+This Python 3 example makes sequential calls on one thread in one process. The server supplies an authenticated caller identifier as owner; key and item are already validated strings. One dictionary entry stores the order contents and number together, and records remain in the dictionary for the run.
requests = {}
def create_order(owner, key, item):
identity = (owner, key)
if identity in requests:
previous_item, order_id = requests[identity]
if previous_item != item:
raise ValueError("KEY_REUSED_WITH_DIFFERENT_REQUEST")
return order_id
order_id = len(requests) + 1
requests[identity] = (item, order_id)
return order_id
first = create_order("alice", "request-1", "book")
retry = create_order("alice", "request-1", "book")
another = create_order("alice", "request-2", "book")
print(first, retry, another) # 1 1 2
The same caller, key, and item return the existing order number; a new key creates a separate order for the same item. Calling create_order("alice", "request-1", "pen") with changed contents under the same key raises ValueError. The (owner, key) pair separates keys belonging to different callers.
The store is an in-memory dictionary: orders and retry records disappear when the process ends. A real server also has concurrent requests and recovery after a failure, which leads to the need for atomic commitment below.
