IRCTC Tatkal: 500 seats, 2 million hands

How a ~500-seat Tatkal quota opens to ~2 million people at 10:00:00 and sells each seat exactly once: a waiting room that admits only what the core can absorb, a dumb atomic Redis counter per pool that says no to 99.975% in milliseconds, a single-writer allocator, a hold-then-pay saga where a payment timeout becomes DEEMED_PENDING, and search kept on a separate plane.