How would you design a ticket booking system like Ticketmaster?

// A hot on-sale: fans fighting over the same seats vs a waiting room that admits fans
// at a fixed rate and lets the server hand out seat holds that expire on a fake clock.
let seed = 42;
const rnd = (n) => { seed = (seed * 48271) % 2147483647; return seed % n; }; // MINSTD, same in every language

console.log("== 1. Load arithmetic (assumptions) ==");
const fans = 1500000, refreshSec = 2;
console.log("fans refreshing every " + refreshSec + " s without a queue: " + fans / refreshSec + " requests/s");
console.log("same fans polling a static queue page every 30 s: " + fans / 30 + " requests/s");
const seats = 70000, perOrder = 4, ordersPerMin = 2000;
console.log("a " + seats + "-seat stadium at " + perOrder + " tickets/order = " + seats / perOrder + " orders, " + seats / perOrder / ordersPerMin + " minutes at " + ordersPerMin + " checkouts/min");

const PAIRS = 500, FANS = 5000;
console.log("\n== 2. Free-for-all: " + FANS + " fans, " + PAIRS + " seat pairs, everyone grabs from the best 10 ==");
let state = new Array(PAIRS).fill("AVAILABLE");
let waiting = FANS, rounds = 0, attempts = 0, wins = 0;
while (waiting > 0 && state.includes("AVAILABLE")) {
  rounds++;
  const best = [];
  for (let i = 0; i < PAIRS && best.length < 10; i++) if (state[i] === "AVAILABLE") best.push(i);
  let won = 0;
  for (let f = 0; f < waiting; f++) {
    const p = best[rnd(best.length)];
    attempts++;
    if (state[p] === "AVAILABLE") { state[p] = "SOLD"; won++; } // UPDATE ... WHERE state = AVAILABLE
  }
  wins += won;
  waiting -= won;
  if (rounds <= 2) console.log("  round " + rounds + ": " + (waiting + won) + " attempts, " + won + " succeed");
}
console.log("  sold out after " + rounds + " rounds: " + attempts + " seat writes for " + wins + " sales, " + ((100 * (attempts - wins)) / attempts).toFixed(1) + "% conflicts");

console.log("\n== 3. Waiting room: admit up to 100 fans/min, server assigns the best free pair ==");
function simulate(HOLD, verbose) {
  seed = 42;
  const st = new Array(PAIRS).fill("AVAILABLE");
  const holds = new Map(); // pair -> {expires, buyAt}; buyAt -1 means the fan walks away
  let nextFan = 0, sold = 0, released = 0, lost = 0, writes = 0, minute = 0;
  while (sold < PAIRS && minute < 120) {
    minute++;
    for (const [p, h] of [...holds.entries()]) {
      if (h.buyAt !== -1 && h.buyAt <= h.expires && h.buyAt <= minute) { st[p] = "SOLD"; sold++; writes++; holds.delete(p); }
      else if (h.expires <= minute) { st[p] = "AVAILABLE"; released++; writes++; if (h.buyAt !== -1) lost++; holds.delete(p); }
    }
    const free = st.filter((x) => x === "AVAILABLE").length;
    const admit = Math.min(100, free, FANS - nextFan);
    for (let i = 0; i < admit; i++) {
      const p = st.indexOf("AVAILABLE"); // best free pair; one allocator per section, so no contention
      st[p] = "HELD";
      writes++;
      nextFan++;
      const buys = rnd(10) < 7; // 70% want to buy and need 1-6 minutes for checkout
      holds.set(p, { expires: minute + HOLD, buyAt: buys ? minute + 1 + rnd(6) : -1 });
    }
    if (verbose && (minute <= 4 || minute % 4 === 0)) console.log("  min " + String(minute).padStart(2) + ": admitted " + String(nextFan).padStart(3) + "  holding " + String(holds.size).padStart(3) + "  sold " + String(sold).padStart(3) + "  released " + String(released).padStart(3));
  }
  return { minute, nextFan, released, lost, writes };
}
const r8 = simulate(8, true);
console.log("  8-min holds: sold out at minute " + r8.minute + ", " + r8.nextFan + " fans admitted, " + (FANS - r8.nextFan) + " told sold out, " + r8.writes + " seat writes, 0 conflicts");
for (const hold of [4, 8, 15]) {
  const r = simulate(hold, false);
  console.log("  hold " + String(hold).padStart(2) + " min: sold out at minute " + String(r.minute).padStart(2) + ", holds released " + String(r.released).padStart(3) + ", buyers cut off mid-checkout " + String(r.lost).padStart(3));
}

console.log("\n== 4. A payment that arrives after its hold expired ==");
const seat = { state: "HELD", holdId: "hold-B" }; // fan A held it first, expired, fan B holds it now
const confirm = (s, holdId) => { if (s.state === "HELD" && s.holdId === holdId) { s.state = "SOLD"; return "SOLD to " + holdId; } return "rejected " + holdId + ", refund the charge"; };
console.log("  late confirm for hold-A: " + confirm(seat, "hold-A"));
console.log("  confirm for hold-B:      " + confirm(seat, "hold-B"));