20 years ago i discovered a project by a university in germany that implemented a well thought through object storage with connections to all sorts of messaging protocols.
it was used as a platform to research collaboration models.
fortunately, at the time some german academic institution offered grants to universities for publishing their projects as Free Software or Open Source, and so this platform was released under the GPL.
what is interesting about the platform is that it not only stores objects like say mongodb, but it also implements user and group management, a hierarchical access control system down to the object level, and messaging. further messaging implements pretty much all communication protocols out there: IRC, XMPP, SMTP, IMAP, and of course HTTP, FTP and more. and no, to get these it does not require a bridge to independent implementations of those protocols, but they are implemented natively into the server, and SMTP or IMAP or the webinterface will all directly access the same stored object.
what's more, the platform allows you to upload your own modules to extend it, which are stored in its object object storage, and can be updated at runtime. it has been used by the university to host various courses for more than 10000 users all handing in assignments at the same time.
i have been using this platform for my own websites pretty much ever since.
the university stopped development on the project more than a decade ago, but i forked it, and eventually added a REST API and implemented multi domain hosting on it so that i could serve multiple websites from the same service.
the code is old, and needs updating. TLS support is outdated, which is something that needs to be fixed before the project can be recommended to anyone else. the built in web templating system is using XSLT, which should be replaced with easier to use alternatives (or simply ignored, as as i do, by building all websites as SPA using the REST API instead. the REST API too, should be updated or replaced by GraphQL.
but aside from these problems the platform is usable like Backend As A Service. ever since adding the REST API, i have not done any custom backend coding, as the platform already provides any features i have ever needed.
the challenge throughout all this time has been to get other developers interested in using such a platform, instead of building yet another CRUD backend from scratch.
i don't currently have time to do any work on it (even its website is down), as i need to focus on paid engagements, but i have not given up hope to be able to revive it and make it popular some day.