jiajunwang commented on a change in pull request #381: Implement the POC work 
greedy constraint based algorithm
URL: https://github.com/apache/helix/pull/381#discussion_r314508590
 
 

 ##########
 File path: 
helix-core/src/main/java/org/apache/helix/controller/rebalancer/waged/algorithm/ConstraintBasedAlgorithm.java
 ##########
 @@ -0,0 +1,108 @@
+package org.apache.helix.controller.rebalancer.waged.algorithm;
+
+/*
+ * 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.
+ */
+
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+
+import org.apache.helix.controller.rebalancer.waged.constraints.HardConstraint;
+import org.apache.helix.controller.rebalancer.waged.constraints.SoftConstraint;
+import org.apache.helix.controller.rebalancer.waged.model.AssignableNode;
+import org.apache.helix.controller.rebalancer.waged.model.AssignableReplica;
+import org.apache.helix.controller.rebalancer.waged.model.ClusterContext;
+import org.apache.helix.controller.rebalancer.waged.model.ClusterModel;
+import org.apache.helix.controller.rebalancer.waged.model.OptimalAssignment;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * The algorithm is based on a given set of constraints
+ * - HardConstraint: Approve or deny the assignment given its condition, any 
assignment cannot
+ * bypass any "hard constraint"
+ * - SoftConstraint: Evaluate the assignment by points/rewards/scores, a 
higher point means a better
+ * assignment
+ * <p>
+ * The goal is to accumulate the most points(rewards) from "soft constraints" 
while avoiding all
+ * "hard constraints"
+ */
+public class ConstraintBasedAlgorithm implements RebalanceAlgorithm {
+  private static final Logger LOG = 
LoggerFactory.getLogger(ConstraintBasedAlgorithm.class);
+  private final List<HardConstraint> _hardConstraints;
+  private final List<SoftConstraint> _softConstraints;
+
+  public ConstraintBasedAlgorithm(List<HardConstraint> hardConstraints,
+      List<SoftConstraint> softConstraints) {
+    _hardConstraints = hardConstraints;
+    _softConstraints = softConstraints;
+  }
+
+  @Override
+  public OptimalAssignment calculate(ClusterModel clusterModel) {
+    ClusterContext clusterContext = clusterModel.getContext();
+    OptimalAssignment optimalAssignment = new 
OptimalAssignment(clusterContext);
+    Map<String, Set<AssignableReplica>> replicasByResource = 
clusterModel.getAssignableReplicaMap();
+    List<AssignableNode> nodes = new 
ArrayList<>(clusterModel.getAssignableNodes().values());
+
+    for (String resource : replicasByResource.keySet()) {
+      for (AssignableReplica replica : replicasByResource.get(resource)) {
+        Optional<AssignableNode> maybeBestNode =
+            getNodeWithHighestPoints(replica, nodes, optimalAssignment);
+        // stop immediately if any replica cannot find best assignable node
+        if (optimalAssignment.hasAnyFailure()) {
+          LOG.error(
+              "Unable to find any available candidate node for partition {}; 
Fail reasons: {}",
+              replica.getPartitionName(), optimalAssignment.getFailures());
+          return optimalAssignment;
+        }
+        maybeBestNode.ifPresent(node -> 
optimalAssignment.addAssignment(replica, node));
+      }
+    }
+
+    return optimalAssignment;
+  }
+
+  private Optional<AssignableNode> getNodeWithHighestPoints(AssignableReplica 
replica,
+      List<AssignableNode> assignableNodes, OptimalAssignment 
currentAssignment) {
+    List<AssignableNode> candidateNodes = assignableNodes.stream()
+        .filter(candidateNode -> _hardConstraints.stream().allMatch(constraint 
-> {
+          boolean isValid = constraint.isAssignmentValid(candidateNode, 
replica, currentAssignment.getClusterContext());
+          if (!isValid) {
+            currentAssignment.trackAssignmentFailure(candidateNode, replica, 
constraint);
 
 Review comment:
   Is this necessary? We basically record all the possible pairs, right?
   So even with a completely valid assignment, you will have (nodeCount - 1) * 
replicaCount error records.
   
   I don't think it is necessary.

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