[paho-dev] "Offline Buffering" for Paho Clients

[Ian Craggs <icraggs@xxxxxxxxxxxxxxxxxxxxxxx>]

One thing that is quite frequently asked about the Paho MQTT clients is 
why can't the application publish messages when not connected to a 
broker.  This seems like an obvious thing to be able to do.

I think the main reason, is that that is the way we have developed MQTT 
clients for as far back as I remember.    But that ability does raise 
other questions for the API implementation, such as:

- how to manage the stored messages (what if the connection information 
is wrong and no messages ever get sent, limiting the storage used, etc)
- how frequently and when to attempt reconnection to the broker

so we have avoided the problem by letting the application handle it.

The Eclipse Kura project has an ostensibly protocol-neutral approach to 
this for Java, although currently I think it implements MQTT only.  One 
of the main interfaces is here:

https://github.com/eclipse/kura/blob/develop/kura/org.eclipse.kura.api/src/main/java/org/eclipse/kura/data/DataService.java

I think it would be good to add offline buffering for the Paho clients, 
but what approach to take?

- for Java, just adopt or adapt the Kura implementation somehow? Just 
say, use Kura?  Or have a more minimal implementation which varies less 
from the current Java API?
- for the others, adopt a Kura-like interface or something more minimal?

I am thinking that the most minimal implementation would simply allow 
calling publish regardless of if a connection was available, with a 
limit on the number of stored messages.

Thoughts?

--
Ian Craggs
icraggs@xxxxxxxxxx                 IBM United Kingdom
Committer on Paho, Mosquitto