Question

Problem Statement

I have two machines, A and B, both running Clojure.

B has some in memory data structure.

A holds an object A_P which is a reference/pointer to some object B_O in B’s memory.

Now, as long as A_P is NOT GC-ed by A, I do not want B_O GC-ed by B.

However, once A_P has been GC-ed by A (and nothing else in A referes to B_O, and nothing else in B refers to B_O), then I want B_O to be elegible to be GC-ed.

Solution in Languages with Destructors

In C++, this is easy — I use destructors. When A_P gets GC-ed, A sends B a msg to decrement the number of external references to B_O, and when that’s 0, and internal refernes to B_0 is also 0, then B_O gets GC-ed.

Solution in Java/Clojure?

Now, I know that Java does not have destructors. However, I’m wondering if Clojure has a way around this problem.

Thanks!

Answer 1

No good solution exists, without a real distributed garbage collector. Even in C++, you cannot do this safely, because you implemented reference counting and pretended it was a real garbage collector; but if two objects point to each other across the machine divide, and are both unreferenced locally, they still both have a nonzero reference count and cannot be collected.