Networks fail mid-request. If you send a market buy and the connection drops before the response arrives, did the order fill? Without protection, retrying can double your position. Exchanges solve this with client order IDs (a.k.a. newClientOrderId, clOrdId, or client_order_id) — a unique string you generate that the exchange stores alongside the order.

When you retry with the same client ID, the exchange returns the existing order instead of creating a new one. This makes your request idempotent.

Generate IDs with a UUID or a monotonic counter persisted to disk.
Never reuse an ID for a different order — some venues reject duplicates outright.
Always reconcile via a GET /orders?clientOrderId=... query after any timeout.

State	Meaning	Action
NEW	Accepted, not yet on book	Wait or cancel
PARTIALLY_FILLED	Some quantity matched	Track remaining
FILLED	Fully executed	Update position
CANCELED / REJECTED	Terminal, no further fills	Log and move on