Built on top of Grape API framework and Goliath web server, this tiny API serves Discourse SSO protocol and talks with SAML2 authentication endpoints.
You should point your Discourse instance to SSO API at https://<host-with-this-API.com>/login, and set the proper SAML endpoint data in config/production.rb (requires restart). Everything else will be handled automatically. The configuration options are rather limited at the moment, but should be pretty self-descriptory.
You can have a look at the attached nginx.conf.example and systemd template unit saml_auth_proxy@.service.example.
With that template unit you can launch multiple instances of a API server, and then use them all in your nginx upstream directive.