Nothing lasts forever.

I've pondered that a lot in my system design which bears some resemblance to the principles of REST.

I have split resources in ephemeral (and mutable), and immutable, reference counted (or otherwise GC-ed), which are persistent while referred to, but collected when no one refers to them.

In a distributed system the former is the default, the latter can exist in little islands of isolated context.

You can't track references throughout the entire world. The only thing that works is timeouts. But those are not reliable. Nor you can exist forever, years after no one needs you. A system needs its parts to be useful, or it dies full of useless parts.