Hi all,

I started working on implementing the MQTT retained message feature on WSO2
MB 3.0.0

*Feature Definition*

In MQTT a publisher can set retain flag in any message it sends to the
broker. By setting the retain flag, the message is kept in the broker even
after delivering to all existing subscription, so when a new subscribers
connect to the broker, they get all the relevant retained messages.

*Proposed Design*

A separate table/column-family will be used to store the last published
message for a topic. This table will be updated every time a message is
published with the retained flag set.

When a new client subscribe to a topic, the table is looked up for matching
topics and retained messages are sent to the new subscriber. This is done
only when initializing the subscription.

Please share your thoughts on this.

-- 
Asanka Abeyweera
Software Engineer
WSO2 Inc.

Phone: +94 712228648
Blog: a5anka.github.io
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