While writing the previous post about our per-test SQLite databases, I was describing how each test gets its own in-memory database, no shared connections, no shared state. And I thought: wait, if nothing is shared, can we just run them all at the same time?
Turns out we could, and our server test suite went from ~4 seconds to ~0.85 seconds for around 800 tests. Zero code changes to the tests themselves. One 25-line Erlang module.
Most web frameworks treat test database isolation as a hard problem. Rails has database_cleaner with three strategies. Django wraps every test in a transaction it rolls back. Phoenix does the same with its SQL sandbox. They all exist because tests share a single database server, and that shared state is the root of flaky tests and ordering dependencies, the kind where a test passes alone but fails in the suite.
Curling IO Version 3 doesn't have this problem. Each test gets its own database. Not a transaction. Not a truncated copy. A completely independent in-memory SQLite database, cloned from a template in microseconds using SQLite's backup API.