title: Mobile app authentication using Token based on OAuth2.0
body: <p>I'm building a REST API using Elixir's Phoenix framework. In the API, I need to authenticate the user by phone number i.e., via sending an SMS OTP code. After authenticating the user, the Auth server sends the Access token and Refresh token to the client. The client(mobile app) stores those tokens locally and sends the Access token in the HTTP header as <code>Authorization: Bearer &lt;Access_Token&gt;</code> in every request to resource server. My actual question is, how do resource server validates the Access token that is received from the mobile app/client?</p>
  <p>Does resource server needs to contact Auth server to validate the Access Token? That would a lot of overhead. Please help me understand RestFull API Authentication.</p>
  <p>Thanks for taking the time to read my question.</p>