Live seat map for a stadium concert on-sale

Your ticketing site sells seats for stadium concerts. When a big show goes on sale, about 100,000 fans have the seat map open at once. The stadium has 50,000 seats. A fan taps a seat to hold it for 8 minutes while they pay. Held and sold seats turn grey on everyone's map.

In the first 8 minutes, fans hold about 6,000 seats a minute. About 1 hold in 6 runs out unpaid. Fans use phones and laptops. A seat that changes should change on every map within 2 seconds. Holding a seat someone else already holds must fail.

How does each map learn which seats changed, and what do you send to each fan?