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# Stop Building Synchronous Web Containers
Seriously, stop it. It's surreally difficult to build a sane ansynchronous service on top of a synchronous API, but building a synchronous service on top of an asynchronous API is easy.
* WSGI: container calls the application as a function, and uses the return
value for the response body. Asynchronous apps generally use a non-WSGI
base (see for example [Bottle](http://bottlepy.org/docs/dev/async.html)).
* Rack: container calls the application as a method, and uses the return
value for the complete response. Asynchronous apps generally use a non-Rack
base (see [this Github ticket](https://github.com/rkh/async-rack/issues/5)).
* Java Servlets: container calls the application as a method, passing a
callback-bearing object as a parameter. The container commits and closes
the response as soon as the application method returns. Asynchronous apps
can use a standard API that operates by _re-invoking_ the servlet method as
needed.
* What does .Net do?
vs
* ExpressJS: container calls the application as a function, passing a
callback-bearing object as a parameter. The application is responsible for
indicating that the response is complete.
## Synchronous web containers are bad API design
* Make the easy parts easy (this works)
* Make the hard parts possible (OH SHIT)
## Writing synchronous adapters for async APIs is easy
def adapter(request, response_callback):
synchronous_response = synchronous_entry_point(request)
return response_callback(synchronous_response)
Going the other way is more or less impossible.
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