I'm very new to hyperledger composer, how do I know the participant is created (or where can I get the participant registry ) and the transaction has been processed? Also, what is "features/sample.feature" used for? Is it for unit test or something?
I respectfully suggest you spend some time completing the tutorials and reading the documentation. You will find the answers to your questions there.
Start here: https://hyperledger.github.io/composer/latest/tutorials/developer-tutorial.html
Then, read this: https://hyperledger.github.io/composer/latest/managing/participantsandidentities.html