Asynchronous Programming

As a real-time data handling framework with user-interface functionalities, beamlime needs to handle asynchronous processes.

This page explains asynchronous characteristic of beamlime and debugging logs.

Debugging Logs

asyncio.get_event_loop vs asyncio.new_event_loop

1. asyncio.get_event_loop get_event_loop will always return the current event loop. If there is no event loop set in the thread, it will create a new one and set it as a current event loop of the thread, and return the loop. Many of asyncio free functions internally use get_event_loop, i.e. asyncio.create_task.

   Things to consider while using asyncio.get_event_loop.

   • asyncio.create_task does not guarantee whether the current loop is/will be alive until the task is done. You may use run_until_complete to make sure the loop is not closed until the task is finished. When you need to throw multiple async calls to the loop, use asyncio.gather to merge all the tasks like in this method.

   • close or stop might accidentally destroy/interrupt other tasks running in the same event loop. i.e. You can accidentally destroy the event loop of a jupyter kernel.

   • 1 :class:RuntimeError if there has been an event loop set in the thread object before but it is now removed.

2. asyncio.new_event_loop asyncio.new_event_loop will always return the new event loop, but it is not set it as a current loop of the thread automatically.

   However, sometimes it is automatically handled within the thread, and it caused errors which was hard to debug under pytest session. For example,

   • The new event loop was not closed properly as it is destroyed.

   • The new event loop was never started until it is destroyed.

   Traceback of pytest did not show where exactly the error is from in those cases. It was resolved by using asyncio.get_event_loop, or manually closing the event loop at the end of the test.

   When to use asyncio.new_event_loop.

   • asyncio.get_event_loop raises RuntimeError 1.

   • Multi-threads.

Please note that the loop object might need to be closed manually.