Overcoming Global State in Tests with Localized State for Async Elixir ExUnit Tests

The article explains a common issue faced by developers when testing Elixir applications: shared or global state interference. JB Steadman introduces a method to 'localize' this global state, enabling each test to operate with its unique state and thus maintaining the ability to run tests asynchronously (`async: true`). The article demonstrates how to use the process dictionary to store data specific to a test's process. It then explains how to access this data in child processes using the `ProcessTree.get/2` function provided by the ProcessTree library. This approach relies on Erlang and Elixir's process hierarchy. The article also discusses the process dictionary's nature and common concerns but emphasizes its appropriate use in this context. It includes Elixirs like Ecto SQL and Logger as examples already taking advantage of the process dictionary for scoping resources to specific processes.