is not FIX too complicated to implement? I found more and more brokers provide REST, few brokers actually provide FIX access to individual traders.
Overall, I personally find it easier to implement and document a FIX API. But a stateless server application is probably easier to scale, and HTTPS traffic could be easier to load balance with commodity hardware. I think the reason for the trend you're seeing is likely a function of how web applications are often built nowadays, as single page applications that render data from a REST API backend. Chances are that a retail broker already had to build and test a REST API backend for their mobile app, browser dashboard or whatever. It stands to reason that it's probably less effort for them to reuse the same backend developers and deployment practices to expose a public API, and provide documentation for said public API. If you're connecting over internet anyway, the only performance consideration I'd stress about is your broker's API request limit and any throttling/blacklisting policies. As others have pointed out, REST is stateless so you'll need to continually poll for order and position state while being economical with your API requests. Yes. As a side effect it's also convenient to hitchhike on ports 80/443 without needing additional routing changes. On C/C++: right, in any case I think it's easier to convince your sysadmins that you need libcurl than a custom build of my-happy-crypto-cpp-beta-sdk. Agreed.