Once Keystone is installed and running, services need to be configured to work with it. To do this, we primarily install and configure middleware for the OpenStack service to handle authentication tasks or otherwise interact with Keystone.
In general:
Clients making calls to the service will pass in an authentication token.
The Keystone middleware will look for and validate that token, taking the appropriate action.
It will also retrieve additional information from the token such as user name, id, tenant name, id, roles, etc...
The middleware will pass those data down to the service as headers.
