Design a Ticketing System (BookMyShow / TicketMaster)

-- Redis Lua script: atomic multi-seat hold
-- KEYS = list of seat lock keys
-- ARGV[1] = user_id, ARGV[2] = TTL in seconds

-- Phase 1: Check all seats are available
for i, key in ipairs(KEYS) do
    if redis.call('EXISTS', key) == 1 then
        return {0, key}  -- FAIL: seat already held, return which seat
    end
end

-- Phase 2: All available → lock all atomically
for i, key in ipairs(KEYS) do
    redis.call('SET', key, ARGV[1], 'EX', ARGV[2])
end

return {1, 'OK'}  -- SUCCESS: all seats held

BEGIN TRANSACTION;
  -- 1. Verify hold is still valid and belongs to this user
  SELECT hold_id, user_id, seat_ids, hold_expires_at 
  FROM seat_holds 
  WHERE hold_id = $1 AND user_id = $2 AND hold_expires_at > NOW()
  FOR UPDATE;
  
  -- 2. Verify payment was authorized
  
  -- 3. Update all seats to BOOKED
  UPDATE seats 
  SET status = 'booked', booked_by = $2, booking_id = $3 
  WHERE show_id = $4 AND seat_id = ANY($5) AND status = 'held' AND held_by = $2;
  
  -- 4. Verify all seats were updated (row count = expected seat count)
  
  -- 5. Insert booking record
  INSERT INTO bookings (booking_id, user_id, show_id, seat_ids, total_amount, payment_id, status, booked_at)
  VALUES ($3, $2, $4, $5, $6, $7, 'confirmed', NOW());
  
  -- 6. Insert audit log
  INSERT INTO booking_audit_log (booking_id, action, actor, details, created_at)
  VALUES ($3, 'CONFIRMED', $2, '{"seats": ...}', NOW());
  
  -- 7. Decrement available seat count
  UPDATE shows SET available_seats = available_seats - $8 WHERE show_id = $4;
  
COMMIT;

# Queue drainer worker loop
while True:
    # Pop next batch of users whose turn it is
    users = redis.zpopmin(f"queue:{event_id}", count=BATCH_SIZE)
    
    for user_id, score in users:
        # Issue a signed booking token (JWT)
        token = jwt.encode({
            "user_id": user_id,
            "event_id": event_id,
            "issued_at": now(),
            "expires_at": now() + timedelta(minutes=10),
            "max_tickets": 4
        }, SECRET_KEY)
        
        # Notify user their turn has arrived
        push_notification(user_id, {"status": "your_turn", "booking_token": token})
    
    sleep(BATCH_SIZE / DRAIN_RATE)

{
  "type": "seat_update",
  "show_id": "show-uuid",
  "updates": [
    {"seat_id": "P-A1", "status": "held"},
    {"seat_id": "P-A2", "status": "held"}
  ]
}

# Runs every 30 seconds
def cleanup_expired_holds():
    # Find expired holds in PostgreSQL
    expired = db.query("""
        SELECT hold_id, show_id, seat_ids 
        FROM seat_holds 
        WHERE status = 'active' AND hold_expires_at < NOW()
    """)
    
    for hold in expired:
        db.begin_transaction()
        db.execute("UPDATE seat_holds SET status = 'expired' WHERE hold_id = %s", hold.hold_id)
        db.execute("""
            UPDATE seats SET status = 'available', held_by = NULL, held_until = NULL 
            WHERE show_id = %s AND seat_id = ANY(%s) AND status = 'held'
        """, hold.show_id, hold.seat_ids)
        db.execute("""
            UPDATE shows SET available_seats = available_seats + %s 
            WHERE show_id = %s
        """, len(hold.seat_ids), hold.show_id)
        db.commit()
        
        # Publish seat-available event
        kafka.produce('seat-status-changes', {
            'show_id': hold.show_id,
            'seats': hold.seat_ids,
            'status': 'available',
            'reason': 'hold_expired'
        })