Phoenix depends on cowboy for HTTP and websockets, and cowboy has a feature to limit websocket frame sizes.

This commit from Nov 2018 exposed the max_frame_size setting in Phoenix. It’s in the master branch, but hasn’t made its way into a release yet.

If you have a proxy/ballancer between world and your backend and you want to save CPU/bandwith on proxy (that is the better way) - you need to check request size on it - not on backend. If you don’t care about proxy - I’m not sure could it be accomplished on earler phases, but you can implement your custom websocket transport and check input message size before deserialization

I am agreeing with everything you have said. My option does not check size, as it is useless after the deserialization. Like I said, I don’t know if they wanted to literally reject a payload based on some size, or if they wanted to do some validations on the payload in order to prevent a resource intensive job from being started for known failures or something.