It's definitely a very rough work in progress but I have used it successfully to deploy my own simplenote server that works with all the clients.
Simperium Server: https://github.com/beaucollins/node-simperium-server
It's in the proof-of-concept stage so it doesn't persist the data on the server.
Protocol for reference: https://github.com/Simperium/simperium-protocol/blob/master/...
> You can mirror all your data to your own backend, but it's not yet possible to host Simperium yourself. We're open sourcing our client libraries, protocol, and a reference service implementation that will allow you to host your own service in the future.
Edit: looks like simperium is some data stashing service (like firebase?). And simplenote is an app built on top of simperium. Did I get that right?
In fact, Simperium was originally created to provide the syncing features of Simplenote and then generalized to work as a service for anyone who wants to sync structured data.
"People will decide if they want to give Automattic money".