A Few Notes on MQTT
MQTT decouples SUBSCRIBERS for PUBLISHERS
- Anyone can PUBLISH and any one can SUBSCRIBE to the data. As long as the data is in the understandable format, the exchange will be smooth.
MQTT allows wildcard subscriptions
Some of the Published data can be as below
scores/football/big12/Texas
scores/football/big12/TexasTech
scores/football/big12/Okalahoma
scores/football/big12/IowaState
Everyone of the above will PUBLISH to the MQTT broker.
The interested parties and their SUBSCRIPTION can be below
Texas Fan => scores/football/big12/Texas
Big12 Fan => scores/football/big12/+
ESPN => scores/#
+ represents single level wild card
# represents multi level wild card
Some of the network properties of MQTT are:
Small Packet Size:
PUBLISH is 2-4 bytes
CONNECT is 14 bytes
HTTP is 0.1 - 1 KB
the headers is binary payload and it is not text
Small clients that has 30KB foot print and as well as huge memory foot print clients can be well supported at the same time
Minimal protocol exchanges between ends. MQTT has a configurable keep alive 2 byte PINGREQ/PINGRES
MQTT supports QoS for reliable messaging:
There are 3 QoS levels
1. QoS 0: at most once (PUBLISH)
- Doesn’t survive failure
- Doesn’t repeat
2. QoS 1: At least once (PUBLISH, PUBLIC ACK)
Survives connection loss
can be duplicated
3. QoS 2 : exactly once (PUBLISH, PUBREC, PUBREL, PUBCOMP)
Service connection loss,
Never duplicated
References:
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