[FSTORE-2025] add ordered primary key index config and fix error code for no featur… (#955)

* add ordered primary key index config and fix error code for no feature group found to handle new backend code of 400 instead of 404.

* address copilot comment

* revert error code change

Author: dhananjay-mk

java/hsfs/src/main/java/com/logicalclocks/hsfs/OnlineConfig.java

@@ -17,15 +17,13 @@
 
 package com.logicalclocks.hsfs;
 
-import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
 
 import java.util.List;
 
 @NoArgsConstructor
-@AllArgsConstructor
 public class OnlineConfig {
 
   @Getter
@@ -35,4 +33,19 @@ public class OnlineConfig {
   @Getter
   @Setter
   private String tableSpace;
+
+  @Getter
+  @Setter
+  private PrimaryKeyIndexType primaryKeyIndexType;
+
+  public OnlineConfig(List<String> onlineComments, String tableSpace) {
+    this.onlineComments = onlineComments;
+    this.tableSpace = tableSpace;
+  }
+
+  public OnlineConfig(List<String> onlineComments, String tableSpace, PrimaryKeyIndexType primaryKeyIndexType) {
+    this.onlineComments = onlineComments;
+    this.tableSpace = tableSpace;
+    this.primaryKeyIndexType = primaryKeyIndexType;
+  }
 }

java/hsfs/src/main/java/com/logicalclocks/hsfs/PrimaryKeyIndexType.java

@@ -0,0 +1,23 @@
+/*
+ *  Copyright (c) 2026. Hopsworks AB
+ *
+ *  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 com.logicalclocks.hsfs;
+
+public enum PrimaryKeyIndexType {
+  HASH,
+  ORDERED
+}

java/hsfs/src/test/java/com/logicalclocks/hsfs/TestOnlineConfig.java

@@ -0,0 +1,67 @@
+/*
+ *  Copyright (c) 2026. Hopsworks AB
+ *
+ *  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 com.logicalclocks.hsfs;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import org.junit.Assert;
+import org.junit.jupiter.api.Test;
+
+class TestOnlineConfig {
+
+  private final ObjectMapper objectMapper = new ObjectMapper();
+
+  @Test
+  void testHashIndexTypeSerializedAndDeserialized() throws Exception {
+    // Arrange
+    OnlineConfig config = new OnlineConfig(null, null, PrimaryKeyIndexType.HASH);
+
+    // Act – round-trip through Jackson
+    String json = objectMapper.writeValueAsString(config);
+    OnlineConfig result = objectMapper.readValue(json, OnlineConfig.class);
+
+    // Assert
+    Assert.assertEquals(PrimaryKeyIndexType.HASH, result.getPrimaryKeyIndexType());
+  }
+
+  @Test
+  void testOrderedIndexTypeSerializedAndDeserialized() throws Exception {
+    // Arrange
+    OnlineConfig config = new OnlineConfig(null, null, PrimaryKeyIndexType.ORDERED);
+
+    // Act
+    String json = objectMapper.writeValueAsString(config);
+    OnlineConfig result = objectMapper.readValue(json, OnlineConfig.class);
+
+    // Assert
+    Assert.assertEquals(PrimaryKeyIndexType.ORDERED, result.getPrimaryKeyIndexType());
+  }
+
+  @Test
+  void testNoConfigPassedDefaultsToHash() throws Exception {
+    // When no primaryKeyIndexType is set the server applies a TTL-driven default
+    // and returns HASH for feature groups without TTL. Simulate that server response.
+    String serverResponse = "{\"primaryKeyIndexType\":\"HASH\"}";
+
+    // Act
+    OnlineConfig result = objectMapper.readValue(serverResponse, OnlineConfig.class);
+
+    // Assert
+    Assert.assertEquals(PrimaryKeyIndexType.HASH, result.getPrimaryKeyIndexType());
+  }
+}