Kafka Stream custom join using state store

Hi,

I didn't reach any people on StackOverflow, so I try here :
https://stackoverflow.com/questions/67694907/kafka-stream-custom-join-using-state-store

I'm really stuck on that part and I have the feeling only the Processor API
can help me.
But since the stream is really complex, I would prefer a confirmation
before start wasting my time on this.

Can anyone help me about this issue please ?

Thanks