Github user tillrohrmann commented on a diff in the pull request:
https://github.com/apache/flink/pull/640#discussion_r29426482
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/executiongraph/Backtracking.java
---
@@ -0,0 +1,252 @@
+/*
+ * 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.runtime.executiongraph;
+
+import akka.actor.ActorRef;
+import akka.dispatch.OnComplete;
+import akka.pattern.Patterns;
+import com.google.common.base.Preconditions;
+import org.apache.flink.runtime.akka.AkkaUtils;
+import org.apache.flink.runtime.execution.ExecutionState;
+import org.apache.flink.runtime.jobgraph.IntermediateResultPartitionID;
+import org.apache.flink.runtime.messages.TaskMessages;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import scala.concurrent.Future;
+
+import java.util.ArrayDeque;
+import java.util.Collection;
+import java.util.Deque;
+import java.util.HashMap;
+import java.util.Map;
+
+
+/**
+ * Backtracking is a mechanism to schedule only those Execution Vertices
of an Execution Graph which
+ * do not have an intermediate result available. This is in contrast to
the simple way of scheduling
+ * a job, where all Execution Vertices are executed starting from the
source. The Backtracking starts
+ * from the sinks and traverses the Execution Graph to the sources. It
only reaches the sources if
+ * no intermediate result could be found on the way.
+ *
+ * @see ExecutionGraph
+ * @see ExecutionVertex
+ * @see Execution
+ */
+public class Backtracking {
+
+ private static final Logger LOG =
LoggerFactory.getLogger(Backtracking.class);
+
+ private final Deque<TaskRequirement> taskRequirements = new
ArrayDeque<TaskRequirement>();
+
+ private final Map<IntermediateResultPartitionID, Boolean>
visitedPartitions = new HashMap<IntermediateResultPartitionID, Boolean>();
+
+ private ScheduleAction scheduleAction;
+ private Runnable postBacktrackingHook;
+
+ public Backtracking(Collection<ExecutionJobVertex> vertices) {
+ Preconditions.checkNotNull(vertices);
+
+ // add all sinks found to the stack
+ for (ExecutionJobVertex ejv : vertices) {
+ if (ejv.getJobVertex().isOutputVertex()) {
+ for (ExecutionVertex ev :
ejv.getTaskVertices()) {
+ if (ev.getExecutionState() ==
ExecutionState.CREATED) {
+ taskRequirements.add(new
TaskRequirement(ev));
+ }
+ }
+ }
+ }
+
+ this.scheduleAction = new ScheduleAction() {
+ @Override
+ public void schedule(ExecutionVertex ev) {}
+ };
+
+ this.postBacktrackingHook = new Runnable() {
+ @Override
+ public void run() {}
+ };
+ }
+
+ /**
+ * Scheduling to be performed when an ExecutionVertex is encountered
that cannot be resumed
+ */
+ public interface ScheduleAction {
+ void schedule(ExecutionVertex ev);
+ }
+
+ /**
+ * A TaskRequirement encaplusates an ExecutionVertex and its
IntermediateResultPartitions which
+ * are required for execution.
+ */
+ private class TaskRequirement {
+
+ private final ExecutionVertex executionVertex;
+ private final Deque<IntermediateResultPartition> pendingInputs
= new ArrayDeque<IntermediateResultPartition>();
+ private final int numInputs;
+
+ private int availableInputs = 0;
+
+ public TaskRequirement(ExecutionVertex executionVertex) {
+ this.executionVertex = executionVertex;
+ this.pendingInputs.addAll(executionVertex.getInputs());
+ this.numInputs = pendingInputs.size();
+ }
+
+ public ExecutionVertex getExecutionVertex() {
+ return executionVertex;
+ }
+
+ public boolean pendingRequirements() {
+ for (IntermediateResultPartition ir : pendingInputs) {
+ Boolean visitedPartition =
visitedPartitions.get(ir.getPartitionId());
+ if (visitedPartition == null) {
+ return true;
+ } else {
+ if (visitedPartition) {
+ availableInputs++;
+ }
+ pendingInputs.remove(ir);
--- End diff --
As far as I know, it's unsafe to modify a collection while iterating over
it if you don't use the remove method of the iterator directly.
Quote from JavaDoc: Iterator.remove() "The behavior of an iterator is
unspecified if the underlying collection is modified while the iteration is in
progress in any way other than by calling this method."
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