Design Ticketmaster / StubHub

GET /api/events?query=taylor+swift&location=dubai&date_from=2025-06-01
Authorization: Bearer {jwt}

Response 200:
{
  "events": [
    {
      "event_id": "evt_abc123",
      "name": "Taylor Swift — Eras Tour",
      "venue": "Dubai Arena",
      "date": "2025-09-15T20:00:00Z",
      "seats_available": 32847,
      "price_range": { "min": 150, "max": 950, "currency": "USD" },
      "queue_enabled": true,
      "sale_starts": "2025-06-01T10:00:00Z"
    }
  ],
  "total": 1,
  "cursor": "..."
}

GET /api/events/{event_id}/availability
Response 200:
{
  "event_id": "evt_abc123",
  "total_seats": 50000,
  "available": 32847,
  "sections": {
    "SEC-A": { "available": 120, "tier": "platinum", "min_price": 950 },
    "SEC-B": { "available": 380, "tier": "gold", "min_price": 450 },
    "SEC-C": { "available": 1200, "tier": "silver", "min_price": 250 }
  },
  "cached_at": "2025-06-01T10:00:02Z",
  "ttl_seconds": 3
}
// NOTE: This is eventually consistent (2-5s stale). OK for display.

POST /api/queue/join
Body: { "event_id": "evt_abc123" }
Response 200: { "queue_token": "qt_xyz789", "status": "waiting" }

GET /api/queue/status?token=qt_xyz789
Response 200:
{
  "position": 847293,
  "now_serving": 450000,
  "estimated_wait": "12 minutes",
  "status": "waiting"   // waiting | admitted | event_sold_out
}

POST /api/inventory/hold
Headers: X-Admission-Token: {queue_admission_token}
Body: {
  "event_id": "evt_abc123",
  "seats": ["SEC-A_ROW-3_SEAT-15", "SEC-A_ROW-3_SEAT-16"],
  "hold_duration_seconds": 480
}
Response 200:
{
  "hold_id": "hold_abc",
  "seats": ["SEC-A_ROW-3_SEAT-15", "SEC-A_ROW-3_SEAT-16"],
  "expires_at": "2025-06-01T10:08:00Z",
  "total_price": 1900.00
}
Response 409: { "error": "seat_taken", "unavailable": ["SEC-A_ROW-3_SEAT-15"] }

POST /api/payments/charge
Headers: Idempotency-Key: {user_id}:{event_id}:{hold_id}
Body: {
  "hold_id": "hold_abc",
  "payment_method_id": "pm_stripe_xyz",
  "total": 1900.00
}
Response 200:
{
  "order_id": "ord_def456",
  "tickets": [
    { "ticket_id": "tkt_001", "seat": "SEC-A_ROW-3_SEAT-15", "qr_code": "..." },
    { "ticket_id": "tkt_002", "seat": "SEC-A_ROW-3_SEAT-16", "qr_code": "..." }
  ],
  "status": "confirmed"
}

-- Thread 1 and Thread 2 both run this simultaneously
SELECT status FROM seats WHERE seat_id = 'A1' AND event_id = 'E1';
-- Both see: AVAILABLE  ← race condition window

UPDATE seats SET status = 'HELD', user_id = 'U1' WHERE seat_id = 'A1';
-- Thread 1 wins
UPDATE seats SET status = 'HELD', user_id = 'U2' WHERE seat_id = 'A1';
-- Thread 2 ALSO succeeds → DOUBLE SOLD

async def hold_seat(event_id, seat_id, user_id):
    redis_key = f"seat:{event_id}:{seat_id}"
    
    # Phase 1: Fast gate
    acquired = await redis.set(redis_key, user_id, nx=True, ex=480)
    if not acquired:
        return HoldResult.SEAT_TAKEN
    
    # Phase 2: Authoritative write
    try:
        rows = await db.execute("""
            UPDATE seats SET status = 'HELD', user_id = %s, 
                   held_until = NOW() + INTERVAL '8 min',
                   version = version + 1
            WHERE seat_id = %s AND event_id = %s
              AND status = 'AVAILABLE'
        """, [user_id, seat_id, event_id])
        
        if rows == 0:
            await redis.delete(redis_key)  # Clean up
            return HoldResult.SEAT_TAKEN
        return HoldResult.SUCCESS
        
    except Exception:
        await redis.delete(redis_key)  # Roll back Redis
        return HoldResult.RETRY

SELECT seat_id FROM seats
WHERE event_id = ? AND status = 'AVAILABLE' AND price_tier = ?
ORDER BY row_number ASC, seat_position ASC  -- front-center is "best"
LIMIT ?
FOR UPDATE SKIP LOCKED  -- Skip rows locked by other transactions

remaining = DECR event:E1:remaining
if remaining >= 0:
    # Purchase succeeds — write to DB async
else:
    INCR event:E1:remaining  # Roll back
    # Sold out