doorkeeper-gem/doorkeeper is an open source project licensed under MIT License which is an OSI approved license.
Similar projects and alternatives to Doorkeeper based on common topics and language
Open Source Identity and Access Management For Modern Applications and Services
The Single Sign-On Multi-Factor portal for web apps
Scout APM - Leading-edge performance monitoring starting at $39/month. Scout APM uses tracing logic that ties bottlenecks to source code so you know the exact line of code causing performance issues and can get back to building a great product faster.
Ultra performant API Gateway with middlewares
A reverse proxy that provides authentication with Google, Github or other providers.
Single Sign On server, OAuth2, Openid Connect, multiple factor authentication with, HOTP/TOTP, FIDO2, TLS Certificates, etc. extensible via plugins
FusionAuth issue submission project
OpenID Certified™ OpenID Connect and OAuth Provider written in Go - cloud native, security-first, open source API security for your infrastructure. SDKs for any language. Compatible with MITREid.
Authelia is an open-source authentication/authorization server with 2FA/SSO
news.ycombinator.com | 2021-03-10
One thing that is missing from this list is open source language specific libraries. Projects such as https://oauthlib.readthedocs.io/en/latest/oauth2/server.html and https://github.com/doorkeeper-gem/doorkeeper
Depending on your use case, for example if you only have one application, you might be better off running something embedded in your app, or independent but using the same runtime/deployment environment. Then, when you are ready to add another app or integration, you should be able to introduce a standalone auth system more easily if appropriate (because all your auth interactions should be relatively standardized). I'm a big fan of standalone auth systems as a way to simplify access control and give a single view of a user/customer, but you can also succeed using open source embedded libraries.
When the moment comes to introduce a standalone system, you should consider a few dimensions (this list pulled from a previous comment of mine: https://news.ycombinator.com/item?id=26360048 ):
* open source or not