Restore messages per client/subscription

[vaLP2eh]

Describe your question

I would like to use MQTTnet to track logging information. Each of my services shall send its logging information to an individual topic. Each message shall be written to a database/cache to have them available for later. The database/cache will be trimmed to get rid of old information.

The thing with writing all messages to a database/cache seems to be easy. But this is my requirement:

When a client subscribes to a topic,, the client should be able to specify in the user properties how far back in history the subscription should "look", i.e. 24h, 7d, ...

How to restore/resend messages for a specific client/subscription?

I read all questions (open/close) of this issue tracker and the wiki and samples, but could not find something that matches my requirements.

Which project is your question related to?

• Client
• Server

[xyderos]

I think this is more of a "broker specific" question as most of the brokers just allow the messages to pass through them. In a more general context, you would need to have a QoS of at least 1 or 2, not having set the clear flag and have some sort of TTL in place which I am not sure mqtt supports that.

A "dirty approach" would be to subscribe to the db/logging whatever service to a topic that would essentially ping you when there is an update and you would execute a request/response scheme.

                  |          SUBSCRIBE TO "NOTIFIER"                   |
                  |------------------------------------------------------------>|
                  |         RECEIVED EVENT "NOTIFIER"                |
Consumer|<------------------------------------------------------------|Producer
                  |QUERY THE DB WITH A PUBLISH MESSAGE|
                  |------------------------------------------------------------>|
                  |RECEIVE RESPONSE IN A DIFFERENT TOPIC|
                  |<------------------------------------------------------------|

But are you sure MQTT is the right way to solve the problem?

[vaLP2eh]

Yes, MQTT is the right way to go for the use case I work on.

I read more in the source code to get a better understanding. It's a pity that there is no documentation for IntelliSense provided by this project. Makes the API difficult to understand. Anyway...

My plan is to use the intercept-subscribe to restore the messages (if needed/requested) for the calling client and the intercept-unsubscribe/-disconnected to cleanup if needed. Should work fine.