Per-key synchronisation inside a single process.py

locks = Map<productId, LockObject>()

function get_lock(productId):
  if productId not in locks:
    locks[productId] = new LockObject()
  return locks[productId]

function purchase(productId):
  lock = get_lock(productId)

  synchronize(lock):                  # one thread per product at a time
    stock = query("SELECT stock FROM products WHERE id = ?", productId)
    if stock <= 0:
      return "Out of stock"

    exec("UPDATE products SET stock = stock - 1 WHERE id = ?", productId)
    return "OK"