Tornado difference between run in executor and defining async methods

Question

I am pretty new to Tornado. I can't understand the difference between using run_on_executor and defining an async method. Is it the same? Is the one multithreaded and the other not?

Thank you in advance.

Answer 1

run_on_executor is for interfacing with blocking non-async code.

You are correct that async code is only executed in a single thread. Maybe an example would illustrate the point.

Let's say your Tornado web service interfaces with a library that makes use of requests to fetch country info for a given IP address. Since requests is a non-async library, calling this function would block the Tornado event loop.

So, you have two options: try to find the replacement for the library that is async-compatible OR run the blocking code in a different thread/process and have your event loop await its result like for normal async code without blocking the event loop. The latter option is run_on_executor which allows you to run the task in different thread or process, and asyncio would "await" its completion.