Avoid duplicate trigger-rule upstream-count queries per scheduling pass

airflow-core/newsfragments/67672.improvement.rst

@@ -0,0 +1 @@
+The scheduler no longer issues the trigger-rule upstream task-instance count query once per downstream task. For tasks that share the same upstreams within a scheduling pass, the count is computed once and reused, cutting database round-trips for DAGs where a mapped upstream feeds many downstream tasks.

airflow-core/src/airflow/models/dagrun.py

@@ -1520,6 +1520,10 @@ def _expand_mapped_task_if_needed(ti: TI) -> Iterable[TI] | None:
                 if new_tis is not None:
                     additional_tis.extend(new_tis)
                     expansion_happened = True
+                    # Expansion changes a mapped task's instance count, which invalidates the
+                    # trigger-rule upstream-count memo on this DepContext (a downstream evaluated
+                    # later in this same pass must see the post-expansion count).
+                    dep_context.upstream_task_id_counts.clear()
             if new_tis is None and schedulable.state in SCHEDULEABLE_STATES:
                 # It's enough to revise map index once per task id,
                 # checking the map index for each mapped task significantly slows down scheduling

airflow-core/src/airflow/ti_deps/dep_context.py

@@ -85,6 +85,19 @@ class DepContext:
     have_changed_ti_states: bool = False
     """Have any of the TIs state's been changed as a result of evaluating dependencies"""
 
+    upstream_task_id_counts: dict[tuple[str, str, frozenset[str]], list[tuple[str, int]]] = attr.ib(
+        factory=dict, init=False
+    )
+    """
+    Per-pass memo of the trigger-rule upstream task-instance counts, keyed by
+    ``(dag_id, run_id, frozenset of direct-upstream task_ids)``.
+
+    Shares the lifetime and snapshot semantics of ``finished_tis`` (one scheduling pass). Only
+    populated for the "simple" case where the count-query predicate is exactly
+    ``task_id IN (upstream_ids)`` and is therefore identical for every downstream sharing the same
+    direct upstreams; the mapped-task-group case uses per-ti map-index predicates and is not cached.
+    """
+
     def ensure_finished_tis(self, dag_run: DagRun, session: Session) -> list[TaskInstance]:
         """
         Ensure finished_tis is populated if it's currently None, which allows running tasks without dag_run.

airflow-core/src/airflow/ti_deps/deps/trigger_rule_dep.py

@@ -20,7 +20,7 @@
 import collections.abc
 import functools
 from collections import Counter
-from collections.abc import Iterator, KeysView, Mapping, Sequence
+from collections.abc import Iterator, KeysView, Mapping
 from typing import TYPE_CHECKING, NamedTuple
 
 from sqlalchemy import and_, func, or_, select
@@ -31,7 +31,6 @@
 from airflow.utils.state import TaskInstanceState
 
 if TYPE_CHECKING:
-    from sqlalchemy.engine import Row
     from sqlalchemy.orm import Session
     from sqlalchemy.sql import ColumnElement
 
@@ -40,7 +39,6 @@
     from airflow.serialization.definitions.taskgroup import SerializedMappedTaskGroup
     from airflow.ti_deps.dep_context import DepContext
     from airflow.ti_deps.deps.base_ti_dep import TIDepStatus
-    from airflow.typing_compat import Unpack
 
 
 class _UpstreamTIStates(NamedTuple):
@@ -372,13 +370,34 @@ def _evaluate_direct_relatives() -> Iterator[TIDepStatus]:
                 upstream = len(upstream_tasks)
                 upstream_setup = sum(1 for x in upstream_tasks.values() if x.is_setup)
             else:
-                # The below type annotation is acceptable on SQLA2.1, but not on 2.0
-                task_id_counts: Sequence[Row[Unpack[tuple[str, int]]]] = session.execute(  # type: ignore[type-arg]
-                    select(TaskInstance.task_id, func.count(TaskInstance.task_id))
-                    .where(TaskInstance.dag_id == ti.dag_id, TaskInstance.run_id == ti.run_id)
-                    .where(or_(*_iter_upstream_conditions(relevant_tasks=upstream_tasks)))
-                    .group_by(TaskInstance.task_id)
-                ).all()
+                # In the simple case, `_iter_upstream_conditions` emits exactly
+                # `task_id IN (upstream_task_ids)` (the matching `get_closest_mapped_task_group()
+                # is None` branch). That predicate, and therefore the resulting counts, are
+                # identical for every downstream that shares the same set of direct upstreams, so
+                # we memoize them on the DepContext and run the query once per pass instead of
+                # once per downstream. The mapped-task-group case uses per-ti map-index predicates
+                # and is left un-memoized. The cache shares finished_tis' per-pass snapshot
+                # semantics; it is cleared in DagRun._get_ready_tis when a mapped task expands and
+                # changes its instance count.
+                cache_key: tuple[str, str, frozenset[str]] | None = None
+                task_id_counts: list[tuple[str, int]] | None = None
+                if task.get_closest_mapped_task_group() is None:
+                    cache_key = (ti.dag_id, ti.run_id, frozenset(upstream_tasks))
+                    task_id_counts = dep_context.upstream_task_id_counts.get(cache_key)
+                if task_id_counts is None:
+                    task_id_counts = [
+                        (task_id, count)
+                        for task_id, count in session.execute(
+                            select(TaskInstance.task_id, func.count(TaskInstance.task_id))
+                            .where(TaskInstance.dag_id == ti.dag_id, TaskInstance.run_id == ti.run_id)
+                            .where(or_(*_iter_upstream_conditions(relevant_tasks=upstream_tasks)))
+                            .group_by(TaskInstance.task_id)
+                        )
+                    ]
+                    if cache_key is not None:
+                        dep_context.upstream_task_id_counts[cache_key] = task_id_counts
+                # `task_id_counts` only contains task_ids matched by `task_id IN (upstream_tasks)`,
+                # so every key is present in `upstream_tasks`; is_setup is re-derived locally.
                 upstream = sum(count for _, count in task_id_counts)
                 upstream_setup = sum(c for t, c in task_id_counts if upstream_tasks[t].is_setup)
 

airflow-core/tests/unit/ti_deps/deps/test_trigger_rule_dep.py

@@ -18,13 +18,16 @@
 from __future__ import annotations
 
 from collections.abc import Iterator
+from contextlib import contextmanager
 from datetime import datetime
 from typing import TYPE_CHECKING
 from unittest import mock
 from unittest.mock import Mock
 
 import pytest
+from sqlalchemy import event
 
+import airflow.settings
 from airflow.models.dag_version import DagVersion
 from airflow.models.taskinstance import TaskInstance
 from airflow.providers.standard.operators.empty import EmptyOperator
@@ -1997,3 +2000,159 @@ def _test_trigger_rule(
     else:
         assert not dep_statuses
     assert ti.state == expected_ti_state
+
+
+@contextmanager
+def _count_upstream_count_queries():
+    """
+    Count only the trigger-rule upstream-count query.
+
+    That query is ``SELECT task_instance.task_id, count(task_instance.task_id) ...
+    GROUP BY task_instance.task_id``; the filter below matches it and nothing else emitted while
+    evaluating the trigger rule for a plain (non-mapped-task-group) downstream.
+    """
+    counter = {"n": 0}
+
+    def _on_execute(conn, cursor, statement, parameters, context, executemany):
+        sql = statement.lower()
+        if "count(" in sql and "group by" in sql and "task_id" in sql and "task_instance" in sql:
+            counter["n"] += 1
+
+    event.listen(airflow.settings.engine, "after_cursor_execute", _on_execute)
+    try:
+        yield counter
+    finally:
+        event.remove(airflow.settings.engine, "after_cursor_execute", _on_execute)
+
+
+def _expand_mapped_task(dr, dag, task_id, states, session):
+    """
+    Materialise ``len(states)`` instances of a mapped ``task_id`` with the given states.
+
+    Handles both shapes: a single unexpanded ``map_index=-1`` placeholder (expand it), or a task
+    already pre-expanded at dagrun creation (just set states on the existing instances).
+    """
+    tis = [ti for ti in dr.get_task_instances(session=session) if ti.task_id == task_id]
+    assert tis, f"no task instances found for {task_id!r}"
+    if len(tis) == 1 and tis[0].map_index == -1:
+        base = tis[0]
+        mapped_task = base.task
+        dag_version = DagVersion.get_latest_version(dag.dag_id)
+        if TYPE_CHECKING:
+            assert dag_version
+        base.map_index = 0
+        base.state = states[0]
+        session.merge(base)
+        for map_index in range(1, len(states)):
+            ti = TaskInstance(
+                mapped_task, run_id=dr.run_id, map_index=map_index, dag_version_id=dag_version.id
+            )
+            ti.state = states[map_index]
+            session.add(ti)
+            ti.dag_run = dr
+    else:
+        tis.sort(key=lambda ti: ti.map_index)
+        assert len(tis) == len(states), f"{task_id!r}: {len(tis)} instances but {len(states)} states given"
+        for ti, state in zip(tis, states):
+            ti.state = state
+            session.merge(ti)
+    session.flush()
+
+
+class TestTriggerRuleUpstreamCountMemo:
+    """The upstream-count query is memoized per scheduling pass (one DepContext) in the simple case."""
+
+    def _make_dag(
+        self, dag_maker, session, *, n_downstreams, src_states, trigger_rule=TriggerRule.ALL_SUCCESS
+    ):
+        @task
+        def src(i):
+            return i
+
+        @task(trigger_rule=trigger_rule)
+        def plain():
+            return 1
+
+        with dag_maker(dag_id="trmemo_simple", session=session) as dag:
+            nums = src.expand(i=list(range(len(src_states))))
+            for k in range(n_downstreams):
+                nums >> plain.override(task_id=f"p{k}")()
+
+        dr = dag_maker.create_dagrun()
+        _expand_mapped_task(dr, dag, "src", src_states, session)
+        session.commit()
+        return dr
+
+    def test_memoized_across_downstreams_sharing_upstream(self, dag_maker, session):
+        """N plain downstreams of the same mapped upstream issue the count query once per pass."""
+        dr = self._make_dag(dag_maker, session, n_downstreams=4, src_states=[SUCCESS, SUCCESS, SUCCESS])
+        dep_context = DepContext()
+        with _count_upstream_count_queries() as counter:
+            for k in range(4):
+                ti = dr.get_task_instance(f"p{k}", session=session)
+                statuses = list(
+                    TriggerRuleDep()._evaluate_trigger_rule(ti=ti, dep_context=dep_context, session=session)
+                )
+                # All three upstreams succeeded -> ALL_SUCCESS is met -> no failing status.
+                assert statuses == []
+        assert counter["n"] == 1
+
+    def test_memoized_count_value_is_correct(self, dag_maker, session):
+        """
+        Guards that the cached value is the real count, not just "present".
+
+        Three upstream instances exist but only two are finished-success; ALL_SUCCESS must NOT be met
+        because ``upstream`` (3) > ``success`` (2). A wrongly-cached count of 2 would let it pass.
+        """
+        dr = self._make_dag(
+            dag_maker,
+            session,
+            n_downstreams=2,
+            src_states=[SUCCESS, SUCCESS, TaskInstanceState.RUNNING],
+        )
+        dep_context = DepContext()
+        with _count_upstream_count_queries() as counter:
+            for k in range(2):
+                ti = dr.get_task_instance(f"p{k}", session=session)
+                statuses = list(
+                    TriggerRuleDep()._evaluate_trigger_rule(ti=ti, dep_context=dep_context, session=session)
+                )
+                assert len(statuses) == 1
+                assert not statuses[0].passed
+        assert counter["n"] == 1
+
+    def test_distinct_upstream_sets_are_not_collapsed(self, dag_maker, session):
+        """Downstreams with different upstream sets get different cache keys -> one query each."""
+
+        @task
+        def src_a(i):
+            return i
+
+        @task
+        def src_b(i):
+            return i
+
+        @task
+        def plain():
+            return 1
+
+        with dag_maker(dag_id="trmemo_keys", session=session) as dag:
+            a = src_a.expand(i=[0, 1])
+            b = src_b.expand(i=[0, 1, 2])
+            a >> plain.override(task_id="pa")()
+            b >> plain.override(task_id="pb")()
+
+        dr = dag_maker.create_dagrun()
+        _expand_mapped_task(dr, dag, "src_a", [SUCCESS, SUCCESS], session)
+        _expand_mapped_task(dr, dag, "src_b", [SUCCESS, SUCCESS, SUCCESS], session)
+        session.commit()
+
+        dep_context = DepContext()
+        with _count_upstream_count_queries() as counter:
+            for task_id in ("pa", "pb"):
+                ti = dr.get_task_instance(task_id, session=session)
+                statuses = list(
+                    TriggerRuleDep()._evaluate_trigger_rule(ti=ti, dep_context=dep_context, session=session)
+                )
+                assert statuses == []
+        assert counter["n"] == 2