A common challenge when testing Elixir apps is dealing with shared/global state. When multiple tests modify/access the same piece of state, in order to ensure the tests don’t interfere with each other, we need to serialize the tests using async: false.

To isolate tests from each other and avoid async: false, here we explore a technique for “localizing” global state. That is, we reduce the scope of in-memory state such that each ExUnit test has its own private copy of the state. With global state thus localized, our ExUnit tests are decoupled and safe for async: true.

This used to be a question with a somewhat unsatisfying answer. No longer. As of OTP 25.0, via this pull request, the parent of every child process is stored in the process info table of the child, and retrievable via process_info/2 (Erlang) and Process.info/2 (Elixir).