https://projects.spring.io/spring-security-oauth/docs/oauth2.html
    !!! need to re-read
https://docs.spring.io/spring-security-oauth2-boot/docs/current/reference/htmlsingle/

client
https://github.com/spring-projects/spring-security/tree/master/oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client
    https://projects.spring.io/spring-security-oauth/docs/oauth2.html
    ! chapter OAuth 2.0 Client
    @EnableOAuth2Client

cloud-client
https://github.com/spring-cloud/spring-cloud-security/tree/master/spring-cloud-security/src/main/java/org/springframework/cloud/security/oauth2/client