There's also software called Constellation[0] from microcosm that exclusively collects "backlinks", which are items which link to a certain thing. e.g. follow records for an account, reposts/replies/likes on a post. If you click on the Backlinks tab when examining something on pdsls.dev you can see a list of them.

It can be self hosted and is much lighter than running a full appview. The author has a public endpoint for it that's keeping up with the whole network running on a single raspberry pi on his home network.

0: https://www.microcosm.blue/

Yes. I describe this in this part of the article: https://overreacted.io/a-social-filesystem/#:~:text=One%20ch...

It's basically event sourcing. You listen to the data you care about from the network and update the local index (DB). There are also tools like Tap (https://docs.bsky.app/blog/introducing-tap) that do the plumbing work and let you backfill automatically.