What is a FIX API? Plus a practical FIX API example
In the world of electronic trading, speed, reliability, and standardisation are critical. The Financial Information eXchange (FIX) Protocol is an industry messaging standard used across the global trading ecosystem. FIX messages are used to communicate trade-related information such as orders, executions, and cancellations in real time.
When developers refer to a "FIX API," they're usually talking about the programming interface exposed by a FIX Engine SDK, which enables integration of the FIX Protocol into proprietary trading platforms, order management systems (OMS), or execution management systems (EMS). These FIX Engine SDKs implement the FIX messaging standard and provide tools to send and receive FIX messages programmatically.
Whether you're developing a low-latency trading application or looking to connect to a new venue, understanding how FIX integration works – and what a sample message looks like – is key to reducing development time, avoiding common errors, and improving performance.
What is the FIX Protocol?
The FIX Protocol was originally developed in the early 1990s to standardise and replace manual methods of order handling and communication in financial markets. It defines a structure for messages using a tag=value format and supports trading in asset classes such as equities, futures, options, and foreign exchange.
FIX messages are sequences of tag-value pairs, where each tag has a specific role. For example, tag 35 indicates the message type, tag 55 represents the security symbol, and tag 54 indicates the order side (buy or sell).
A closer look at FIX messages
Here's an example of a FIX message that represents a new limit order:
8=FIX.4.2|9=176|35=D|49=SENDER|56=TARGET|34=2|52=20240319-14:35:00|11=12345|21=1|55=AAPL|54=1|38=100|40=2|44=150.50|10=128|
This message uses the FIX 4.2 version and is of type "D" (New Order - Single). It contains details about the sender, target, timestamp, instrument, side, quantity, order type, price, and checksum. Each field is defined in the FIX Protocol and must follow a specific format. The final field (tag 10) is the checksum, which validates the message integrity.
For clarity, the pipe symbol | is used here to represent the standard FIX field delimiter (ASCII "SOH" character, 0x01). When sent over the wire, this delimiter separates each tag=value pair.
FIX API example: placing a limit order
Let’s take a simplified example of submitting a buy limit order for 1,000 shares of MSFT at $300 using the FIX message format.
8=FIX.4.4|9=112|35=D|49=CLIENT12|56=VENUE1|34=4|52=20240319-15:01:00.000|11=ORD123|21=1|55=MSFT|54=1|38=1000|40=2|44=300.00|59=0|10=099|
This message uses FIX 4.4 and includes all required fields for order placement. Once received, the venue will respond with an Execution Report (MsgType=8) confirming the order status.
If you'd like to validate or experiment with sample FIX messages, try our free online FIX Parser tool, which allows you to check message structure and field accuracy.
Why use a FIX Engine SDK?
Implementing the FIX Protocol from scratch is challenging and error-prone. FIX Engine SDKs handle protocol compliance, message encoding/decoding, session management, and error handling, allowing development teams to focus on trading logic and integration.
At OnixS, we provide ultra-low latency FIX Engine SDKs in C++, .NET, and Java. These SDKs are used by proprietary trading firms, banks, and exchanges around the world to FIX-enable their trading infrastructure.
Key benefits of using our FIX Engine SDKs include:
- Performance: Optimised for latency-sensitive trading.
- Ease of integration: Simple APIs for embedding into existing platforms.
- Protocol support: Full support for FIX versions including FIX 4.2, 4.4, and FIXT.1.1.
- Tools: Free access to development and operational tools like the OnixS FIX Dictionary and FIX Analyser.
- Support and updates: Continuous improvements and expert technical assistance.
Learn more with our technical resources
For a more detailed look at FIX message structure, field types, and validation rules, check out our FIX Message Guide. It explains how messages are built, how repeating fields work, how checksums are calculated, and much more.
Need help integrating FIX into your trading systems? Contact us to explore our FIX Engine SDKs and development tools. You can also request a free 30-day evaluation to test performance and compatibility in your environment.
