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Concurrency Review

Names the shared mutable state first, because every concurrency bug in the code follows from it.

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CategoryCodingForDevelopersTested onClaudeChatGPT

Running it, start to finish

  1. List every piece of shared mutable state before analysing anything.
  2. Fix the check-then-act cases with atomic operations or constraints.
  3. Verify the synchronisation holds across multiple instances.

What you get back

The output this produces, every time.

  • Enumerates shared mutable state first, which makes the rest of the analysis tractable.
  • Finds every check-then-act shape, the most common source of production races.
  • Asks whether synchronisation survives multiple instances, which is where scaling breaks things.

Getting better results

Where this usually goes wrong, and how to avoid it.

  • Include the state you forget is state. Lazy singletons, memoised values and counters are shared mutable state and are where the surprising races live.
  • Prefer a database constraint. For exists-then-create, a unique constraint cannot be forgotten by a future caller the way a lock can.
  • Check locks held across I/O. It is correct and it destroys throughput, and it only shows up under load.

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