And one last question before I head off (as in this is asynchronous): rpm-ostree can maintain a cache of downloaded rpms and metadata. How would I model this in Pants? As a source-ish thing that I’d write back out?

You might look into a processes

append_only_cache

. The writer of that cache must be multi-process safe though

Following on from what @bitter-ability-32190 said, I can talk you through our JVM cache support sometime, as it’s a reasonable clear example of exactly what you’re asking for