Support Retained Message for QoS=0 with a config option

[BigGitWorld]

Hi. I'm going to use save last state of some topics with the retained message concept. So my question is : Does the Moquette version 0.12.1 support Retained Message? and if the answer is yes, Can you tell me what class is used for this purpose? Thank you.

[andsel]

Yes, it support since old versions.

Here is, for example, one of sides where the broker stores the retained: https://github.com/moquette-io/moquette/blob/d8a59f135b2cf0896e04f6f02483b0841920fe60/broker/src/main/java/io/moquette/broker/PostOffice.java#L195-L202

Look at implementation of IRetainedRepository: https://github.com/moquette-io/moquette/blob/d8a59f135b2cf0896e04f6f02483b0841920fe60/broker/src/main/java/io/moquette/broker/IRetainedRepository.java#L27

[BigGitWorld]

Thank you @andsel I checked the source code and see the following code in the PostOffice for a published message with QoS=0 :

void receivedPublishQos0(Topic topic, String username, String clientID, ByteBuf payload, boolean retain,
                             MqttPublishMessage msg) {
        ...
        if (retain) {
            // QoS == 0 && retain => clean old retained
            retainedRepository.cleanRetained(topic);
        }
        ...
    }

Why do you clean retained message with QoS=0?

I think its better to add a config option to let developers to choose one of the below different behaviors :

• If they want to behave like as QoS=1,2 and save retained message with QoS=0 also.
• If they want to clean previously saved message in the broker, they have to publish a retained message with an empty payload (an empty string)

what do you think about this config option?

[andsel]

This come from http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html normative [MQTT-3.3.1-7] which states:

If the Server receives a QoS 0 message with the RETAIN flag set to 1 it MUST discard any message previously retained for that topic. It SHOULD store the new QoS 0 message as the new retained message for that topic, but MAY choose to discard it at any time - if this happens there will be no retained message for that topic.

[BigGitWorld]

I know it. But I changed that behavior to be same as QoS=1,2 in the Moquette broker. In fact almost all of my IoT devices are using QoS=0 and don't want to change their QoS. So I changed the broker behavior to save messages for all QoS values if the retained flag is set to true.

[andsel]

I feel your pain, but fixing this with the flag on Moquette broker make me rise some questions:

• patching the broker to go out of specs instead of client, seems like an antipattern
• seems to patch for only specific use case