The latest version of my invoicing app lets the user install CouchDB on their desktop PC and they can choose to use their local CouchDB only. This "local-first" approach runs offline only and provides close to native app performance.

They can also choose to "live sync" their local CouchDB with my Cloud based CouchDB and use the app online on all their devices.

They can also configure CouchDB to live sync their DB with 3rd party services like Couchbase and Cloudant and create backups and snapshots of all their data on those 3rd party services with their local CouchDB.

It has probably been done before (what hasn't?), but it would be unusual.

(a) the customer needs to open his DB box to the outside world. Yes, VPNs, encryption, whatever. Customer's IT operations won't like it. And that is probably enough to kill it, unless someone powerful in the organisation really throws his weight behind it.

(b) you introduce another round trip, and thus latency. Hosting the application close to the data is more natural.

That would be the case in which the customer wants to self host everything, but the company is only willing to let the customer self host the data. So (a) would not be so much a big deal for the customer

I don't understand what you're saying. The data is on my server, inside my own network. The application runs on your server, on your network. Between us is the Internet.

Your application needs access to my data.

Why wouldn't I need to open up my server and network to the outside world? You're outside.