Table of Contents
What is MQTT
MQTT Data Flow by Examples
Based on this image the following examples can show typical data flows
Example: 1 Publisher, no Subscriber
- Client X connected
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)
Example: 1 Publisher, 1 Subscriber
- Client A connected
- Client B connected and subscripes Topic 2
If Client A publishes any data with Topic 2 and Client B is notified instantly about change of Topic 2.
Example: Many Publisher, Many Subscriber
- Client A, C, E connected
- Client B connected and subscripes Topic 2
- Client F connected and subscripes Topic 3
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.
Example: Many Publisher, 1 Subscriber with Wildcard
- Client A, C, E connected
- Client D connected and subscripes Topic with '#'
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.
Example: Request and Answer
- Client E connected and subscripes Topic 4
- Client F connected and subscripes Topic 3
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.
Example: QoS-Flag - Quality of Service
- Client G connected
- Client H connected and subscripes Topic 5 with QoS
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.
Example: Remain-Flag
- Client Y was connected long time ago
- Client Z connects now and subscripes Topic 6
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 |
Topic Wording
There is no law that you have to name your topics in a special way. So the following names are (principle) correct:
- /MyHome/Kitchen/Temperature
- /MYHOME/KITCHEN/TEMPERATURE
- MyHome/Kitchen/Temperature
- MyHome.Kitchen.Temperature
- MyHomeKitchenTemperature
- MHKT
- /Temperature/Kitchen/MyHome
- 42
But there are some guidelines for names that all clinets can filter and handle subscriptions in an easy way:
- The topic begins with a '/'
- The topic do NOT end with a '/'
- A '/' seperates topic levels
- The topic levels starts with the most commen level
So the recommented name is:
- /MyHome/Kitchen/Temperature
Subscription Wildcard Examples
Exact Match
On subscribing “/MyHome/Kitchen/Temperature” you get:
- /MyHome/Kitchen/Temperature
/MyHome/Kitchen/Humity/MyHome/Kitchen/Fridge/Temperature/MyHome/Livingroom/Temperature/MyHome/Livingroom/Humity/MyGarden/Air/Temperature/MyGarden/Pool/Temperature
# Wildcard
On subscribing “/#“ you get:
- /MyHome/Kitchen/Temperature
- /MyHome/Kitchen/Humity
- /MyHome/Kitchen/Fridge/Temperature
- /MyHome/Livingroom/Temperature
- /MyHome/Livingroom/Humity
- /MyGarden/Air/Temperature
- /MyGarden/Pool/Temperature
On subscribing ”/MyHome/#“ you get:
- /MyHome/Kitchen/Temperature
- /MyHome/Kitchen/Humity
- /MyHome/Kitchen/Fridge/Temperature
- /MyHome/Livingroom/Temperature
- /MyHome/Livingroom/Humity
/MyGarden/Air/Temperature/MyGarden/Pool/Temperature
On subscribing ”/MyHome/Kitchen/#“ you get:
- /MyHome/Kitchen/Temperature
- /MyHome/Kitchen/Humity
- /MyHome/Kitchen/Fridge/Temperature
/MyHome/Livingroom/Temperature/MyHome/Livingroom/Humity/MyGarden/Air/Temperature/MyGarden/Pool/Temperature
+ Wildcard
On subscribing ”/MyHome/+/Temperature” you get:
- /MyHome/Kitchen/Temperature
/MyHome/Kitchen/Humity/MyHome/Kitchen/Fridge/Temperature- /MyHome/Livingroom/Temperature
/MyHome/Livingroom/Humity/MyGarden/Air/Temperature/MyGarden/Pool/Temperature
On subscribing “/+/+/Temperature” you get:
- /MyHome/Kitchen/Temperature
/MyHome/Kitchen/Humity/MyHome/Kitchen/Fridge/Temperature- /MyHome/Livingroom/Temperature
/MyHome/Livingroom/Humity- /MyGarden/Air/Temperature
- /MyGarden/Pool/Temperature
On subscribing “/+/+/+/Temperature” you get:
/MyHome/Kitchen/Temperature/MyHome/Kitchen/Humity- /MyHome/Kitchen/Fridge/Temperature
/MyHome/Livingroom/Temperature/MyHome/Livingroom/Humity/MyGarden/Air/Temperature/MyGarden/Pool/Temperature
