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Lessons (2)

• A WhatsApp OTP Is Only Single Use If You Make It So

  … AND consumed at IS NULL; -- both requests see "unused" -- application code decides it is fine UPDATE otp codes …

• 500 Users Upload Documents at Once: Design the Background Job Pipeline

  … This is the same atomic-check-and-set idea used for idempotent payments and single-use OTPs. The …