When imperfect systems are good: Bluesky's lossy timelines

Note that all of this reflects design decisions on Bluesky's closed-source "AppView" server—any federated servers interacting with Bluesky would need to construct their own timelines, and do not get the benefit of the work described here.

What reason does Bluesky give for not opening up their AppView code?

Another notable component that is closed source is the discovery feed generator, where at least there is some reason.

The App View frontend is open source: https://github.com/bluesky-social/social-app

Much of the backend is open source as well: https://github.com/bluesky-social/atproto/tree/main/packages

What is not are the extra services they run to provide a better and faster UX. Even if it was open source, it likely costs 10s of thousands to run per month (they have moved largely to "onprem" hardware instead of the cloud aiui)

that's not the appview, that's the client

App View is a bit fuzzy of a term. To me it seems like a combination of frontend, backend, custom lexicon, and supporting services. There isn't really another place in the spec or design where clients or browsers fit in, which do in fact provide a view of the network via an app.