[question] Is there a way to list all topics any client is subscribed to?

I’d like to know I the broker can get a list of all topics at least one client is subscribed to.

I need to compute some stuff before publishing the data, therefore I’d like to eliminate unnecessary computation.

Imagine that we have several topics the clients can subscribe to and each of these topics is appended with /i where i is an integer 1..n (e.g. topic_a/1, topic_b/50). If no client is subscribed to topic_a/2, there is no reason to compute the data and publish it.

Thanks in advance! 😉

[robertsLando]

When I publish two (or more) retained messages for the same topic, only the first one is send to new clients, while I want to send only the last (latest) message and forget about the others.

This statement is incorrect. New clients will always get last retained message sent to a specific topic, that’s what a retained message is, when a new one comes it overrides the previous

[robertsLando]

The script is wrong:

client.on('machine/1/light_indicator/current', (topic, payload) => {
  console.log(topic, ':', payload)
  client.unsubscribe('machine/1/light_indicator/current')
})

Should be

client.on('message', function(topic, message, packet) {
  if (topic === 'machine/1/light_indicator/current') {
    console.log(topic, ':', payload)
    client.unsubscribe('machine/1/light_indicator/current')
  }
});