roryqi commented on code in PR #12234:
URL: https://github.com/apache/gravitino/pull/12234#discussion_r3704227853


##########
core/src/main/java/org/apache/gravitino/RelationUpdate.java:
##########
@@ -0,0 +1,129 @@
+/*
+ * 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.gravitino;
+
+import com.google.common.base.Preconditions;
+import java.util.Arrays;
+
+/**
+ * Describes a relation update from a source entity to target endpoints. 
Target endpoint identity
+ * and relation-edge attributes are carried by {@link RelationEdgeTarget}.
+ */
+public final class RelationUpdate {
+
+  private static final RelationEdgeTarget[] EMPTY_TARGETS = new 
RelationEdgeTarget[0];
+
+  private final SupportsRelationOperations.Type relationType;
+  private final NameIdentifier sourceIdentifier;
+  private final Entity.EntityType sourceEntityType;
+  private final RelationEdgeTarget[] targetsToAdd;
+  private final RelationEdgeTarget[] targetsToRemove;
+
+  private RelationUpdate(
+      SupportsRelationOperations.Type relationType,
+      NameIdentifier sourceIdentifier,
+      Entity.EntityType sourceEntityType,
+      RelationEdgeTarget[] targetsToAdd,
+      RelationEdgeTarget[] targetsToRemove) {
+    this.relationType = relationType;
+    this.sourceIdentifier = sourceIdentifier;
+    this.sourceEntityType = sourceEntityType;
+    this.targetsToAdd = copyTargets(targetsToAdd, "targetsToAdd");
+    this.targetsToRemove = copyTargets(targetsToRemove, "targetsToRemove");
+  }
+
+  /**
+   * Creates a relation update.
+   *
+   * @param relationType The type of relation.
+   * @param sourceIdentifier The identifier of the source entity whose 
relations are being updated.
+   * @param sourceEntityType The source entity type.
+   * @param targetsToAdd Target endpoints to associate with the source entity.
+   * @param targetsToRemove Target endpoints to disassociate from the source 
entity.
+   * @return A relation update.
+   */
+  public static RelationUpdate of(
+      SupportsRelationOperations.Type relationType,
+      NameIdentifier sourceIdentifier,
+      Entity.EntityType sourceEntityType,
+      RelationEdgeTarget[] targetsToAdd,
+      RelationEdgeTarget[] targetsToRemove) {
+    Preconditions.checkArgument(relationType != null, "relationType must not 
be null");
+    Preconditions.checkArgument(sourceIdentifier != null, "sourceIdentifier 
must not be null");
+    Preconditions.checkArgument(sourceEntityType != null, "sourceEntityType 
must not be null");
+    return new RelationUpdate(
+        relationType, sourceIdentifier, sourceEntityType, targetsToAdd, 
targetsToRemove);
+  }
+
+  /**
+   * @return The type of relation.
+   */
+  public SupportsRelationOperations.Type relationType() {
+    return relationType;
+  }
+
+  /**
+   * @return The source entity identifier.
+   */
+  public NameIdentifier sourceIdentifier() {
+    return sourceIdentifier;
+  }
+
+  /**
+   * @return The source entity type.
+   */
+  public Entity.EntityType sourceEntityType() {
+    return sourceEntityType;
+  }
+
+  /**
+   * @return Target endpoints to associate with the source entity.
+   */
+  public RelationEdgeTarget[] targetsToAdd() {
+    return targetsToAdd.clone();
+  }
+
+  /**
+   * @return Target endpoints to disassociate from the source entity.
+   */
+  public RelationEdgeTarget[] targetsToRemove() {
+    return targetsToRemove.clone();

Review Comment:
   Correct, this is a shallow array copy. For this API that is sufficient 
because `RelationEdgeTarget` is immutable: it is final, has private final 
fields, a private constructor, no mutators, and only contains immutable 
endpoint/value data. I clarified the Javadocs in `0662d7171` to say the copy 
protects the array container from external mutation, and added a unit test that 
mutating the input arrays or returned arrays does not affect the stored 
`RelationUpdate`.



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