Based on this image the following examples can show typical data flows
If Client X publishes any data with Topic 1 and nobody is listening (nobody has subscriped Topic 1) the data is lost.
(Exception: see Topics with Retain Flag)
If Client A publishes any data with Topic 2 and Client B is notified instantly about change of Topic 2.
Any Client can publish any data with any Topic at anytime. The subscribing Clients are notified instantly about change of the Topics they subscripes and only that.
Any Client can publish any data with any Topic at anytime. A Client can subcripe with '+' and/or '#' in the Topic name. The Client is notified instantly about any change of all Topics which matches the wildcard Topic name.
Every Client can be publisher and subscriber. For sneding a request/command one Topic can be used. For the answer an other Topic is used.
Note: This can be solved with only one Tpoic but the publisher would receive his own request/command.
A Client can publish data with any Topic with flag QoS set. This garantees that the data arrives the broker.
A Client can subscripe with flag QoS set. This garantees that the data arrives the clinet.
| QoS | Meaning |
|---|---|
| 0 | no garantee (default) |
| 1 | garantee that data arrives |
| 2 | garantee that data arrives only once |
Note: Each publisher and each subscriber can user the QoS flag without affecting the others.
Ages ago: A Client publish any data with a Topic and Remain flag set. The broker stores the Topic with data in its own database/file. The publishing Client died…
Now: An other Client ist started and subscripes a Topic. The Client is notified instantly about the stored data of the Topic
| Remain | Meaning |
|---|---|
| 0 | fire and forget |
| 1 | saved on broker |
There is no law that you have to name your topics in a special way. So the following names are (principle) correct:
But there are some guidelines for names that all clinets can filter and handle subscriptions in an easy way:
So the recommented name is:
On subscribing “/MyHome/Kitchen/Temperature” you get:
On subscribing “/#“ you get:
On subscribing ”/MyHome/#“ you get:
On subscribing ”/MyHome/Kitchen/#“ you get:
On subscribing ”/MyHome/+/Temperature” you get:
On subscribing “/+/+/Temperature” you get:
On subscribing “/+/+/+/Temperature” you get: