MQTT 笔记

MQTT

 

MQTT Over WebSockets
Websockets allows you to receive MQTT data directly into a web browser.

 

Q- Are messages stored on the broker?
A- Yes but only temporarily. Once they have been sent to all subscribers they are then discarded. But see next question.

 

With a non clean session the broker will remember client subscriptions and may hold undelivered messages for the client.

 

The last Will Message is stored on the broker and sent to any subscribing client (to that topic) if the connection to the publisher fails.
If the publisher disconnects normally the last Will Message is not sent.

 

There is no direct connection between a publisher and subscriber.

 

All clients can publish and subscribe.

 

MQTT clients publish a keepalive message at regular intervals (usually 60 seconds) which tells the broker that the client is still connected.

 

If you attempt to connect to an MQTT broker with the same name as an existing client then the existing client connection is dropped.
Because most MQTT clients will attempt to reconnect following a disconnect this can result in a loop of disconnect and connect.

 

MQTT specifies Quality of Service (QoS) levels to support message reliability.
0 - at most once, 1 - at least once, and 2 - exactly once.

 

Bridge: A connection between two MQTT brokers