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feat: add a concurrency limit for named child runs - #1154

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feat/child-run-concurrency-limit
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feat/child-run-concurrency-limit

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@vdusek vdusek commented Sep 30, 2026 •

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Stacked on #1153.

Adds Actor.set_child_run_limits(max_concurrent_runs=...). While that many named child runs are READY, RUNNING, ABORTING or TIMING-OUT, a named Actor.start, Actor.call, Actor.start_task or Actor.call_task that would start or resurrect a run waits for one of them to finish. Reattaching to a recorded run never waits, and runs without a run_name aren't counted.

The count comes from the child run registry, so children started before a migration hold their slots too. A child's status is fetched again before counting once it's more than 10 seconds old, so a finished run nobody waited on frees its slot. Actor.call and Actor.call_task free the slot as soon as their run finishes. A start holds its slot until the run is recorded, which keeps concurrent starts under different names within the limit.

The limit lives in memory, so the Actor sets it again after a migration. Once the parent receives ABORTING, a start still waiting for a slot raises a RuntimeError without starting a run. Waiting further would block the abort of the other children.

Closes: #1138

✍️ Drafted by Claude Code

@vdusek vdusek added the t-tooling Issues with this label are in the ownership of the tooling team. label Sep 30, 2026
@vdusek vdusek self-assigned this Sep 30, 2026
@vdusek
vdusek added this pull request to stack #1152 September 30, 2026 09:49
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codecov Bot commented Sep 30, 2026 •

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.37%. Comparing base (4ff9c35) to head (3447fab).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files
@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
+ Coverage   93.25%   93.37%   +0.11%     
==========================================
  Files          54       54              
  Lines        3766     3834      +68     
==========================================
+ Hits         3512     3580      +68     
  Misses        254      254              
Flag Coverage Δ
e2e 31.58% <33.33%> (+0.06%) ⬆️
integration 51.12% <33.33%> (-0.29%) ⬇️
unit 90.27% <100.00%> (+0.17%) ⬆️

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@vdusek
vdusek force-pushed the feat/child-run-concurrency-limit branch from 5bbadc2 to 033624c Compare October 6, 2026 13:20
@vdusek
vdusek force-pushed the feat/child-run-concurrency-limit branch from 7322d8e to 7c3d462 Compare October 9, 2026 15:11
@vdusek
vdusek force-pushed the feat/child-run-concurrency-limit branch from 7c3d462 to 6f9300e Compare October 15, 2026 12:35
Base automatically changed from feat/abort-child-runs-with-parent to master October 29, 2026 13:19
@vdusek
vdusek force-pushed the feat/child-run-concurrency-limit branch from 6f9300e to c2f5a87 Compare October 29, 2026 13:38
@vdusek
vdusek marked this pull request as ready for review October 29, 2026 14:26
@apify-service-account apify-service-account added the tested Temporary label used only programatically for some analytics. label Oct 29, 2026
@vdusek
vdusek requested a review from barjin October 29, 2026 14:28
@vdusek
vdusek requested a review from Pijukatel October 29, 2026 14:29

@Pijukatel Pijukatel left a comment

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I am struggling a lot with the variable naming in this PR. Reading this with fresh eyes the variable names gave me a lot of false leads instead of building a helpful mental model.

The usage of word slot here had a very confusing effect on me. One time it was for an asyncio.Condition and the second time for a very custom context manager that uses the first one inside...
On top of it, slot has its own established meaning in Python, but here it is not related in any manner.

Comment thread src/apify/_child_runs.py
"""Client the run under each name was last started or looked up with. Lost on a migration."""
self._max_concurrent_runs: int | None = None
"""How many recorded runs may be active at once, or `None` for no limit."""
self._slots = asyncio.Condition()

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This sounds more a like an iterable, not a condition name. Can you please use some condition like name.

Maybe something like _slots_available ?

Comment thread src/apify/_actor.py
"""
return self._child_run_registry.run_clients(self.apify_client)

def set_child_run_limits(self, *, max_concurrent_runs: int | None) -> None:

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The named in the description is critical. I would probably put it to the method name, because without reading the description you will get a wrong idea from just the method name. It is pretty uggly and explicit, but better than giving a wrong idea:
set_named_child_run_limits

Comment thread src/apify/_child_runs.py
"""How many recorded runs may be active at once, or `None` for no limit."""
self._slots = asyncio.Condition()
"""Serializes counting the active runs, and wakes the starts waiting for a slot when one may have freed up."""
self._starting: set[str] = set()

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This sounds like a state, not a set. Can we have a noun, so that we know what is starting?

Maybe something like _starting_names?

Comment thread src/apify/_child_runs.py
self._observe(name, run)
if record.status != run.status:
await self._save(name, record.model_copy(update={'status': run.status}))
async with self._slots:

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To me the naming is extremely confusing. We have these similarly named context managers. async with self._slots, async with self._slot, but they are quite something different.

When having a singluar item _slot, than I expect that _slots will be just some structure holding multiple of _slot, but here it is just completelly different object.

Comment thread src/apify/_child_runs.py
"""Serializes counting the active runs, and wakes the starts waiting for a slot when one may have freed up."""
self._starting: set[str] = set()
"""Names holding a slot for a start or resurrection in flight, not counted from the records yet."""
self._observed: dict[str, tuple[str, float]] = {}

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observed what? The name for this will be even more difficult, but observed tells too little...

Maybe timestamped_run_statuses?

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Limit the number of concurrent child runs

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