Fix api
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Not to be confused with api trading.
FIX API is an interface through which a trading system exchanges messages with a broker or venue using the FIX protocol, a session-based, tag-value messaging standard originally developed for equity order routing. The API exposes the protocol's message types — such as new order single, execution report and market data request — to an application, rather than presenting a graphical order ticket. Access, session configuration and permitted message types are set by the counterparty and vary by broker, venue and regulatory regime.
How a FIX session works
A FIX session is a persistent, sequenced connection between two parties, typically over TCP with optional TLS. Each message carries a header with fields such as SenderCompID, TargetCompID and MsgSeqNum, followed by a body of tagged fields and a checksum trailer. Both sides maintain sequence numbers; a gap triggers a resend request rather than silent loss.
Common message types include:
- NewOrderSingle (D) — submit an order.
- ExecutionReport (8) — acknowledge, fill or reject an order.
- OrderCancelRequest (F) — request cancellation.
- MarketDataRequest (V) — subscribe to quotes.
Session parameters, supported FIX versions and available order types are specified by the broker or venue and differ between them.
Worked example: sequencing and fills
A client sends an order and receives an acknowledgement followed by a partial fill. Sequence numbers advance with every message in each direction.
Operational considerations
Because FIX is a session protocol, disconnections, sequence gaps and duplicate messages must be handled explicitly. Many implementations use a heartbeat interval and a test request to detect a dead session, and a logon message to re-establish sequence state.
Certification against a counterparty's test environment is commonly required before production use. The exact FIX version, required fields, session hours and drop-copy arrangements are not universal and are agreed with each broker or venue.
Often confused with
- api trading
- API trading is the general practice of trading through any programmatic interface, which may be REST, WebSocket or a proprietary protocol, whereas a FIX API specifically uses the FIX message standard; the visible sign is whether messages are tag-value FIX strings such as 35=D rather than JSON or query parameters.