Is it possible to have nostrdb live on an Umbrel for example, and make all Damus instances (phone, tablet, pc) talk to it on a local network. 

Like a shared DB always connected even if apps are closed. 

 You can just use strfry or nostr-rs-relay, those are great for personal relays. nostrdb is more meant as an in-process relay for high performance use cases 

 I’ll look into that. Thanks.