/*
* Copyright 2018 LinkedIn Corp.
*
* Licensed 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 azkaban.dag;
import static java.util.Objects.requireNonNull;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
/**
* A builder to build DAGs.
*
* <p>Use the {@link DagBuilder#createNode} method to create NodeBuilder instances. Call
* methods on NodeBuilder to add dependencies among them. Call the {@link DagBuilder#build()} method
* to build a Dag.
*/
public class DagBuilder {
private final String name;
private final DagProcessor dagProcessor;
private final List<NodeBuilder> builders = new ArrayList<>();
private final Set<String> nodeNamesSet = new HashSet<>();
public DagBuilder(final String name, final DagProcessor dagProcessor) {
requireNonNull(name, "The name of the DagBuilder can't be null");
this.name = name;
requireNonNull(name, "The dagProcessor of the DagBuilder can't be null");
this.dagProcessor = dagProcessor;
}
public NodeBuilder createNode(final String name, final NodeProcessor nodeProcessor) {
final NodeBuilder builder = new NodeBuilder(name, nodeProcessor, this);
this.builders.add(builder);
if (this.nodeNamesSet.contains(name)) {
throw new DagException(String.format("Node names in %s need to be unique. The name "
+ "(%s) already exists.", this, name));
}
this.nodeNamesSet.add(name);
return builder;
}
/**
* Builds the dag.
*
* <p>Once this method is called, subsequent calls via NodeBuilder to modify the nodes's
* relationships in the dag will have no effect on the returned Dag object.
* </p>
*
* @return the Dag reflecting the current state of the DagBuilder
*/
public Dag build() {
checkCircularDependencies();
final Dag dag = new Dag(this.name, this.dagProcessor);
final Map<NodeBuilder, Node> builderNodeMap = createBuilderToNodeMap(dag);
updateNodesRelationships(builderNodeMap);
return dag;
}
/**
* Checks if the builder contains nodes that form a circular dependency ring.
*
* <p>The depth first algorithm is described in this article
* <a href="https://en.wikipedia.org/wiki/Topological_sorting">https://en.wikipedia.org/wiki/Topological_sorting</a>
* </p>
*
* @throws DagException if true
*/
private void checkCircularDependencies() {
class CircularDependencyChecker {
// The nodes that need to be visited
private final Set<NodeBuilder> toVisit = new HashSet<>(DagBuilder.this.builders);
// The nodes that have finished traversing all their parent nodes
private final Set<NodeBuilder> finished = new HashSet<>();
// The nodes that are waiting for their parent nodes to finish visit.
private final Set<NodeBuilder> ongoing = new HashSet<>();
// One sample of nodes that form a circular dependency
private final List<NodeBuilder> sampleCircularNodes = new ArrayList<>();
/**
* Checks if the builder contains nodes that form a circular dependency ring.
*
* @throws DagException if true
*/
private void check() {
while (!this.toVisit.isEmpty()) {
final NodeBuilder node = removeOneNodeFromToVisitSet();
if (checkNode(node)) {
final String msg = String.format("Circular dependency detected. Sample: %s",
this.sampleCircularNodes);
throw new DagException(msg);
}
}
}
/**
* Removes one node from the toVisit set and returns that node.
*
* @return a node
*/
private NodeBuilder removeOneNodeFromToVisitSet() {
final Iterator<NodeBuilder> iterator = this.toVisit.iterator();
final NodeBuilder node = iterator.next();
iterator.remove();
return node;
}
/**
* Checks if the node is part of a group of nodes that form a circular dependency ring.
*
* <p>If true, the node will be added to the sampleCircularNodes list</p>
*
* @param node node to check
* @return true if it is
*/
private boolean checkNode(final NodeBuilder node) {
if (this.finished.contains(node)) {
return false;
}
if (this.ongoing.contains(node)) {
this.sampleCircularNodes.add(node);
return true;
}
this.toVisit.remove(node);
this.ongoing.add(node);
for (final NodeBuilder parent : node.getParents()) {
if (checkNode(parent)) {
this.sampleCircularNodes.add(node);
return true;
}
}
this.ongoing.remove(node);
this.finished.add(node);
return false;
}
}
final CircularDependencyChecker checker = new CircularDependencyChecker();
checker.check();
}
/**
* Creates nodes using information stored in the current list of builders.
*
* <p>New nodes are created here to ensure they don't change even if their corresponding
* NodeBuilders are modified after the {@link DagBuilder#build()} is called.
*
* @param dag the dag to associate the nodes with
* @return the map from NodeBuilder to Node
*/
private Map<NodeBuilder, Node> createBuilderToNodeMap(final Dag dag) {
final Map<NodeBuilder, Node> builderNodeMap = new HashMap<>();
for (final NodeBuilder builder : this.builders) {
final Node node = builder.build(dag);
builderNodeMap.put(builder, node);
}
return builderNodeMap;
}
private void updateNodesRelationships(final Map<NodeBuilder, Node> builderNodeMap) {
for (final NodeBuilder builder : this.builders) {
addParentNodes(builder, builderNodeMap);
}
}
/**
* Adds parent nodes to the node associated with the builder.
*/
private void addParentNodes(final NodeBuilder builder,
final Map<NodeBuilder, Node> builderToNodeMap) {
final Node node = builderToNodeMap.get(builder);
for (final NodeBuilder parentBuilder : builder.getParents()) {
final Node parentNode = builderToNodeMap.get(parentBuilder);
// The NodeBuilders should have checked if the NodeBuilders belong to the same DagBuilder.
assert (parentNode != null);
node.addParent(parentNode);
}
}
@Override
public String toString() {
return String.format("DagBuilder (%s)", this.name);
}
}