How do you test that operations remain correct under Race Conditions and balance conflicts?

Short answer

Send multiple simultaneous requests with Promise.all to perform conflicting operations (such as booking the same seat or withdrawing a balance twice), and check that only one succeeds.

Explanation

Ordinary sequential tests do not expose race conditions. To test database locks or distributed transactions, send a batch of concurrent requests with Promise.all against the same resource. Verify that only one returns 200, the others fail with a 409 Conflict or 422, and the final balance is consistent and accurate.

Code example

const results = await Promise.all([
  withdraw(userId, 100),
  withdraw(userId, 100)
]);

Common mistakes

• Running the concurrent operations in an ordinary for loop, which executes them sequentially and fails to expose race conditions.

Follow-up questions

• What causes Flaky Tests, and how do you make tests deterministic (Determinism)?

Sources

• Node.js docs — Test runner