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When a user signs in they will be redirected to your backend(with a token) where you can provide the core functionality of your SaaS offering.
I've setup a starter Rails App that you can use to build the core of your SaaS app. I've also setup a deploy to Heroku button so you can get up and running right away. MVPaaS and this starter app makes it really easy to turn your side project into a complete SaaS business.
I'm also offering monthly development retainers targeted at non-technical entrepreneurs.
Thanks!