⚡️ Parallelize dependency resolution #13756
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archfz
commented
Jun 2, 2025
| @@ -29,7 +29,7 @@ async def get_sub_counter( | |||
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| @app.get("/sub-counter-no-cache/") | |||
| async def get_sub_counter_no_cache( | |||
| subcount: int = Depends(super_dep), | |||
| subcount: int = Depends(dep_counter), | |||
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Here I had to change this to the topmost dep, because one level lower already resolves just a tiny bit later and in that case this test is not valid anymore.
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Tested this in a medium side production backend with fastapi setup. Found several issues, more notably around caching and exception handling. Fixed these, however these warrant for additional test cases. |
archfz
commented
Jun 3, 2025
| ensure_cache(None) | ||
| return sub_dependant.name, None, exc.errors, None | ||
| except BaseException as resolution_exc: | ||
| ensure_cache(resolution_exc) |
archfz
commented
Jun 3, 2025
fastapi/dependencies/utils.py
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| for name, result, sub_errors, sub_exception in results: | ||
| # Ensures order of exception based on dependency order. | ||
| if sub_exception: | ||
| raise sub_exception |
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This resolves a serious performance issue in fastapi: async dependencies resolving sequentially. #639
I have attempted to pick up this previous work #3902 but I thought it was too complicated. So I am giving it a more simpler localized approach. Two things to mention:
Testing wise I'd need suggestions. Not sure how to test this and I also expect that many tests already cover the changes. Asserting that things are done in parallel would be beneficial but I'd need help with that.
EDIT:
Added in the meantime an option, just like cache, on the dependency definition called parallelizable. This is by default opt-out style, meaning it is True. Maybe to prevent breaking changes we can ship this with opt-in style. Let community experiment with it and in time if no issues are creeping up it can be considered to switch to opt-out style with a major release. For Security however I am thinking that maybe we should have it always False.