Design a Ticketing System (BookMyShow)

Requirements

Type	Requirement
Functional	Browse events and venues; select seats on an interactive map; reserve and purchase tickets
Functional	Each seat can only be booked by one user at a time (no double-booking)
Non-functional	10M DAU; 100K concurrent users during peak (popular event launch)
Non-functional	Seat reservation must be consistent — eventual consistency is not acceptable
Non-functional	Reserve-to-payment window: 10 minutes (after which the seat is released)

The Core Challenge: Seat Reservation

Ticketing is a classic high-contention write problem. At event launch, thousands of users simultaneously try to reserve the same limited seats. Standard optimistic locking at the DB layer will cause massive retry storms.

Flow:

1. User selects seat — Frontend shows real-time seat availability via WebSocket/SSE
2. Reserve (10-min hold) — Acquire Redis lock on seat ID; write reservation to DB with expiry
3. Payment — User completes payment within 10 min; redirect to payment gateway
4. Confirm or Release — Payment webhook: confirm reservation. Timeout: release seat.

async function reserveSeat(seatId: string, userId: string): Promise<boolean> {
  // Atomic: SET if Not eXists + 10-min expiry
  const lockKey = `seat:lock:${seatId}`;
  const reserved = await redis.set(lockKey, userId, 'NX', 'EX', 600);

  if (!reserved) return false; // Seat already held by someone else

  // Write pending reservation to DB
  await db.reservations.create({
    seatId, userId, status: 'pending',
    expiresAt: new Date(Date.now() + 600_000),
  });

  // Schedule expiry cleanup (via job queue)
  await queue.add('release-seat', { seatId }, { delay: 600_000 });

  return true;
}

Handling Flash Sales

• Queue-based admission — a virtual waiting room queues users; only N are let into the reservation flow at once
• Pre-shard seats — partition seats into buckets; each bucket handled by a separate DB instance to reduce contention
• Read replicas for browsing — seat availability reads from replicas (with 1–2s staleness acceptable for display)
• CDN for static content — venue maps, images served from edge to protect origin