Hi @caiboyu,
I always recommend ONF’s NG-SDN tutorial. If you want to understand how ONOS works (different to a controller like Ryu), do all exercises, 1 to 8 and do not skip steps.
I did respond to a question about ONOS a few days ago in the best way I could … considering that my understanding is not perfect ab out everything of how ONOS and P4 switches are programmed. My response will make sense once you have done most of the exercises.
Cheers,