JSON is very inefficient both in bytes (32 bit price is 4 bytes in binary and could be 7+ bytes as string, think "1299.99" for example) and CPU: to parse "1299.99" you need burn a lot of cycles, and if it is number of cents stored as native 4-byte number you need 3 shifts and 4 binary ors at most, if you need to change endianness, and in most cases it is simple memory copy of 4 bytes, 1-2 CPU cycle.
When you have binary protocol, you could skip fields which you are not interested in as simple as "offset = offset + <filed-size>" (where <filed-size> is compile-time constant!) and in JSON you need to parse whole thing anyway.
Difference between converting binary packet to internal data structure and parsing JSON with same data to same structure could be ten-fold easily, and you need to be very creative to parse JSON without additional memory allocations (it is possible, but code becomes very dirty and fragile), and memory allocation and/or deallocation costs a lot, both in GC languages and languages with manual memory management.
This is typical (from NASDAQ http://www.nasdaqtrader.com/content/technicalsupport/specifi... ):
Prices are integer fields. When converted to a decimal format, prices are in fixed point format with 6 whole number places followed by 4 decimal digits. The maximum price in OUCH 4.2 is $199,999.9900 (decimal, 7735939C hex). When entering market orders for a cross, use the special price of $214,748.3647 (decimal, 7FFFFFFF hex).
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.
FAST is not particularly common in Europe
the large European venues use fixed-width binary encoding (LSE group, Euronext, CBOE Europe)
https://msgpack.org/index.html
Good protobuf vs msgpack comparison: https://medium.com/@hugovs/the-need-for-speed-experimenting-...
https://uploads-ssl.webflow.com/5ba40927ac854d8c97bc92d7/5bf...
If you're optimizing for latency JSON is pretty terrible, but most people who use it are optimizing for interoperability and ease of development. It works just fine for that, and you can recover decent bandwidth just by compressing it.
