How to register a global state for actix_web? : I am a new user of `Rust` and `actix_web` crate. I am trying to set up a global state for my HttpServer, and it seems like `register_data` is the proper API. (I could be wrong about this API).

But from the [doc][1], it is not clear to me how to create a single instance of Application Data shared by all HttpServer threads. Here is my code piece:
```
    HttpServer::new(|| {
        App::new()
            .register_data(web::Data::new(Mutex::new(MyServer::new())))
            .service(web::resource("/myservice").route(web::post().to(my_service)))
            .service(web::resource("/list").to(list_service))
    })
```

In the POST handler `my_service`, I update the state of `MyServer` and in the GET handler `list_service`, it will print out the state. 

While `my_service` is successful in storing the state, the `list_service` only prints empty output. My question is: 

How do I know if `actix` `HttpServer` created a single instance of `MyServer` or not? If not, how can I ensure it creates a single instance?  

Thanks.


  [1]: https://docs.rs/actix-web/1.0.9/actix_web/web/struct.Data.html