"old skool" exchanges uses either FIX (old and really vernose), FAST (binary encoding for FIX) or custom fixed-layout protocols. Most big USA exchanges uses cu

"old skool" exchanges uses either FIX (old and really vernose), FAST (binary encoding for FIX) or custom fixed-layout protocols. Most big USA exchanges uses custom fixed-layout protocols, where each message is described in documentation, but not in machine-readable way. European ones still use FAST. I didn't seen FIX in the wild for data feeds, but it is used for brokers, to submit orders to exchange (our company didn't do this part, we only consume feeds). I don't know why, but all Crypto Exchanges use JSON, not protobufs or something like this, and didn't publish any formal schemes. Fun fact: one crypto exchange put GZIP'ed and base64'ed JSON data into JSON which pushed to websocket, to save bandwidth. IMHO, it is peak of bad design.