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Fixing flaky parallel tests

I'm currently working on a large Laravel project with about 1.900 tests. Unfortunately, running those tests in sequence takes about ten minutes, and nobody has time for that. Fortunately, Laravel comes with a parallel test runner (, which makes use of all available cores on your machine and cuts the time down to about 2:30 minutes. After I've written about 1.850 tests, running the whole suite sometimes resulted in tests failing. When running those failing tests, they're passing. I was stunned, not knowing where to start to fix this annoyance. Consulting the PHPUnit docs (, I came across the processIsolation attribute, which resides in the phpunit.xml configuration file. The documentation reads

This attribute configures whether each test should be run in a separate PHP process for increased isolation.

<?xml version="1.0" encoding="UTF-8"?>
-   processIsolation="false"
+   processIsolation="true"

This simple change seems to fix my problem; now, all tests pass all the time.

Update: Although setting processIsolation to true fixes tests flaking out, the run time is about ten minutes and therefore utterly useless. I found out that running ./vendor/bin/paratest instead php artisan test --parallel really fixes my problems. I'm currently on the brink of submitting an issue to the Laravel repository, but it isn't effortless to create a proof of concept to attach to the submission.