What is the correct method to call a synchronous task from a

Question

What is the correct method to call a synchronous task from an asynchronous flow in Prefect 2 When trying to this with the attached code our asynchronous `async task` s will execute but our `sync task` will never start and the flow just seems to hang The intention here is to run some asynchronous tasks gather the results then send those results to a synchronous or blocking task that can be executed sequentially or submitted to a concurrent task runner The equivalent code works in a standard Python script if we remove our Prefect decorators ```from prefect import flow task import asyncio time task async def async task n await asyncio sleep 1 return n task def sync task res time sleep 1 print res flow async def async flow res = await asyncio gather async task n for n in range 0 3 1 sync task res if name == main asyncio run async flow ``` See output in replies

Ilya Galperin · Answer

Our output in Prefect `2 6 0` reads like this then the execution just hangs it looks like the `sync task` never starts ```13 22 40 277 | INFO | prefect engine Created flow run tourmaline pronghorn for flow async flow 13 22 41 313 | INFO | Flow run tourmaline pronghorn Created task run async task 1da93ad6 0 for task async task 13 22 41 314 | INFO | Flow run tourmaline pronghorn Executing async task 1da93ad6 0 immediately 13 22 41 349 | INFO | Flow run tourmaline pronghorn Created task run async task 1da93ad6 2 for task async task 13 22 41 350 | INFO | Flow run tourmaline pronghorn Executing async task 1da93ad6 2 immediately 13 22 41 370 | INFO | Flow run tourmaline pronghorn Created task run async task 1da93ad6 1 for task async task 13 22 41 370 | INFO | Flow run tourmaline pronghorn Executing async task 1da93ad6 1 immediately 13 22 42 872 | INFO | Task run async task 1da93ad6 0 Finished in state Completed 13 22 42 942 | INFO | Task run async task 1da93ad6 2 Finished in state Completed 13 22 42 962 | INFO | Task run async task 1da93ad6 1 Finished in state Completed ```

Khuyen Tran · Answer

This is a bug that we are working on fixing This is the workaround for now ```from prefect import flow task import asyncio time task async def async task n await asyncio sleep 1 return n task def sync task res time sleep 1 res append 1 flow async def async flow res = await asyncio gather async task n for n in range 0 3 1 return res flow def sync flow res return sync task res if name == main res = asyncio run async flow sync flow res ``` Basically you create one flow for asynchronous tasks and one flow for synchronous tasks

Ilya Galperin · Answer

Thanks for the response Khuyen This work around unfortunately seems less than ideal unless I m misunderstanding we would need to potentially create a pretty large number of flows if we have some workloads that need to mix a lot of synchronous and asynchronous functionality Is it better possible to do this using sub flows instead Separately is this bug logged anywhere we can track and are there any work arounds that do not require us to create separate flows for workloads that require both async and sync functionality

Khuyen Tran · Answer

< Mason Menges> Maybe you know where the issues are

Mason Menges · Answer

There are actually several around mixing Async and Sync tasks these are two that I know of currently That said I know for certain theres active work going on to improve the Orion engine reliability and mixing async and synchronous tasks is a specific problem we re tackling in that process

Ilya Galperin · Answer

Thanks Mason and Khuyen Do we need to create a new issue for the hanging bug I m describing It doesn t seem like either of these two issues are the same as what I m describing unless I m misunderstanding them

Ilya Galperin · Answer

It looks like there is another issue here of mixing async and sync tasks in an async flow but using submit method rather than just calling a task directly which apparently doesn t hang and throws an error <https github com PrefectHQ prefect issues 6318>

Mason Menges · Answer

This is a pretty clear reproducible example so I think that d be useful if you d be willing to smile

Ilya Galperin · Answer

Will do thanks Mason

Ilya Galperin · Answer

FYI ticket created here <https github com PrefectHQ prefect issues 7551>