You can go as simple as using 'nak serve' and that will spin up a relay, you can even expose it to clearnet, the events will live in memory. If you want more customization use khatru which is what is running behind nak serve. Also rely is good and simple. Both are written in go, which is relatively simple to read and write. Then if you want other languages there are relays in rust, ts, whatnot. You can find other implementations in nostr.net
