Determinism by Design

Nondeterminism is a dependency. Code that calls the wall clock, the OS entropy pool, or the scheduler directly has hard-wired an uncontrollable input — and everything downstream inherits it: tests that need sleeps and retries, failures that vanish under observation, "run it 100 times" as a verification strategy. The fix is the same as for any dependency: inject it. Route every nondeterministic source through a seam the caller controls, wire the real source in production, and wire a controlled one everywhere reproducibility matters.

What this buys compounds: hermetic tests that cannot flake, failures that replay from a single seed, forced interleavings that turn race conditions into on-demand reproductions, and differential runs where two implementations see byte-identical inputs.

Nondeterminism is a dependency — inject it

Code that calls the wall clock, the OS entropy pool, or the scheduler directly has hard-wired an uncontrollable input, and everything downstream inherits it: tests that need sleeps and retries, failures that vanish under observation, "run it 100 times" as a verification strategy. The fix is the fix for any dependency: route every nondeterministic source through a seam the caller controls, wire the real source in production, and wire a controlled one wherever reproducibility matters.