How would you design a hotel reservation system?

// Hotel inventory: per (hotel, room type, date) counters, the double-booking race,
// all-or-nothing multi-night stays, idempotent retries and the overbooking trade-off.
console.log("== 1. Capacity (assumptions) ==");
const rooms = 1000000, occupancy = 0.7, avgNights = 3;
const perDay = Math.round((rooms * occupancy) / avgNights);
const tps = perDay / 86400;
console.log("reservations/day = 1,000,000 x 0.7 / 3 nights = " + perDay);
console.log("reservations/s = " + tps.toFixed(2) + ", booking page QPS ~ " + (tps * 10).toFixed(1) + ", room detail QPS ~ " + (tps * 100).toFixed(0));
console.log("inventory rows = 5,000 hotels x 20 room types x 730 days = " + 5000 * 20 * 730);

// inventory table: key "hotel|type|date" -> {total, reserved, version}
const inv = new Map();
const key = (d) => "H1|KING|" + d;
const reset = () => { for (const [d, r] of [["12-23", 6], ["12-24", 9], ["12-25", 4]]) inv.set(key(d), { total: 10, reserved: r, version: 0 }); };
const show = () => ["12-23", "12-24", "12-25"].map((d) => d + "=" + inv.get(key(d)).reserved + "/10").join("  ");

console.log("\n== 2. Two guests, one room left on 12-24 ==");
reset();
let row = inv.get(key("12-24"));
const rows = [];
for (let i = 0; i < 9; i++) rows.push("old-" + i); // 9 existing reservation rows
const seen1 = rows.length, seen2 = rows.length; // both SELECT count(*) before either INSERTs
if (seen1 < row.total) rows.push("guest-A");
if (seen2 < row.total) rows.push("guest-B");
console.log("naive SELECT count then INSERT: both saw " + seen1 + " of 10, rows now " + rows.length + " -> DOUBLE BOOKED");
reset();
row = inv.get(key("12-24"));
const condUpdate = (r, n) => { if (r.reserved + n <= r.total) { r.reserved += n; r.version++; return 1; } return 0; };
console.log("conditional UPDATE ... WHERE reserved + 1 <= total: T1 rows=" + condUpdate(row, 1) + ", T2 rows=" + condUpdate(row, 1) + ", reserved = " + row.reserved);
reset();
row = inv.get(key("12-24"));
const v1 = row.version, v2 = row.version;
const casUpdate = (r, ver) => { if (r.version !== ver) return "version conflict"; if (r.reserved + 1 > r.total) return "sold out"; r.reserved++; r.version++; return "booked"; };
console.log("optimistic version check: T1 " + casUpdate(row, v1) + ", T2 " + casUpdate(row, v2) + ", T2 retry " + casUpdate(row, row.version));

console.log("\n== 3. A 3-night stay (12-23 to 12-26) when 12-24 is full ==");
reset();
inv.get(key("12-24")).reserved = 10;
const nights = ["12-23", "12-24", "12-25"];
console.log("before:        " + show());
for (const d of nights) { if (!condUpdate(inv.get(key(d)), 1)) break; }
console.log("night by night: " + show() + "   <- 12-23 leaked a room");
reset();
inv.get(key("12-24")).reserved = 10;
function bookStay(ds, n) { // one transaction: check every night, then apply all
  if (!ds.every((d) => inv.get(key(d)).reserved + n <= inv.get(key(d)).total)) return false;
  for (const d of ds) condUpdate(inv.get(key(d)), n);
  return true;
}
console.log("one transaction: booked=" + (bookStay(nights, 1) ? "yes" : "no") + "  " + show());
inv.get(key("12-24")).reserved = 9;
console.log("after a cancel:  booked=" + (bookStay(nights, 1) ? "yes" : "no") + "  " + show());

console.log("\n== 4. Retry with the same reservation id ==");
reset();
const reservations = new Map();
function reserve(resId, ds) {
  if (reservations.has(resId)) return "existing " + resId;
  if (!bookStay(ds, 1)) return "sold out";
  reservations.set(resId, ds);
  return "created " + resId;
}
console.log(reserve("R-7f3a", nights) + " | timeout, client retries: " + reserve("R-7f3a", nights) + " | " + show());

console.log("\n== 5. Overbooking 100 rooms, 5% no-shows, room 200, walking a guest costs 400 ==");
function outcome(booked, pShow) {
  let walks = 0, filled = 0;
  for (let k = 0; k <= booked; k++) {
    let c = 1; // binomial coefficient C(booked, k) as a float
    for (let i = 0; i < k; i++) c = (c * (booked - i)) / (i + 1);
    const p = c * Math.pow(pShow, k) * Math.pow(1 - pShow, booked - k);
    walks += p * Math.max(0, k - 100);
    filled += p * Math.min(k, 100);
  }
  return [filled, walks];
}
for (const n of [100, 102, 104, 106, 108, 110]) {
  const [filled, walks] = outcome(n, 0.95);
  const net = 200 * filled - 400 * walks;
  console.log("  accept " + n + ": rooms filled " + filled.toFixed(2) + ", guests walked " + walks.toFixed(2) + ", expected net " + net.toFixed(0));
}