Currently, client and server APIs are basically:

fn start_request(Headers, request_body: Stream<Part>) -> Stream<Part>; 

The client invokes this function and server provides a callback for that function.

• this API is not very easy API to use, because Stream is not trivial to implement (request for a client and response for a server)
• httpbis library requires to spawn a separate task just to "pull" from a provided stream, because of that httpbis performs additional work when user implementation already has a task which provides data

The proposed API should be like this:

// client: fn start_request(Headers) -> (Sink<Part>, Stream<Part>); // return a request sink and response stream // server fn start_request(Headers, request: Stream<Part>, response: Sink<Part>); 

Note h2 library and grpc-rs use similar APIs.