQTT.

---

Regards.

Zhangxuan Xu
plu...@gmail.com

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[Roger Light]

Hi Xu,

> But what I want is publisher decide who would receive the message. So it's
> totally different.

The principle isn't completely different, but I think you're
approaching it from the opposite direction. You seem to be thinking
along the lines of "as a publisher, how do I force clients to receive
this message?". MQTT takes the view if a subscriber wants information,
they will subscribe to the topics that will provide it for them and
the publisher will publish to that topic. If you don't trust your
subscribers to subscribe to the correct topics, you can't assume that
they will process your message anyway. If you do trust your clients to
subscribe to the correct topics, then there is no need to force
messages upon them.

Cheers,

Roger