Github backend for CMS

[jimafisk]

We're currently building out the CMS with a GitLab backend, mainly because GitLab supports PKCE. This allows you to edit your site in the browser and have it write back to your GitLab repository without having to run your own server.

GitHub does not support PKCE so even if we built an API to work with their OAuth provider, you would need to run a server to authenticate to GitHub (or pay someone to run the server for you). See NetlifyCMS's docs about having to run an intermediary server, which you could do with one of these OAuth clients.

If you want a Github backend for Plenti, please Upvote and Comment on:

• Support PKCE flow for OAuth apps community/community#15752

[jimafisk]

Maybe device flow could be used as a workaround since PKCE is still unsupported: https://github.com/orgs/community/discussions/54568#discussioncomment-5820902