National Vaccine Appointment & Administration System

🌱 How It Started Few Years ago, I had a system design interview. The interviewer gave me this scenario: "Design a national vaccine appointment booking system. Millions of citizens need to register and book slots. Clinics must administer the doses. The government needs audit logs and fraud prevention." My first thought was simple just let people book a slot, check the stock, and confirm. I drew a basic flow on the whiteboard and felt pretty good about it. Then the interviewer started asking harder questions. "What if two people try to book the last slot at the same time?" "What if the clinic runs out of doses after the booking is already confirmed?" "How do you undo things if eligibility check fails in the middle?" I didn't have good answers. I only designed for the happy path. That interview stuck in my mind. Months later, I was doing research on inventory reservation patterns for an internet credit purchase system , and I realized the same ideas could have helped me in that interview.