The bundle provides drivers for Doctrine ORM and Doctrine MongoDB. Though sometimes you might want to use the bundle with a custom or in-house written storage. For that, the bundle has support for custom storage. Once set, setting manager options in fos_oauth_server.service section becomes mandatory.
Here's an example of custom configuration:
# config/packages/fos_oauth_server.yaml
fos_oauth_server:
db_driver: custom
service:
user_provider: 'user_provider_manager_service_id'
client_manager: 'client_provider_manager_service_id'
access_token_manager: 'access_token_manager_service_id'
refresh_token_manager: 'refresh_token_manager_service_id'
auth_code_manager: 'auth_code_manager_service_id'