API rate limit

An API rate limit is a cap on how many requests an account may send to a broker's programming interface within a period. Requests beyond it are refused or delayed, and the limit is usually not one number: different endpoints are counted separately, and the allowance for reading prices is typically not the allowance for sending orders. Handling refusals badly is the more expensive half. Retrying immediately makes the situation worse, because the retries are counted too. More seriously, a refusal is not always distinguishable from other failures, and an order request that timed out on the client side may still have been received: treating no answer as no order is how a system sends the same order twice. Anything that trades through an interface needs to establish what actually happened before acting again, and to slow itself down rather than push against a limit it has already reached.