How to do a COVID-19 Self Test (rapid antigen test)

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System Design: Building a Bulletproof Payment Gateway! 🚀 Basic payment setups fail during network timeouts. To block catastrophic duplicate charges, enforce client-generated idempotency keys (UUIDs). The server saves this key first; if a duplicate request arrives, it immediately returns the cached result instead of re-charging the buyer. Never overwrite database rows to update payment statuses. Instead, implement an immutable ledger where every state change—authorized, captured, or refunded—cre