/*
* 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 Dag dag;
private final Map<String, Node> nameToNodeMap = new HashMap<>();
// The builder can only be used to build a DAG once to prevent modifying an existing DAG after it
// is built.
private boolean isBuilt = false;
/**
* A builder for building a DAG.
*
* @param name name of the DAG
* @param dagProcessor the associated DagProcessor
*/
public DagBuilder(final String name, final DagProcessor dagProcessor) {
requireNonNull(name, "The name of the DagBuilder can't be null");
requireNonNull(dagProcessor, "The dagProcessor of the DagBuilder can't be null");
this.dag = new Dag(name, dagProcessor);
}
/**
* Creates a new node and adds it to the DagBuilder.
*
* @param name name of the node
* @param nodeProcessor node processor associated with this node
* @return a new node
* @throws DagException if the name is not unique in the DAG.
*/
public Node createNode(final String name, final NodeProcessor nodeProcessor) {
checkIsBuilt();
if (this.nameToNodeMap.get(name) != null) {
throw new DagException(String.format("Node names in %s need to be unique. The name "
+ "(%s) already exists.", this, name));
}
final Node node = new Node(name, nodeProcessor, this.dag);
this.nameToNodeMap.put(name, node);
return node;
}
/**
* Throws an exception if the {@link DagBuilder#build()} method has been called.
*/
private void checkIsBuilt() {
if (this.isBuilt) {
final String msg = String
.format("The DAG (%s) is built already. Can't create new nodes.", this);
throw new DagException(msg);
}
}
/**
* Add a parent node to a child node. All the names should have been registered with this builder
* with the {@link DagBuilder#createNode(String, NodeProcessor)} call.
*
* @param childNodeName name of the child node
* @param parentNodeName name of the parent node
*/
public void addParentNode(final String childNodeName, final String parentNodeName) {
checkIsBuilt();
final Node child = this.nameToNodeMap.get(childNodeName);
if (child == null) {
throw new DagException(String.format("Unknown child node (%s). Did you create the node?",
childNodeName));
}
final Node parent = this.nameToNodeMap.get(parentNodeName);
if (parent == null) {
throw new DagException(
String.format("Unknown parent node (%s). Did you create the node?", parentNodeName));
}
child.addParent(parent);
}
/**
* 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() {
checkIsBuilt();
checkCircularDependencies();
this.isBuilt = true;
return this.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<Node> toVisit = new HashSet<>(DagBuilder.this.nameToNodeMap.values());
// The nodes that have finished traversing all their parent nodes
private final Set<Node> finished = new HashSet<>();
// The nodes that are waiting for their parent nodes to finish visit.
private final Set<Node> ongoing = new HashSet<>();
// One sample of nodes that form a circular dependency
private final List<Node> 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 Node 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 Node removeOneNodeFromToVisitSet() {
final Iterator<Node> iterator = this.toVisit.iterator();
final Node 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 Node 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 Node 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();
}
@Override
public String toString() {
return String.format("DagBuilder (%s)", this.dag.getName());
}
}