Grape gem is the core of API service, which bulits API endpoints and handles each exception. Devise gem’s role is the user management system. Doorkeeper gem manages oauth clients, which are related with users in Devise gem, and record oauth clients' app id, secret, scopes, authorization codes and access tokens. Doorkeeper takes responsibilities about giving, revoking, and refreshing authorization codes and access tokens. Wine_bouncer gem’s role is a guard of