[GitHub] [lucene-solr] shalinmangar commented on a change in pull request #1417: SOLR-12847: Auto-create a policy rule that corresponds to maxShardsPerNode

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shalinmangar commented on a change in pull request #1417: SOLR-12847: Auto-create a policy rule that corresponds to maxShardsPerNode
URL: https://github.com/apache/lucene-solr/pull/1417#discussion_r406735984
 
 

 ##########
 File path: solr/core/src/java/org/apache/solr/cloud/api/collections/CreateCollectionCmd.java
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 @@ -407,14 +466,81 @@ public void call(ClusterState clusterState, ZkNodeProps message, NamedList resul
           .assignPullReplicas(numPullReplicas)
           .onNodes(nodeList)
           .build();
-      Assign.AssignStrategyFactory assignStrategyFactory = new Assign.AssignStrategyFactory(cloudManager);
+      // if Strategy.POLICY is in use AND maxShardsPerNode was specified then for back-compat it needs to
+      // be translated into a rule (see SOLR-12847).
+      // Modify the autoscaling rules as needed BEFORE actually calling assign, setting configToRestore as
+      // a side-effect to restore the original config if the creation fails
+      if ((strategy == Assign.AssignStrategyFactory.Strategy.POLICY) &&
+          (maxReplicasPerNode != null && maxReplicasPerNode != Integer.MAX_VALUE)) {
+        maybeAddMaxReplicasRule(cloudManager, maxReplicasPerNode, docCollection, configToRestore);
+      }
       Assign.AssignStrategy assignStrategy = assignStrategyFactory.create(clusterState, docCollection);
       replicaPositions = assignStrategy.assign(cloudManager, assignRequest);
       sessionWrapper.set(PolicyHelper.getLastSessionWrapper(true));
     }
     return replicaPositions;
   }
 
+  /**
+   * Optionally add a policy that implements 'maxShardsPerNode' and set the collection to use this policy.
+   * @param cloudManager SolrCloudManager
+   * @param maxReplicasPerNode max number of replicas per node
+   * @param docCollection collection (its properties may be modified as a side-effect)
+   * @param configToRestore if AutoScalingConfig has been changed as a result of this method the original
+   *                        value is set here in order to restore it in case of failures
+   */
+  public static void maybeAddMaxReplicasRule(SolrCloudManager cloudManager, int maxReplicasPerNode, DocCollection docCollection,
+                                                AtomicReference<AutoScalingConfig> configToRestore) throws IOException, InterruptedException {
+    AutoScalingConfig initialConfig = cloudManager.getDistribStateManager().getAutoScalingConfig();
+    Policy policy = initialConfig.getPolicy();
+    String policyName = docCollection.getPolicyName();
+    if (policyName == null) {
+      policyName = CollectionAdminParams.AUTO_PREFIX + docCollection.getName();
+      docCollection.getProperties().put(Policy.POLICY, policyName);
+    }
+    Map<String, List<Clause>> policies = policy.getPolicies();
+    List<Clause> clauses = policies.get(policyName);
+    maxReplicasPerNode++; // only < supported
+    Clause c = Clause.create("{'replica': '<" + maxReplicasPerNode +"' , 'shard': '#ANY', 'node': '#ANY'}");
+    boolean alreadyExists = false;
+    if (clauses != null) {
+      for (Clause clause : clauses) {
+        if (clause.equals(c)) {
+          alreadyExists = true;
+          break;
+        }
+        if (clause.getReplica() != null) {
+          throw new Assign.AssignmentException("Both an existing policy=" + policyName + " and " +
 
 Review comment:
   This block will always execute if were to add a default policy like the one on SOLR-12845. We should test if there is a similar clause in the policy where similar means that only the value of N in `replica < N` is different than maxShardsPerNode

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