Distributed lock across instances.py

function acquire_distributed_lock(key, ttl_ms) -> (ok, token):
  # e.g. SET key token NX PX ttl_ms in Redis
  ...

function release_distributed_lock(key, token):
  # delete key only if stored token matches
  ...

function purchase(productId):
  (ok, token) = acquire_distributed_lock("lock:product:" + productId, ttl_ms=5000)
  if not ok:
    return "Busy, try again"

  try:
    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"
  finally:
    release_distributed_lock("lock:product:" + productId, token)