Orders per second (OPS) is a measure of the rate at which order messages, meaning new orders, changes and cancellations, are sent by a trading participant or processed by an exchange. It is used in two directions. For a venue it describes capacity: the throughput its gateways and matching engine can sustain without delay. For a participant it describes how intensively a connection is being used.
Exchanges assign each trading connection a maximum message rate and apply a throttle, a form of rate limiting, that delays or rejects orders above it. Firms engaged in high-frequency trading buy extra connections to raise their allowance. The figure also appears in regulation. Indian exchanges use an orders-per-second threshold in their rules for algorithmic trading, with orders above it treated as algorithmic and subject to registration. Message rates are monitored together with the order-to-trade ratio as part of algorithmic trading controls.