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Posted to issues@flink.apache.org by GitBox <gi...@apache.org> on 2020/10/21 08:24:04 UTC
[GitHub] [flink] godfreyhe commented on a change in pull request #13692: [FLINK-19624][table-planner-blink] Update deadlock break-up algorithm to cover more cases
godfreyhe commented on a change in pull request #13692:
URL: https://github.com/apache/flink/pull/13692#discussion_r509079899
##########
File path: flink-table/flink-table-planner-blink/src/test/scala/org/apache/flink/table/planner/plan/nodes/exec/TestingExecNode.scala
##########
@@ -0,0 +1,156 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.plan.nodes.exec
+
+import org.apache.flink.api.dag.Transformation
+import org.apache.flink.table.api.TableConfig
+import org.apache.flink.table.planner.calcite.{FlinkContextImpl, FlinkRelOptClusterFactory, FlinkRexBuilder, FlinkTypeFactory, FlinkTypeSystem}
+import org.apache.flink.table.planner.delegation.BatchPlanner
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchPhysicalRel
+
+import org.apache.calcite.plan.hep.{HepPlanner, HepProgram}
+import org.apache.calcite.plan.{Convention, RelOptCluster, RelOptCost, RelOptPlanner, RelOptQuery, RelOptTable, RelTraitSet}
+import org.apache.calcite.rel.`type`.{RelDataType, RelDataTypeSystem}
+import org.apache.calcite.rel.core.CorrelationId
+import org.apache.calcite.rel.{RelCollation, RelNode, RelShuttle, RelVisitor, RelWriter}
+import org.apache.calcite.rel.metadata.{Metadata, RelMetadataQuery}
+import org.apache.calcite.rex.{RexNode, RexShuttle}
+import org.apache.calcite.util.{ImmutableBitSet, Litmus}
+
+import java.util
+
+/**
+ * [[BatchExecNode]] for testing purpose.
+ */
+class TestingExecNode extends BatchExecNode[BatchPlanner] with BatchPhysicalRel {
Review comment:
rename to `TestingBatchExecNode` ?
also extends from `AbstractRelNode`, which has implemented many methods.
##########
File path: flink-table/flink-table-planner-blink/src/main/java/org/apache/flink/table/planner/plan/reuse/InputPriorityConflictResolver.java
##########
@@ -0,0 +1,353 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.plan.reuse;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.streaming.api.datastream.DataStream;
+import org.apache.flink.streaming.api.transformations.ShuffleMode;
+import org.apache.flink.table.planner.plan.nodes.exec.AbstractExecNodeExactlyOnceVisitor;
+import org.apache.flink.table.planner.plan.nodes.exec.BatchExecNode;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecEdge;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecNode;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecBoundedStreamScan;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecExchange;
+import org.apache.flink.table.planner.plan.trait.FlinkRelDistribution;
+import org.apache.flink.util.Preconditions;
+
+import org.apache.calcite.rel.RelNode;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+import java.util.Set;
+import java.util.TreeMap;
+
+/**
+ * This class contains algorithm to detect and resolve input priority conflict in an {@link ExecNode} graph.
+ *
+ * <p>Some batch operators (for example, hash join and nested loop join) have different priorities for their inputs.
+ * When some operators are reused, a deadlock may occur due to the conflict in these priorities.
+ *
+ * <p>For example, consider the SQL query:
+ * <pre>
+ * WITH
+ * T1 AS (SELECT a, COUNT(*) AS cnt1 FROM x GROUP BY a),
+ * T2 AS (SELECT d, COUNT(*) AS cnt2 FROM y GROUP BY d)
+ * SELECT * FROM
+ * (SELECT cnt1, cnt2 FROM T1 LEFT JOIN T2 ON a = d)
+ * UNION ALL
+ * (SELECT cnt1, cnt2 FROM T2 LEFT JOIN T1 ON d = a)
+ * </pre>
+ *
+ * <p>When sub-plan reuse are enabled, we'll get the following physical plan:
+ * <pre>
+ * Union(all=[true], union=[cnt1, cnt2])
+ * :- Calc(select=[CAST(cnt1) AS cnt1, cnt2])
+ * : +- HashJoin(joinType=[LeftOuterJoin], where=[=(a, d)], select=[a, cnt1, d, cnt2], build=[right])
+ * : :- HashAggregate(isMerge=[true], groupBy=[a], select=[a, Final_COUNT(count1$0) AS cnt1], reuse_id=[2])
+ * : : +- Exchange(distribution=[hash[a]])
+ * : : +- LocalHashAggregate(groupBy=[a], select=[a, Partial_COUNT(*) AS count1$0])
+ * : : +- Calc(select=[a])
+ * : : +- LegacyTableSourceScan(table=[[default_catalog, default_database, x, source: [TestTableSource(a, b, c)]]], fields=[a, b, c])
+ * : +- HashAggregate(isMerge=[true], groupBy=[d], select=[d, Final_COUNT(count1$0) AS cnt2], reuse_id=[1])
+ * : +- Exchange(distribution=[hash[d]])
+ * : +- LocalHashAggregate(groupBy=[d], select=[d, Partial_COUNT(*) AS count1$0])
+ * : +- Calc(select=[d])
+ * : +- LegacyTableSourceScan(table=[[default_catalog, default_database, y, source: [TestTableSource(d, e, f)]]], fields=[d, e, f])
+ * +- Calc(select=[cnt1, CAST(cnt2) AS cnt2])
+ * +- HashJoin(joinType=[LeftOuterJoin], where=[=(d, a)], select=[d, cnt2, a, cnt1], build=[right])
+ * :- Reused(reference_id=[1])
+ * +- Reused(reference_id=[2])
+ * </pre>
+ *
+ * <p>Note that the first hash join needs to read all results from the hash aggregate whose reuse id is 1
+ * before reading the results from the hash aggregate whose reuse id is 2, while the second hash join requires
+ * the opposite. This physical plan will thus cause a deadlock.
+ *
+ * <p>This class maintains a topological graph in which an edge pointing from vertex A to vertex B indicates
+ * that the results from vertex A need to be read before those from vertex B. A loop in the graph indicates
+ * a deadlock, and we resolve such deadlock by inserting a {@link BatchExecExchange} with batch shuffle mode.
+ *
+ * <p>For a detailed explanation of the algorithm, see appendix of the
+ * <a href="https://docs.google.com/document/d/1qKVohV12qn-bM51cBZ8Hcgp31ntwClxjoiNBUOqVHsI">design doc</a>.
+ */
+@Internal
+public class InputPriorityConflictResolver {
+
+ private final List<ExecNode<?, ?>> sinks;
+
+ private TopologyGraph graph;
+
+ public InputPriorityConflictResolver(List<ExecNode<?, ?>> sinks) {
Review comment:
nit: an `ExecNode` may be not `BatchExecSink` here, use roots ?
##########
File path: flink-table/flink-table-planner-blink/src/test/java/org/apache/flink/table/planner/plan/reuse/InputPriorityConflictResolverTest.java
##########
@@ -0,0 +1,180 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.plan.reuse;
+
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecEdge;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecNode;
+import org.apache.flink.table.planner.plan.nodes.exec.TestingExecNode;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecExchange;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Tests for {@link InputPriorityConflictResolver}.
+ */
+public class InputPriorityConflictResolverTest {
+
+ private Tuple2<InputPriorityConflictResolver.TopologyGraph, TestingExecNode[]> buildTopologyGraph() {
+ // 0 -> 1 -> 2 --------> 5
+ // \-> 3 -> 4 -/
+ // \-> 6 -> 7
+ TestingExecNode[] nodes = new TestingExecNode[8];
+ for (int i = 0; i < nodes.length; i++) {
+ nodes[i] = new TestingExecNode();
+ }
+ nodes[1].addInput(nodes[0]);
+ nodes[2].addInput(nodes[1]);
+ nodes[3].addInput(nodes[1]);
+ nodes[4].addInput(nodes[3]);
+ nodes[5].addInput(nodes[2]);
+ nodes[5].addInput(nodes[4]);
+ nodes[6].addInput(nodes[3]);
+ nodes[7].addInput(nodes[6]);
+
+ return Tuple2.of(
+ new InputPriorityConflictResolver.TopologyGraph(Arrays.asList(nodes[5], nodes[7])),
+ nodes);
+ }
+
+ @Test
+ public void testTopologyGraphCanReach() {
+ Tuple2<InputPriorityConflictResolver.TopologyGraph, TestingExecNode[]> tuple2 = buildTopologyGraph();
+ InputPriorityConflictResolver.TopologyGraph graph = tuple2.f0;
+ TestingExecNode[] nodes = tuple2.f1;
+
+ String[] canReach = new String[] {
+ "11111111",
+ "01111111",
+ "00100100",
+ "00011111",
+ "00001100",
+ "00000100",
+ "00000011",
+ "00000001"};
+ for (int i = 0; i < 8; i++) {
+ for (int j = 0; j < 8; j++) {
+ if (canReach[i].charAt(j) == '1') {
+ Assert.assertTrue(graph.canReach(nodes[i], nodes[j]));
+ } else {
+ Assert.assertFalse(graph.canReach(nodes[i], nodes[j]));
+ }
+ }
+ }
+ }
+
+ @Test
+ public void testTopologyGraphLink() {
+ Tuple2<InputPriorityConflictResolver.TopologyGraph, TestingExecNode[]> tuple2 = buildTopologyGraph();
+ InputPriorityConflictResolver.TopologyGraph graph = tuple2.f0;
+ TestingExecNode[] nodes = tuple2.f1;
+
+ Assert.assertTrue(graph.link(nodes[2], nodes[4]));
+ Assert.assertTrue(graph.link(nodes[3], nodes[5]));
+ Assert.assertTrue(graph.link(nodes[5], nodes[6]));
+ Assert.assertFalse(graph.link(nodes[7], nodes[2]));
+ Assert.assertFalse(graph.link(nodes[7], nodes[4]));
+ Assert.assertTrue(graph.link(nodes[0], nodes[7]));
+ }
+
+ @Test
+ public void testTopologyGraphUnlink() {
+ Tuple2<InputPriorityConflictResolver.TopologyGraph, TestingExecNode[]> tuple2 = buildTopologyGraph();
+ InputPriorityConflictResolver.TopologyGraph graph = tuple2.f0;
+ TestingExecNode[] nodes = tuple2.f1;
+
+ graph.unlink(nodes[2], nodes[5]);
+ Assert.assertTrue(graph.canReach(nodes[0], nodes[5]));
+ graph.unlink(nodes[4], nodes[5]);
+ Assert.assertFalse(graph.canReach(nodes[0], nodes[5]));
+ graph.unlink(nodes[3], nodes[6]);
+ Assert.assertFalse(graph.canReach(nodes[0], nodes[7]));
+ }
+
+ @Test
+ public void testCalculateAncestors() {
+ // 0 ------P----> 1 -E--> 2
+ // \-----P----> 3 -P-/
+ // 4 -E-> 5 -P-/ /
+ // 6 -----E-----/
+ TestingExecNode[] nodes = new TestingExecNode[7];
+ for (int i = 0; i < nodes.length; i++) {
+ nodes[i] = new TestingExecNode();
+ }
+ nodes[1].addInput(nodes[0]);
+ nodes[2].addInput(nodes[1], ExecEdge.builder().damBehavior(ExecEdge.DamBehavior.END_INPUT).build());
+ nodes[2].addInput(nodes[3]);
+ nodes[3].addInput(nodes[0]);
+ nodes[3].addInput(nodes[5]);
+ nodes[3].addInput(nodes[6], ExecEdge.builder().damBehavior(ExecEdge.DamBehavior.END_INPUT).build());
+ nodes[5].addInput(nodes[4], ExecEdge.builder().damBehavior(ExecEdge.DamBehavior.END_INPUT).build());
+
+ InputPriorityConflictResolver resolver = new InputPriorityConflictResolver(Collections.singletonList(nodes[2]));
+ List<ExecNode<?, ?>> ancestors = resolver.calculateAncestors(nodes[2]);
+ Assert.assertEquals(2, ancestors.size());
+ Assert.assertTrue(ancestors.contains(nodes[0]));
+ Assert.assertTrue(ancestors.contains(nodes[5]));
+ }
+
+ @Test
+ public void testDetectAndResolve() {
+ // 0 --------(P0)----> 1 --(P0)-----------> 7
+ // \ \-(P0)-> 2 -(P0)--/
+ // \-------(P0)----> 3 --(P1)-----------/
+ // \------(P0)----> 4 --(P10)---------/
+ // \ / /
+ // \ 8 -(P0)-< /
+ // \ \ /
+ // \--(E0)----> 5 --(P10)-----/
+ // 6 ---------(P100)----------------/
Review comment:
add some comments to explain `Pxx`, `Exx`
##########
File path: flink-table/flink-table-planner-blink/src/main/java/org/apache/flink/table/planner/plan/reuse/InputPriorityConflictResolver.java
##########
@@ -0,0 +1,353 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.plan.reuse;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.streaming.api.datastream.DataStream;
+import org.apache.flink.streaming.api.transformations.ShuffleMode;
+import org.apache.flink.table.planner.plan.nodes.exec.AbstractExecNodeExactlyOnceVisitor;
+import org.apache.flink.table.planner.plan.nodes.exec.BatchExecNode;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecEdge;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecNode;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecBoundedStreamScan;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecExchange;
+import org.apache.flink.table.planner.plan.trait.FlinkRelDistribution;
+import org.apache.flink.util.Preconditions;
+
+import org.apache.calcite.rel.RelNode;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+import java.util.Set;
+import java.util.TreeMap;
+
+/**
+ * This class contains algorithm to detect and resolve input priority conflict in an {@link ExecNode} graph.
+ *
+ * <p>Some batch operators (for example, hash join and nested loop join) have different priorities for their inputs.
+ * When some operators are reused, a deadlock may occur due to the conflict in these priorities.
+ *
+ * <p>For example, consider the SQL query:
+ * <pre>
+ * WITH
+ * T1 AS (SELECT a, COUNT(*) AS cnt1 FROM x GROUP BY a),
+ * T2 AS (SELECT d, COUNT(*) AS cnt2 FROM y GROUP BY d)
+ * SELECT * FROM
+ * (SELECT cnt1, cnt2 FROM T1 LEFT JOIN T2 ON a = d)
+ * UNION ALL
+ * (SELECT cnt1, cnt2 FROM T2 LEFT JOIN T1 ON d = a)
+ * </pre>
+ *
+ * <p>When sub-plan reuse are enabled, we'll get the following physical plan:
+ * <pre>
+ * Union(all=[true], union=[cnt1, cnt2])
+ * :- Calc(select=[CAST(cnt1) AS cnt1, cnt2])
+ * : +- HashJoin(joinType=[LeftOuterJoin], where=[=(a, d)], select=[a, cnt1, d, cnt2], build=[right])
+ * : :- HashAggregate(isMerge=[true], groupBy=[a], select=[a, Final_COUNT(count1$0) AS cnt1], reuse_id=[2])
+ * : : +- Exchange(distribution=[hash[a]])
+ * : : +- LocalHashAggregate(groupBy=[a], select=[a, Partial_COUNT(*) AS count1$0])
+ * : : +- Calc(select=[a])
+ * : : +- LegacyTableSourceScan(table=[[default_catalog, default_database, x, source: [TestTableSource(a, b, c)]]], fields=[a, b, c])
+ * : +- HashAggregate(isMerge=[true], groupBy=[d], select=[d, Final_COUNT(count1$0) AS cnt2], reuse_id=[1])
+ * : +- Exchange(distribution=[hash[d]])
+ * : +- LocalHashAggregate(groupBy=[d], select=[d, Partial_COUNT(*) AS count1$0])
+ * : +- Calc(select=[d])
+ * : +- LegacyTableSourceScan(table=[[default_catalog, default_database, y, source: [TestTableSource(d, e, f)]]], fields=[d, e, f])
+ * +- Calc(select=[cnt1, CAST(cnt2) AS cnt2])
+ * +- HashJoin(joinType=[LeftOuterJoin], where=[=(d, a)], select=[d, cnt2, a, cnt1], build=[right])
+ * :- Reused(reference_id=[1])
+ * +- Reused(reference_id=[2])
+ * </pre>
+ *
+ * <p>Note that the first hash join needs to read all results from the hash aggregate whose reuse id is 1
+ * before reading the results from the hash aggregate whose reuse id is 2, while the second hash join requires
+ * the opposite. This physical plan will thus cause a deadlock.
+ *
+ * <p>This class maintains a topological graph in which an edge pointing from vertex A to vertex B indicates
+ * that the results from vertex A need to be read before those from vertex B. A loop in the graph indicates
+ * a deadlock, and we resolve such deadlock by inserting a {@link BatchExecExchange} with batch shuffle mode.
+ *
+ * <p>For a detailed explanation of the algorithm, see appendix of the
+ * <a href="https://docs.google.com/document/d/1qKVohV12qn-bM51cBZ8Hcgp31ntwClxjoiNBUOqVHsI">design doc</a>.
+ */
+@Internal
+public class InputPriorityConflictResolver {
+
+ private final List<ExecNode<?, ?>> sinks;
+
+ private TopologyGraph graph;
+
+ public InputPriorityConflictResolver(List<ExecNode<?, ?>> sinks) {
+ Preconditions.checkArgument(
+ sinks.stream().allMatch(sink -> sink instanceof BatchExecNode),
+ "InputPriorityConflictResolver can only be used for batch jobs.");
+ this.sinks = sinks;
+ }
+
+ public void detectAndResolve() {
+ // build an initial topology graph
+ graph = new TopologyGraph(sinks);
+
+ // check and resolve conflicts about input priorities
+ AbstractExecNodeExactlyOnceVisitor inputPriorityVisitor = new AbstractExecNodeExactlyOnceVisitor() {
+ @Override
+ protected void visitNode(ExecNode<?, ?> node) {
+ visitInputs(node);
+ checkInputPriorities(node);
+ }
+ };
+ sinks.forEach(n -> n.accept(inputPriorityVisitor));
+ }
+
+ private void checkInputPriorities(ExecNode<?, ?> node) {
+ // group inputs by input priorities
+ TreeMap<Integer, List<Integer>> inputPriorityGroupMap = new TreeMap<>();
+ Preconditions.checkState(
+ node.getInputNodes().size() == node.getInputEdges().size(),
+ "Number of inputs nodes does not equal to number of input edges for node " +
+ node.getClass().getName() + ". This is a bug.");
+ for (int i = 0; i < node.getInputNodes().size(); i++) {
Review comment:
nit: use `node.getInputEdges().size()`, consistent with the body of `for`
##########
File path: flink-table/flink-table-planner-blink/src/main/java/org/apache/flink/table/planner/plan/reuse/InputPriorityConflictResolver.java
##########
@@ -0,0 +1,353 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.plan.reuse;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.streaming.api.datastream.DataStream;
+import org.apache.flink.streaming.api.transformations.ShuffleMode;
+import org.apache.flink.table.planner.plan.nodes.exec.AbstractExecNodeExactlyOnceVisitor;
+import org.apache.flink.table.planner.plan.nodes.exec.BatchExecNode;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecEdge;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecNode;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecBoundedStreamScan;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecExchange;
+import org.apache.flink.table.planner.plan.trait.FlinkRelDistribution;
+import org.apache.flink.util.Preconditions;
+
+import org.apache.calcite.rel.RelNode;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+import java.util.Set;
+import java.util.TreeMap;
+
+/**
+ * This class contains algorithm to detect and resolve input priority conflict in an {@link ExecNode} graph.
+ *
+ * <p>Some batch operators (for example, hash join and nested loop join) have different priorities for their inputs.
+ * When some operators are reused, a deadlock may occur due to the conflict in these priorities.
+ *
+ * <p>For example, consider the SQL query:
+ * <pre>
+ * WITH
+ * T1 AS (SELECT a, COUNT(*) AS cnt1 FROM x GROUP BY a),
+ * T2 AS (SELECT d, COUNT(*) AS cnt2 FROM y GROUP BY d)
+ * SELECT * FROM
+ * (SELECT cnt1, cnt2 FROM T1 LEFT JOIN T2 ON a = d)
+ * UNION ALL
+ * (SELECT cnt1, cnt2 FROM T2 LEFT JOIN T1 ON d = a)
+ * </pre>
+ *
+ * <p>When sub-plan reuse are enabled, we'll get the following physical plan:
+ * <pre>
+ * Union(all=[true], union=[cnt1, cnt2])
+ * :- Calc(select=[CAST(cnt1) AS cnt1, cnt2])
+ * : +- HashJoin(joinType=[LeftOuterJoin], where=[=(a, d)], select=[a, cnt1, d, cnt2], build=[right])
+ * : :- HashAggregate(isMerge=[true], groupBy=[a], select=[a, Final_COUNT(count1$0) AS cnt1], reuse_id=[2])
+ * : : +- Exchange(distribution=[hash[a]])
+ * : : +- LocalHashAggregate(groupBy=[a], select=[a, Partial_COUNT(*) AS count1$0])
+ * : : +- Calc(select=[a])
+ * : : +- LegacyTableSourceScan(table=[[default_catalog, default_database, x, source: [TestTableSource(a, b, c)]]], fields=[a, b, c])
+ * : +- HashAggregate(isMerge=[true], groupBy=[d], select=[d, Final_COUNT(count1$0) AS cnt2], reuse_id=[1])
+ * : +- Exchange(distribution=[hash[d]])
+ * : +- LocalHashAggregate(groupBy=[d], select=[d, Partial_COUNT(*) AS count1$0])
+ * : +- Calc(select=[d])
+ * : +- LegacyTableSourceScan(table=[[default_catalog, default_database, y, source: [TestTableSource(d, e, f)]]], fields=[d, e, f])
+ * +- Calc(select=[cnt1, CAST(cnt2) AS cnt2])
+ * +- HashJoin(joinType=[LeftOuterJoin], where=[=(d, a)], select=[d, cnt2, a, cnt1], build=[right])
+ * :- Reused(reference_id=[1])
+ * +- Reused(reference_id=[2])
+ * </pre>
+ *
+ * <p>Note that the first hash join needs to read all results from the hash aggregate whose reuse id is 1
+ * before reading the results from the hash aggregate whose reuse id is 2, while the second hash join requires
+ * the opposite. This physical plan will thus cause a deadlock.
+ *
+ * <p>This class maintains a topological graph in which an edge pointing from vertex A to vertex B indicates
+ * that the results from vertex A need to be read before those from vertex B. A loop in the graph indicates
+ * a deadlock, and we resolve such deadlock by inserting a {@link BatchExecExchange} with batch shuffle mode.
+ *
+ * <p>For a detailed explanation of the algorithm, see appendix of the
+ * <a href="https://docs.google.com/document/d/1qKVohV12qn-bM51cBZ8Hcgp31ntwClxjoiNBUOqVHsI">design doc</a>.
+ */
+@Internal
+public class InputPriorityConflictResolver {
+
+ private final List<ExecNode<?, ?>> sinks;
+
+ private TopologyGraph graph;
+
+ public InputPriorityConflictResolver(List<ExecNode<?, ?>> sinks) {
+ Preconditions.checkArgument(
+ sinks.stream().allMatch(sink -> sink instanceof BatchExecNode),
+ "InputPriorityConflictResolver can only be used for batch jobs.");
+ this.sinks = sinks;
+ }
+
+ public void detectAndResolve() {
+ // build an initial topology graph
+ graph = new TopologyGraph(sinks);
+
+ // check and resolve conflicts about input priorities
+ AbstractExecNodeExactlyOnceVisitor inputPriorityVisitor = new AbstractExecNodeExactlyOnceVisitor() {
+ @Override
+ protected void visitNode(ExecNode<?, ?> node) {
+ visitInputs(node);
+ checkInputPriorities(node);
+ }
+ };
+ sinks.forEach(n -> n.accept(inputPriorityVisitor));
+ }
+
+ private void checkInputPriorities(ExecNode<?, ?> node) {
+ // group inputs by input priorities
+ TreeMap<Integer, List<Integer>> inputPriorityGroupMap = new TreeMap<>();
+ Preconditions.checkState(
+ node.getInputNodes().size() == node.getInputEdges().size(),
+ "Number of inputs nodes does not equal to number of input edges for node " +
+ node.getClass().getName() + ". This is a bug.");
+ for (int i = 0; i < node.getInputNodes().size(); i++) {
+ int priority = node.getInputEdges().get(i).getPriority();
+ inputPriorityGroupMap.computeIfAbsent(priority, k -> new ArrayList<>()).add(i);
+ }
+
+ // add edges between neighboring priority groups
+ List<List<Integer>> inputPriorityGroups = new ArrayList<>(inputPriorityGroupMap.values());
+ for (int i = 0; i + 1 < inputPriorityGroups.size(); i++) {
+ List<Integer> higherGroup = inputPriorityGroups.get(i);
+ List<Integer> lowerGroup = inputPriorityGroups.get(i + 1);
+
+ for (int higher : higherGroup) {
+ for (int lower : lowerGroup) {
+ addTopologyEdges(node, higher, lower);
+ }
+ }
+ }
+ }
+
+ private void addTopologyEdges(ExecNode<?, ?> node, int higherInput, int lowerInput) {
+ ExecNode<?, ?> higherNode = node.getInputNodes().get(higherInput);
+ ExecNode<?, ?> lowerNode = node.getInputNodes().get(lowerInput);
+ List<ExecNode<?, ?>> lowerAncestors = calculateAncestors(lowerNode);
+
+ List<Tuple2<ExecNode<?, ?>, ExecNode<?, ?>>> linkedEdges = new ArrayList<>();
+ for (ExecNode<?, ?> ancestor : lowerAncestors) {
+ if (graph.link(higherNode, ancestor)) {
+ linkedEdges.add(Tuple2.of(higherNode, ancestor));
+ } else {
+ // a conflict occurs, resolve it by adding a batch exchange
+ // and revert all linked edges
+ if (lowerNode instanceof BatchExecExchange) {
+ BatchExecExchange exchange = (BatchExecExchange) lowerNode;
+ exchange.setRequiredShuffleMode(ShuffleMode.BATCH);
+ } else {
+ node.replaceInputNode(lowerInput, (ExecNode) createExchange(node, lowerInput));
+ }
+
+ for (Tuple2<ExecNode<?, ?>, ExecNode<?, ?>> linkedEdge : linkedEdges) {
+ graph.unlink(linkedEdge.f0, linkedEdge.f1);
+ }
+ return;
+ }
+ }
+ }
+
+ /**
+ * Find the ancestors by going through PIPELINED edges.
+ */
+ @VisibleForTesting
+ List<ExecNode<?, ?>> calculateAncestors(ExecNode<?, ?> node) {
+ List<ExecNode<?, ?>> ret = new ArrayList<>();
+ AbstractExecNodeExactlyOnceVisitor ancestorVisitor = new AbstractExecNodeExactlyOnceVisitor() {
+ @Override
+ protected void visitNode(ExecNode<?, ?> node) {
+ List<ExecEdge> inputEdges = node.getInputEdges();
+ boolean hasAncestor = false;
+
+ for (int i = 0; i < inputEdges.size(); i++) {
+ // we only go through PIPELINED edges
+ if (inputEdges.get(i).getDamBehavior().stricterOrEqual(ExecEdge.DamBehavior.END_INPUT)) {
+ continue;
+ }
+ hasAncestor = true;
+ node.getInputNodes().get(i).accept(this);
+ }
+
+ if (!hasAncestor) {
+ ret.add(node);
+ }
+ }
+ };
+ node.accept(ancestorVisitor);
+ return ret;
+ }
+
+ private BatchExecExchange createExchange(ExecNode<?, ?> node, int idx) {
Review comment:
could you provide more test cases to cover different branches ?
##########
File path: flink-table/flink-table-planner-blink/src/main/java/org/apache/flink/table/planner/plan/reuse/InputPriorityConflictResolver.java
##########
@@ -0,0 +1,353 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.table.planner.plan.reuse;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.annotation.VisibleForTesting;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.streaming.api.datastream.DataStream;
+import org.apache.flink.streaming.api.transformations.ShuffleMode;
+import org.apache.flink.table.planner.plan.nodes.exec.AbstractExecNodeExactlyOnceVisitor;
+import org.apache.flink.table.planner.plan.nodes.exec.BatchExecNode;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecEdge;
+import org.apache.flink.table.planner.plan.nodes.exec.ExecNode;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecBoundedStreamScan;
+import org.apache.flink.table.planner.plan.nodes.physical.batch.BatchExecExchange;
+import org.apache.flink.table.planner.plan.trait.FlinkRelDistribution;
+import org.apache.flink.util.Preconditions;
+
+import org.apache.calcite.rel.RelNode;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+import java.util.Set;
+import java.util.TreeMap;
+
+/**
+ * This class contains algorithm to detect and resolve input priority conflict in an {@link ExecNode} graph.
+ *
+ * <p>Some batch operators (for example, hash join and nested loop join) have different priorities for their inputs.
+ * When some operators are reused, a deadlock may occur due to the conflict in these priorities.
+ *
+ * <p>For example, consider the SQL query:
+ * <pre>
+ * WITH
+ * T1 AS (SELECT a, COUNT(*) AS cnt1 FROM x GROUP BY a),
+ * T2 AS (SELECT d, COUNT(*) AS cnt2 FROM y GROUP BY d)
+ * SELECT * FROM
+ * (SELECT cnt1, cnt2 FROM T1 LEFT JOIN T2 ON a = d)
+ * UNION ALL
+ * (SELECT cnt1, cnt2 FROM T2 LEFT JOIN T1 ON d = a)
+ * </pre>
+ *
+ * <p>When sub-plan reuse are enabled, we'll get the following physical plan:
+ * <pre>
+ * Union(all=[true], union=[cnt1, cnt2])
+ * :- Calc(select=[CAST(cnt1) AS cnt1, cnt2])
+ * : +- HashJoin(joinType=[LeftOuterJoin], where=[=(a, d)], select=[a, cnt1, d, cnt2], build=[right])
+ * : :- HashAggregate(isMerge=[true], groupBy=[a], select=[a, Final_COUNT(count1$0) AS cnt1], reuse_id=[2])
+ * : : +- Exchange(distribution=[hash[a]])
+ * : : +- LocalHashAggregate(groupBy=[a], select=[a, Partial_COUNT(*) AS count1$0])
+ * : : +- Calc(select=[a])
+ * : : +- LegacyTableSourceScan(table=[[default_catalog, default_database, x, source: [TestTableSource(a, b, c)]]], fields=[a, b, c])
+ * : +- HashAggregate(isMerge=[true], groupBy=[d], select=[d, Final_COUNT(count1$0) AS cnt2], reuse_id=[1])
+ * : +- Exchange(distribution=[hash[d]])
+ * : +- LocalHashAggregate(groupBy=[d], select=[d, Partial_COUNT(*) AS count1$0])
+ * : +- Calc(select=[d])
+ * : +- LegacyTableSourceScan(table=[[default_catalog, default_database, y, source: [TestTableSource(d, e, f)]]], fields=[d, e, f])
+ * +- Calc(select=[cnt1, CAST(cnt2) AS cnt2])
+ * +- HashJoin(joinType=[LeftOuterJoin], where=[=(d, a)], select=[d, cnt2, a, cnt1], build=[right])
+ * :- Reused(reference_id=[1])
+ * +- Reused(reference_id=[2])
+ * </pre>
+ *
+ * <p>Note that the first hash join needs to read all results from the hash aggregate whose reuse id is 1
+ * before reading the results from the hash aggregate whose reuse id is 2, while the second hash join requires
+ * the opposite. This physical plan will thus cause a deadlock.
+ *
+ * <p>This class maintains a topological graph in which an edge pointing from vertex A to vertex B indicates
+ * that the results from vertex A need to be read before those from vertex B. A loop in the graph indicates
+ * a deadlock, and we resolve such deadlock by inserting a {@link BatchExecExchange} with batch shuffle mode.
+ *
+ * <p>For a detailed explanation of the algorithm, see appendix of the
+ * <a href="https://docs.google.com/document/d/1qKVohV12qn-bM51cBZ8Hcgp31ntwClxjoiNBUOqVHsI">design doc</a>.
+ */
+@Internal
+public class InputPriorityConflictResolver {
+
+ private final List<ExecNode<?, ?>> sinks;
+
+ private TopologyGraph graph;
+
+ public InputPriorityConflictResolver(List<ExecNode<?, ?>> sinks) {
+ Preconditions.checkArgument(
+ sinks.stream().allMatch(sink -> sink instanceof BatchExecNode),
+ "InputPriorityConflictResolver can only be used for batch jobs.");
+ this.sinks = sinks;
+ }
+
+ public void detectAndResolve() {
+ // build an initial topology graph
+ graph = new TopologyGraph(sinks);
+
+ // check and resolve conflicts about input priorities
+ AbstractExecNodeExactlyOnceVisitor inputPriorityVisitor = new AbstractExecNodeExactlyOnceVisitor() {
+ @Override
+ protected void visitNode(ExecNode<?, ?> node) {
+ visitInputs(node);
+ checkInputPriorities(node);
+ }
+ };
+ sinks.forEach(n -> n.accept(inputPriorityVisitor));
+ }
+
+ private void checkInputPriorities(ExecNode<?, ?> node) {
+ // group inputs by input priorities
+ TreeMap<Integer, List<Integer>> inputPriorityGroupMap = new TreeMap<>();
+ Preconditions.checkState(
+ node.getInputNodes().size() == node.getInputEdges().size(),
+ "Number of inputs nodes does not equal to number of input edges for node " +
+ node.getClass().getName() + ". This is a bug.");
+ for (int i = 0; i < node.getInputNodes().size(); i++) {
+ int priority = node.getInputEdges().get(i).getPriority();
+ inputPriorityGroupMap.computeIfAbsent(priority, k -> new ArrayList<>()).add(i);
+ }
+
+ // add edges between neighboring priority groups
+ List<List<Integer>> inputPriorityGroups = new ArrayList<>(inputPriorityGroupMap.values());
+ for (int i = 0; i + 1 < inputPriorityGroups.size(); i++) {
+ List<Integer> higherGroup = inputPriorityGroups.get(i);
+ List<Integer> lowerGroup = inputPriorityGroups.get(i + 1);
+
+ for (int higher : higherGroup) {
+ for (int lower : lowerGroup) {
+ addTopologyEdges(node, higher, lower);
+ }
+ }
+ }
+ }
+
+ private void addTopologyEdges(ExecNode<?, ?> node, int higherInput, int lowerInput) {
+ ExecNode<?, ?> higherNode = node.getInputNodes().get(higherInput);
+ ExecNode<?, ?> lowerNode = node.getInputNodes().get(lowerInput);
+ List<ExecNode<?, ?>> lowerAncestors = calculateAncestors(lowerNode);
+
+ List<Tuple2<ExecNode<?, ?>, ExecNode<?, ?>>> linkedEdges = new ArrayList<>();
+ for (ExecNode<?, ?> ancestor : lowerAncestors) {
+ if (graph.link(higherNode, ancestor)) {
+ linkedEdges.add(Tuple2.of(higherNode, ancestor));
+ } else {
+ // a conflict occurs, resolve it by adding a batch exchange
+ // and revert all linked edges
+ if (lowerNode instanceof BatchExecExchange) {
+ BatchExecExchange exchange = (BatchExecExchange) lowerNode;
+ exchange.setRequiredShuffleMode(ShuffleMode.BATCH);
+ } else {
+ node.replaceInputNode(lowerInput, (ExecNode) createExchange(node, lowerInput));
+ }
+
+ for (Tuple2<ExecNode<?, ?>, ExecNode<?, ?>> linkedEdge : linkedEdges) {
+ graph.unlink(linkedEdge.f0, linkedEdge.f1);
+ }
+ return;
+ }
+ }
+ }
+
+ /**
+ * Find the ancestors by going through PIPELINED edges.
+ */
+ @VisibleForTesting
+ List<ExecNode<?, ?>> calculateAncestors(ExecNode<?, ?> node) {
+ List<ExecNode<?, ?>> ret = new ArrayList<>();
+ AbstractExecNodeExactlyOnceVisitor ancestorVisitor = new AbstractExecNodeExactlyOnceVisitor() {
+ @Override
+ protected void visitNode(ExecNode<?, ?> node) {
+ List<ExecEdge> inputEdges = node.getInputEdges();
+ boolean hasAncestor = false;
+
+ for (int i = 0; i < inputEdges.size(); i++) {
+ // we only go through PIPELINED edges
+ if (inputEdges.get(i).getDamBehavior().stricterOrEqual(ExecEdge.DamBehavior.END_INPUT)) {
+ continue;
+ }
+ hasAncestor = true;
+ node.getInputNodes().get(i).accept(this);
+ }
+
+ if (!hasAncestor) {
+ ret.add(node);
+ }
+ }
+ };
+ node.accept(ancestorVisitor);
+ return ret;
+ }
+
+ private BatchExecExchange createExchange(ExecNode<?, ?> node, int idx) {
+ RelNode inputRel = (RelNode) node.getInputNodes().get(idx);
+
+ FlinkRelDistribution distribution;
+ ExecEdge.RequiredShuffle requiredShuffle = node.getInputEdges().get(idx).getRequiredShuffle();
+ if (requiredShuffle.getType() == ExecEdge.ShuffleType.HASH) {
+ distribution = FlinkRelDistribution.hash(requiredShuffle.getKeys(), true);
+ } else if (requiredShuffle.getType() == ExecEdge.ShuffleType.BROADCAST) {
+ // should not occur
+ throw new IllegalStateException(
+ "Trying to resolve input priority conflict on broadcast side. This is not expected.");
+ } else if (requiredShuffle.getType() == ExecEdge.ShuffleType.SINGLETON) {
+ distribution = FlinkRelDistribution.SINGLETON();
+ } else {
+ distribution = FlinkRelDistribution.ANY();
+ }
+
+ BatchExecExchange exchange = new BatchExecExchange(
+ inputRel.getCluster(),
+ inputRel.getTraitSet().replace(distribution),
+ inputRel,
+ distribution);
+ exchange.setRequiredShuffleMode(ShuffleMode.BATCH);
+ return exchange;
+ }
+
+ /**
+ * A data structure storing the topological information of an {@link ExecNode} graph.
+ */
+ @VisibleForTesting
+ static class TopologyGraph {
+ private final Map<ExecNode<?, ?>, TopologyNode> nodes;
+ private int visitTag;
+
+ TopologyGraph(List<ExecNode<?, ?>> sinks) {
+ this.nodes = new HashMap<>();
+ this.visitTag = 0;
+
+ // we first link all edges in the original exec node graph
+ AbstractExecNodeExactlyOnceVisitor visitor = new AbstractExecNodeExactlyOnceVisitor() {
+ @Override
+ protected void visitNode(ExecNode<?, ?> node) {
+ for (ExecNode<?, ?> input : node.getInputNodes()) {
+ link(input, node);
+ }
+ visitInputs(node);
+ }
+ };
+ sinks.forEach(n -> n.accept(visitor));
+ }
+
+ /**
+ * Link an edge from `from` node to `to` node if no loop will occur after adding this edge.
+ * Returns if this edge is successfully added.
+ */
+ boolean link(ExecNode<?, ?> from, ExecNode<?, ?> to) {
+ TopologyNode fromNode = getTopologyNode(from);
+ TopologyNode toNode = getTopologyNode(to);
+
+ if (canReach(toNode, fromNode)) {
+ // invalid edge, as `to` is the predecessor of `from`
+ return false;
+ } else {
+ // link `from` and `to`
+ fromNode.outputs.add(toNode);
+ toNode.inputs.add(fromNode);
+ return true;
+ }
+ }
+
+ /**
+ * Remove the edge from `from` node to `to` node. If there is no edge between them then do nothing.
+ */
+ void unlink(ExecNode<?, ?> from, ExecNode<?, ?> to) {
+ TopologyNode fromNode = getTopologyNode(from);
+ TopologyNode toNode = getTopologyNode(to);
+
+ fromNode.outputs.remove(toNode);
+ toNode.inputs.remove(fromNode);
+ }
+
+ @VisibleForTesting
+ boolean canReach(ExecNode<?, ?> from, ExecNode<?, ?> to) {
+ TopologyNode fromNode = getTopologyNode(from);
+ TopologyNode toNode = getTopologyNode(to);
+ return canReach(fromNode, toNode);
+ }
+
+ private boolean canReach(TopologyNode from, TopologyNode to) {
+ Queue<TopologyNode> queue = new LinkedList<>();
+ from.tag = --visitTag;
+ queue.offer(from);
+
+ while (!queue.isEmpty()) {
+ TopologyNode node = queue.poll();
+ if (to.equals(node)) {
+ return true;
+ }
+
+ for (TopologyNode next : node.outputs) {
+ if (next.tag == visitTag) {
+ continue;
+ }
+ next.tag = visitTag;
+ queue.offer(next);
+ }
+ }
+
+ return false;
+ }
+
+ private TopologyNode getTopologyNode(ExecNode<?, ?> execNode) {
+ // NOTE: We treat different `BatchExecBoundedStreamScan`s with same `DataStream` object as the same
+ if (execNode instanceof BatchExecBoundedStreamScan) {
+ DataStream<?> currentStream =
+ ((BatchExecBoundedStreamScan) execNode).boundedStreamTable().dataStream();
+ for (Map.Entry<ExecNode<?, ?>, TopologyNode> entry : nodes.entrySet()) {
+ ExecNode<?, ?> key = entry.getKey();
+ if (key instanceof BatchExecBoundedStreamScan) {
+ DataStream<?> existingStream =
+ ((BatchExecBoundedStreamScan) key).boundedStreamTable().dataStream();
+ if (existingStream.equals(currentStream)) {
+ return entry.getValue();
+ }
+ }
+ }
+
+ TopologyNode result = new TopologyNode();
+ nodes.put(execNode, result);
+ return result;
+ } else {
+ return nodes.computeIfAbsent(execNode, k -> new TopologyNode());
+ }
+ }
+ }
+
+ /**
+ * A node in the {@link TopologyGraph}.
+ */
+ private static class TopologyNode {
+ private final Set<TopologyNode> inputs = new HashSet<>();
+ private final Set<TopologyNode> outputs = new HashSet<>();
+
+ private int tag;
Review comment:
make this pojo stateless, we can use a map to represent the usage of the `tag`
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