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Thanks for contributing to Prefect!
Please describe your work and make sure your PR:
CHANGELOG.md
(if appropriate)docs/outline.toml
for API reference docs (if appropriate)Note that your PR will not be reviewed unless all three boxes are checked.
What does this PR change?
This PR refactors how we manage background heartbeats from task / flow runs.
Why is this PR important?
It turns out that the old implementation of heartbeating led to non-deterministic deadlocks when using dask
worker_client
s. Ultimately this boiled down to some complicated interaction between dask / requests / our Timer implementation. These deadlock situations would ultimately show themselves as Zombie Tasks in Prefect Cloud. The new implementation uses aThreadPoolExecutor
to manage the thread and its associated state, which empirically has resolved the issue!