Maybe they are; as far as I know, the protocol they use to communicate with the worker isn't specified, all they say is "implement a function with this signature, then import our SDK, and it will call your function". This gives them the flexibility to switch protocols at will, at the cost of having to implement these SDKs.
That said, building a FCGI "bridge" isn't hard, it's just that nobody cared enough to do it.
Basically: your instance POSTS to an endpoint to get/reply to requests, one at a time.
I can't wait until you can specify that a lambda instance can handle a certain # of requests in parallel. Ex: things just blocked on IO/DB. That would make better use of your RAM.
