A real production bug from a payment service where class-level Ruby constants are evaluated once at class load time. If a database lookup returns nil at boot (due to autoload timing, deploy race conditions, or replica lag), the constant is permanently set to nil for the entire process lifetime — silently writing NULL to the database on every subsequent call. The fix replaces load-time constant assignment with lazy per-instance caching using the Singleton pattern and find_by! (which raises on missing rows instead of returning nil), ensuring the cache is never poisoned with a missing-row nil.
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The QuestionThe Symptom in ProductionThe Root Cause: The Class Body Runs OnceWhy Only Some Pods, Why Only SometimesThe FixWhy It’s a Good Interview Question1.4K Impressions