xml/xsd is preferred as the schema definition, but the format "on the wire" is mostly tagvalue ASCII over TCP sockets these days (or binary using SBE if its latency sensitive e.g. HFT). JSON is relatively new in the FIX world. Some applications could encode fix in the front-end/browser and I can see some advantages of using FIX JSON in those scenarios and that's why we added support for it in FIXParser https://fixparser.dev
It's been like 10 years for me but all the systems I worked with just use the classic ASCII format; the last thing you'd want to do is inflate the messages with XML or JSON. Has that changed?
exactly, it is the same. basically the simplest way to parse is to split the message into key/value pairs separated by \x01 (SOH), and then inflate/enrich the message with field names and enumerations to make the message more understandable. the protocol is deceptively simple and the complexity moved to the validation instead, here's where the specification + versioning + vendor specific fields become complex because the devil is definitively in the details
Mine was actually a question as well. My thought was because of nano second accuracy, json won't be preferred. XML causes no issues here just the bloat.