Is parallel system tests a good default?

I have this in as a “how do I do this?” question over here: How to disable test parallelization *just* for system tests?

But it’s actually kind of a newbie WTF as welll. I’m running the out-of-box rails generate scaffold-generated system tests for a resource using chromedriver, and at least 50% of the time one or more tests fail. Running with PARALLEL_WORKERS=1 fixes it.

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Interestingly this is not just a Rails problem. When I was playing with writing a web scraper in Rust a while back, Rust’s test parallelization caused ALLLLLLLL kinds of failures. The root cause there was that the chromedriver process was a contended resource between threads.

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I have encountered this WTF. I found running with: :threads seemed to resolve it.

class ActiveSupport::TestCase
  parallelize(workers: :number_of_processors, with: :threads)
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