Alternate of @cyclone.web.asynchronous in tornado enviroment

Question

We are shifting our code infra from cyclone to tornado now. Previously we were using @cyclone.web.asynchronous for one of our api for non blocking async call in cyclone(so that we do not block the UI). What is the alternative of this in tornado, @tornado.web.asynchronous is not working in tornado 6.1. My code for cyclone is someting like this

class ABCHandler(cyclone.web.RequestHandler):

    @cyclone.web.asynchronous
    def post(self):
    some_validation()
    # Spawn a thread to import the files and leave the post method
    # asynchronous decorator will keep the request open to write the response on, 
    #  once the import is complete
    file_handler.start()  ---- this is a thread that do all the heavy work and in this method we are 
                               closing the request with self.finish

Class file_handler():
     run(self):
         {
          ---do some heavy work, like importing a file
          self.importing_a_large_file()
          self.set_status(status)
          self.write(json_response)
          self.finish()
       
}   

What can be its tornado equivalent method.

I tried various things like adding gencouroutine decorater, change method name to async but nothing seems to work.

Answer 1

Use Python's async def coroutines.

You can't use regular threading with Tornado, so you'll have to use the run_in_executor method. It will run the code in a separate thread but will allow you to wait for the result without blocking.

class ABCHandler(tornado.web.RequestHandler):
    async def post(self):
        loop = tornado.ioloop.IOLoop.current()

        some_data = await loop.run_in_executor(executor=None, func=blocking_func)

       self.write(some_data)


# this is a blocking code
# you don't have to create a separate thread for this
# because `run_in_executor` will take care of that.
def blocking_func():
    # do blocking things here
    return some_data

Answer 2

It sounds like cyclone.web.asynchronous is equivalent to tornado.web.asynchronous, so it may be better for you to first move from cyclone to Tornado 5.1 (where the asynchronous decorator was still supported) then to move to coroutines and Tornado 6.x in a separate step. (Or if cyclone supports coroutines, move to coroutines on cyclone before switching to Tornado).

If you try to move from cyclone.web.asynchronous to Tornado 6 with native coroutines all in one jump, it will be a very difficult refactoring. Also, your sample code looks like it calls methods like RequestHandler.finish from another thread. I'm not sure if that's allowed in cyclone, but it's definitely not in Tornado.