1

I think we can just use publish and pubrcv to meet the QoS2.

  1. ClientA-> Server publish packet
  2. Server -> ClientA pubrecv packet

When the server recv the publish packet, save to db, then the server publish to other clients, eg. ClientB. Even if we recv two same publish packets from ClientA, the server check the db, and know this is repeated message, so not publish to the ClientB.

So I don't think need 4 packets.

Do my logic is correct?

user501836
  • 1,106
  • 1
  • 12
  • 20
  • I guess it uses 4 packets to avoid the sender to resend the initial packet and not just only to avoid processing it in the receiver. – Hicaro Dec 26 '16 at 14:12

1 Answers1

6

There protocol uses the two exchanges of packets in order to provide the exactly-once semantics of QoS 2 messaging.

C --- PUBLISH --> S
*1
C <-- PUBREC  --- S
*2
C --- PUBREL  --> S
*3
C <-- PUBCOMP --- S
*4

When the server receives the PUBLISH it stores the ID and forwards the message on. When the server receives the PUBREL it can then delete the ID.

If the connection breaks at *1, the client does not know if the server received the message or not. It resends the PUBLISH (containing the full message payload). If the server had already received the message it just needs to respond with the PUBREC.

If the connection breaks at *2, the client may or may not have received the PUBREC. If it didn't, it will resend the PUBLISH. Otherwise it will send the PUBREL.

If the connection breaks at *3, the client does not know if the server received the message or not. It resends the PUBREL - which does not contain full message payload.

If the connection breaks at *4 and the client hasn't received the PUBCOMP it can resend the PUBREL.

There are two observations for why the two exchanges are needed:

  1. the server is not required to remember every message it has ever seen. There is a very well defined period for it to store the message ID. The two exchanges allow both sides to have certainty that the message has been delivered exactly once.
  2. the client does not need to resend the PUBLISH multiple times (unless the connection is interrupted at *1. Given the protocol is intended to minimise network traffic, this is an important feature.
knolleary
  • 9,777
  • 2
  • 28
  • 35
  • I got it. MQTT is define not use database, all the data is in the memory, so need 4 packets, if use Database to store the message, can use 2 packets. – user501836 Dec 27 '16 at 01:37
  • The use of a database or not is irrelevant. Your scheme suggests the broker must store all messages forever in order to avoid duplicates. That does not scale and is not practical. – knolleary Dec 27 '16 at 14:42
  • Because I want to create my IM service with MQTT, so need to save all the messages. MQTT is very useful. – user501836 Dec 28 '16 at 03:11