If a registry is using some kind of CDN or proxy which caches access to the index files, then it is recommended that registries implement some form of cache invalidation when the files are updated. If these caches are not updated, then users may not be able to access new crates until the cache is cleared.
this doesn't make any sense if you think caching is about data, because you could obviously just serve a progressive stream of new crates, bucketed into discrete timesteps for the purpose of easier caching
https://doc.rust-lang.org/cargo/reference/source-replacement.html#local-registry-sources
A “local registry source” is intended to be a subset of another registry source, but available on the local filesystem (aka vendoring). Local registries are downloaded ahead of time, typically sync’d with a Cargo.lock, and are made up of a set of *.crate files and an index like the normal registry is.
this is obviously a backdoor. but why? for what purpose?