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What is MQTT

… see Wikipedia

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 subscribed Topic 1) the data is lost.

(Exception: see Topics with Retain Flag)

Example: 1 Publisher, 1 Subscriber

Client A connected
Client B connected and subscribes 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 subscribes Topic 2
Client F connected and subscribes Topic 3

Any Client can publish any data with any Topic at anytime. The subscriping 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 subscribes 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 subscribes Topic 4
Client F connected and subscribes Topic 3

Every Client can be publisher and subscriper. 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 subscribes 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 subscriper can user the QoS flag without affecting the others.

Example: Remain-Flag

Client Y was connected long time ago
Client Z connects now and subscribes 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

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