Siemens SIMATIC CC712 Operating Instructions Manual page 69

● Retain
From topics/groups with the "Retain" flag, the broker always saves the last message.
When a subscriber subscribes to a new topic or when the connection with a subscriber
returns after being terminated, the broker sends the last message on each topic with
Retain flag to the subscriber.
If the connection between the device and the broker is terminated, the broker publishes
the last message of the topic for each registered subscriber. Triggers for publishing the
last message can be:
– A planned connection termination with the "Clean session" option disabled
– A connection abort
– Network faults
You can set the Retain flag for all or for individual topics/groups (option enabled in single
row).
The higher-level check box activates the function for all topics/groups of the table.
● QoS
You use the "Quality of Service" parameter to specify the transfer performance of the
messages for this topic:
– QoS 0
– QoS 1
– QoS 2
When a connection is aborted, the data frames are buffered for QoS 1 and QoS 2. See
also the section "Connection abort" below.
SIMATIC CC712
Operating Instructions, 05/2019, C79000-G8976-C503-01
Transfer no more than once
The device sends the topic once to the broker. The device does not expect an
acknowledgment. If the topic is not received by the broker, it is lost.
Transfer at least once
The device sends the topic to the broker until it receives a PUBACK packet as
acknowledgment from the broker.
Transfer exactly once
The device sends the topic and waits until it receives the two-step acknowledgment
from the broker as specified.
This version represents the highest level of quality, but it is also associated with the
highest administrative burden for the client as well as the server.
Configuration
4.9 Cloud configuration
69