ref-
http://www.steves-internet-guide.com/understanding-mqtt-qos-1/
The QOS levels are a way of guaranteeing message delivery and they refer to the connection between a broker and a client.
In this two part tutorial we will look in detail at the message flow when publishing using all three QOS levels.
We also look at the advantages and disadvantages of using the various levels
QOS 0 – Once
This is the fastest method and requires only 1 message. It is also the most unreliable transfer mode.
The message is not stored on the sender, and is not acknowledged.
The message will be delivered only once, or not at all.
Once the message has been sent by the client it is deleted from the outbound message queue.
Therefore with this QOS level there is no possibility of duplicate messages.
QOS 1 – At Least Once
This level guarantees that the message will be delivered at least once, but may be delivered more than once.
Publishing with QOS of 1 requires 2 messages.
The sender sends a message and waits for an acknowledgement (PUBACK).
If it receives an acknowledgement then it notifies the client app, (thus the callback) and deletes the message from the outbound queue..
If it doesn’t receive an acknowledgement it will resend the message with the DUP flag set (Duplicate Flag).
The message will continue to be resent at regular intervals, until the sender receives an acknowledgement.
If the message is being sent to the broker then the broker will forward that message to subscribers even though the duplicate flag is set.
Therefore subscribers can receive the same message multiple times.
QOS 2 – Only Once
This level guarantees that the message will be delivered only once. You keep them in line by messageId.
This is the slowest method as it requires 4 messages.
1- The sender sends a message and waits for an acknowledgement (PUBREC)
2 -The receiver sends a PUBREC message
3. If the sender doesn’t receive an acknowledgement ( PUBREC) it will resend the message with the DUP flag set.
4. When the sender receives an acknowledgement message PUBREC it then sends a message release message (PUBREL). The message can be deleted from the queue.
5. If the receiver doesn’t receive the PUBREL it will resend the PUBREC message
5. When the receiver receives the PUBREL message it can now forward the message onto any subscribers.
6. The receiver then send a publish complete (PUBCOMP) .
7. If the sender doesn’t receive the PUBCOMP message it will resend the PUBREL message.
8. When the sender receives the PUBCOMP the process is complete and it can delete the message from the outbound queue, and also the message state.