Feature request: publish to topic excluding specific client(s)

[jtalbourdet]

Hi,

I'm currently using Mochi MQTT as an embedded broker and writing a custom hook to handle bulk messages.

In this hook, I intercept messages sent to the topic bulk and split them into separate messages, publishing them to sensor/... topics. Everything works fine, except for one missing feature: I would like to publish to a topic while excluding the client who sent the original message.

Here’s what I'm trying to do:

1. Client A publishes to bulk with a JSON payload (e.g. { "temp": 22, "hum": 60 })
2. My hook publishes:
   • sensor/temp → 22
   • sensor/hum → 60
3. But I want to avoid sending those messages back to client A.

So far, I’ve tried:

• Using srv.Publish(...): works, but always delivers to all subscribers, including the source.
• Using InjectPacket(...) with a fake client, but it's difficult to construct a valid *mqtt.Client from outside the package (some fields are private, some required interfaces are internal).

Question:

Is there currently a supported way to publish a message to a topic and exclude one or more clients from receiving it?

If not, would you consider adding an API like:

server.PublishExclude(topic string, payload []byte, retain bool, qos byte, excludeIDs []string)

or perhaps allow InjectPacket() to accept a special internal "server" client that bypasses the sender?

Thanks a lot for the great work on this broker — it's lightweight, performant, and very developer-friendly 🙌

Best regards,

[thedevop]

Hi @jtalbourdet,

You can use OnSelectSubscribers(subs *Subscribers, pk packets.Packet) to alter the subscribers the message will be published to.

You can use cl := s.NewClient(nil, "local", "inline", true) to create a client to use InjectPacket(...). See #428.

[kabili207]

You can use OnSelectSubscribers(subs *Subscribers, pk packets.Packet) to alter the subscribers the message will be published to.

OnSelectSubscribers is only called for shared subscriptions, an MQTT 5 feature that is used for client load balancing and is not useful for filtering clients from the list of receivers.

What you can do is use the OnACLCheck(cl *mqtt.Client, topic string, write bool) bool hook. This is called when a client subscribes, when a client publishes, AND when a packet is published to a client. This does require some careful planning and requires you to set up your topics to include some kind of ID that is also in the client ID. The flow basically goes like this:

• Clients subscribe to sensor/+/temp
• Client A sends packet to bulk
• Your hook does it's processing and sends to sensor/client_a/temp
• OnACLCheck parses the topic, extracts the client ID, and compares it to the ID of the mqtt.Client. If it's a match, return false. If it's the + wildcard, just do the normal ACL checks you're already doing.

I had to do something very similar in my own project, and this worked perfectly