vinishjail97 commented on code in PR #935:
URL: https://github.com/apache/incubator-xtable/pull/935#discussion_r4226522649


##########
xtable-core/src/main/java/org/apache/xtable/index/Index.java:
##########
@@ -0,0 +1,66 @@
+/*
+ * 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.xtable.index;
+
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Row;
+
+/**
+ * A secondary index over a table in another format, backed by XTable's Hudi 
conversion. It maps the
+ * values of an indexed column to the data file and row position that holds 
each value, so an engine
+ * can locate the rows to merge or delete without a full table join.
+ *
+ * @param <T> The table type of the source format (for example an Iceberg 
{@code Table})
+ */
+public interface Index<T> {
+  /** Result column with the full path of the data file that holds the row. */
+  String FILE_COLUMN = "_file";

Review Comment:
   Is the interface specific to iceberg? _delta can have the same index 
structure? So let's make it really format agnostic? 



##########
xtable-core/src/main/java/org/apache/xtable/hudi/engine/HudiExecutionEngineProvider.java:
##########
@@ -0,0 +1,59 @@
+/*
+ * 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.xtable.hudi.engine;
+
+import java.util.List;
+import java.util.Map;
+
+import org.apache.hudi.client.BaseHoodieWriteClient;
+import org.apache.hudi.client.WriteStatus;
+import org.apache.hudi.common.engine.HoodieEngineContext;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.util.Option;
+import org.apache.hudi.config.HoodieWriteConfig;
+import org.apache.hudi.table.HoodieTable;
+
+/**
+ * Abstracts the Hudi engine (Java or Spark) used by the Hudi conversion 
target to commit the
+ * registered files and to maintain the table and its metadata table indexes.
+ */
+public interface HudiExecutionEngineProvider {
+
+  /** Returns the engine context used for file listing, index generation and 
table services. */
+  HoodieEngineContext getEngineContext();
+
+  /** Creates the engine specific {@link HoodieTable} for cleaning and 
archival. */
+  HoodieTable<?, ?, ?, ?> createTable(

Review Comment:
   Do we need `HoodieTable<?, ?, ?, ?>` or just returning `HoodieTable` is 
enough?  



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(
+          "Failed to sync the secondary indexes of columns "
+              + indexedColumns
+              + " of table "
+              + tableLocation
+              + ": "
+              + syncResult.getTableFormatSyncStatus());
+    }
+    Set<String> completedPartitions = getCompletedIndexPartitions();
+    List<String> notIndexed =
+        indexedColumns.stream()
+            .filter(
+                column ->
+                    
!completedPartitions.contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + column))
+            .collect(Collectors.toList());
+    if (!notIndexed.isEmpty()) {
+      throw new IllegalStateException(
+          "The secondary indexes of columns "
+              + notIndexed
+              + " of table "
+              + tableLocation
+              + " do not exist. The indexes are built when the index is 
created, and adding a"
+              + " column to an existing index is not supported yet.");
+    }
+    log.info(
+        "Synced the secondary indexes of columns {} of table {}", 
indexedColumns, tableLocation);
+  }
+
+  /**
+   * Returns the metadata table partitions whose build has completed. A 
partition directory can
+   * exist while the build is in flight or after it failed, so the directory 
alone does not show
+   * that the index is usable.
+   */
+  private Set<String> getCompletedIndexPartitions() {
+    try {
+      return getMetaClient().getTableConfig().getMetadataPartitions();
+    } catch (TableNotFoundException e) {
+      return Collections.emptySet();
+    }
+  }
+
+  private HoodieTableMetaClient getMetaClient() {
+    HoodieStorage storage =
+        new HoodieHadoopStorage(dataPath, 
javaSparkContext.hadoopConfiguration());
+    return 
HoodieTableMetaClient.builder().setStorage(storage).setBasePath(dataPath).build();
+  }
+
+  private ConversionConfig getConversionConfig(Table icebergTable) {
+    TargetTable targetTable =
+        TargetTable.builder()
+            .name(icebergTable.name())
+            .basePath(dataPath)
+            .formatName(TableFormat.HUDI)
+            .additionalProperties(targetTableProperties)
+            .build();
+    return ConversionConfig.builder()
+        .sourceTable(sourceTable)
+        .targetTables(Collections.singletonList(targetTable))
+        .build();
+  }
+
+  @Override
+  public Dataset<Row> lookup(Table icebergTable, Dataset<Row> keys, String 
columnName) {
+    Preconditions.checkArgument(
+        indexedColumns.contains(columnName),
+        "Column %s is not one of the indexed columns %s",
+        columnName,
+        indexedColumns);
+    StructType resultSchema =
+        new StructType()
+            .add(columnName, DataTypes.StringType, false)
+            .add(FILE_COLUMN, DataTypes.StringType, false)
+            .add(POSITION_COLUMN, DataTypes.LongType, false);
+    // the secondary index stores the values of the indexed column as strings
+    JavaRDD<String> secondaryKeys =
+        keys.where(col(columnName).isNotNull())
+            .select(col(columnName).cast(DataTypes.StringType))
+            .as(Encoders.STRING())
+            .toJavaRDD();
+    HoodieTableMetaClient metaClient = getMetaClient();
+    HoodieStorage storage = metaClient.getStorage();
+    if (!metaClient
+        .getTableConfig()
+        .getMetadataPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName)) {
+      throw new IllegalStateException(
+          "The secondary index of column "
+              + columnName
+              + " of table "
+              + tableLocation
+              + " is not built");
+    }
+    HoodieMetadataConfig metadataConfig =
+        
HoodieMetadataConfig.newBuilder().enable(true).withSecondaryIndexEnabled(true).build();
+    try (HoodieBackedTableMetadata tableMetadata =
+        (HoodieBackedTableMetadata)
+            metaClient
+                .getTableFormat()
+                .getMetadataFactory()
+                .create(engineContext, storage, metadataConfig, dataPath)) {
+      HoodiePairData<String, String> recordKeysBySecondaryKey =
+          tableMetadata.readSecondaryIndexDataTableRecordKeysWithKeys(
+              HoodieJavaRDD.of(secondaryKeys), 
PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+      // capture the field so the closure does not serialize the index instance
+      String dataPath = this.dataPath;
+      JavaRDD<Row> lookupResults =
+          toJavaPairRDD(recordKeysBySecondaryKey)
+              .map(
+                  recordKeyBySecondaryKey ->
+                      toLookupResult(
+                          dataPath, recordKeyBySecondaryKey._1(), 
recordKeyBySecondaryKey._2()));
+      return sparkSession.createDataFrame(lookupResults, resultSchema);
+    }
+  }
+
+  /**
+   * Hudi returns an RDD backed result for keys in an RDD, but a list backed 
result when there are
+   * no keys to look up, for example when every key is null.
+   */
+  private JavaPairRDD<String, String> toJavaPairRDD(HoodiePairData<String, 
String> pairData) {
+    if (pairData instanceof HoodieJavaPairRDD) {
+      return HoodieJavaPairRDD.getJavaPairRDD(pairData);
+    }
+    List<Tuple2<String, String>> pairs =
+        pairData.collectAsList().stream()
+            .map(pair -> new Tuple2<>(pair.getKey(), pair.getValue()))
+            .collect(Collectors.toList());
+    return javaSparkContext.parallelizePairs(pairs);
+  }
+
+  private static Row toLookupResult(String dataPath, String secondaryKey, 
String recordKey) {
+    // record keys generated by Hudi for files without record keys are 
"<relative path>_<row
+    // position>"
+    int positionSeparator = recordKey.lastIndexOf('_');
+    if (positionSeparator == -1) {
+      throw new IllegalArgumentException(
+          "Expected a record key of the form <file path>_<row position> but 
got " + recordKey);
+    }
+    String relativeFilePath = recordKey.substring(0, positionSeparator);
+    return RowFactory.create(
+        secondaryKey,
+        dataPath.endsWith("/") ? dataPath + relativeFilePath : dataPath + "/" 
+ relativeFilePath,

Review Comment:
   `dataPath.endsWith("/") ? dataPath + relativeFilePath : dataPath + "/" + 
relativeFilePath`
   Isn't there an existing util function for this? 



##########
xtable-core/src/main/java/org/apache/xtable/index/Index.java:
##########
@@ -0,0 +1,66 @@
+/*
+ * 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.xtable.index;
+
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Row;
+
+/**
+ * A secondary index over a table in another format, backed by XTable's Hudi 
conversion. It maps the
+ * values of an indexed column to the data file and row position that holds 
each value, so an engine
+ * can locate the rows to merge or delete without a full table join.
+ *
+ * @param <T> The table type of the source format (for example an Iceberg 
{@code Table})
+ */
+public interface Index<T> {
+  /** Result column with the full path of the data file that holds the row. */
+  String FILE_COLUMN = "_file";
+
+  /** Result column with the zero based position of the row within the file. */
+  String POSITION_COLUMN = "_pos";
+
+  /**
+   * Checks whether a secondary index exists for the given column.
+   *
+   * @param columnName The indexed column
+   * @return true when a build of the index has completed
+   */
+  boolean doesIndexExist(String columnName);
+
+  /**
+   * Brings the secondary indexes of the configured columns up to date with 
the current state of the
+   * table. The indexed columns are part of the index configuration, not of 
the sync.
+   *
+   * @param table The table to index
+   */
+  void syncIndex(T table);
+
+  /**
+   * Looks up the given keys in the secondary index of a column.
+   *
+   * @param table The table the index belongs to
+   * @param keys The values to look up, in the column named {@code columnName}
+   * @param columnName The indexed column
+   * @return one row for every row of the table that holds a key, with the key 
in the column named

Review Comment:
   Make the java doc not coupled to implementation. 



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));

Review Comment:
   This should be a function of it's own instead of writing consecutive 
statements like this? 



##########
xtable-core/src/main/java/org/apache/xtable/hudi/HudiTargetConfig.java:
##########
@@ -35,22 +39,130 @@ public class HudiTargetConfig {
    */
   public static final String HUDI_TABLE_VERSION = 
"xtable.hudi.target.table_version";
 
+  /**
+   * Engine that writes the Hudi commit and the metadata table indexes. 
Supported values are {@code
+   * java} (default) and {@code spark}. The Spark engine reuses the active 
Spark session and
+   * distributes the index generation, which is recommended when the record 
index or a secondary
+   * index is enabled because those indexes read every registered data file.
+   */
+  public static final String EXECUTION_ENGINE = 
"xtable.hudi.target.execution.engine";
+
+  public static final String EXECUTION_ENGINE_JAVA = "java";
+  public static final String EXECUTION_ENGINE_SPARK = "spark";
+
+  /**
+   * Comma separated columns to build a Hudi secondary index on, one index per 
column. Setting it
+   * also enables the global record index, which the secondary index depends 
on. The indexes are
+   * stored in the Hudi metadata table under the target table path and require 
table version 9. The
+   * indexes are built when the Hudi table is created; a column added to the 
list of an existing
+   * table is not indexed yet.
+   */
+  public static final String SECONDARY_INDEX_COLUMNS = 
"xtable.hudi.target.secondary.index.columns";
+
+  /**
+   * Lower and upper bound on the number of file groups of the record index. 
Both must be set
+   * together; when unset the Hudi defaults apply.
+   */
+  public static final String RECORD_INDEX_MIN_FILEGROUP_COUNT =
+      "xtable.hudi.target.metadata.record.index.min.filegroup.count";
+
+  public static final String RECORD_INDEX_MAX_FILEGROUP_COUNT =
+      "xtable.hudi.target.metadata.record.index.max.filegroup.count";
+
+  /** Parallelism used when generating the secondary index records. */
+  public static final String SECONDARY_INDEX_PARALLELISM =
+      "xtable.hudi.target.metadata.index.secondary.parallelism";
+
   static final HoodieTableVersion DEFAULT_TABLE_VERSION = 
HoodieTableVersion.SIX;
 
   HoodieTableVersion tableVersion;
+  boolean sparkEngine;
+  List<String> secondaryIndexColumns;
+  Optional<Integer> recordIndexMinFileGroupCount;
+  Optional<Integer> recordIndexMaxFileGroupCount;
+  Optional<Integer> secondaryIndexParallelism;
 
   public static HudiTargetConfig fromProperties(Properties properties) {
+    Properties targetProperties = properties == null ? new Properties() : 
properties;
+    HoodieTableVersion tableVersion = parseTableVersion(targetProperties);
+    boolean sparkEngine = parseSparkEngine(targetProperties);
+    List<String> secondaryIndexColumns =
+        Arrays.stream(targetProperties.getProperty(SECONDARY_INDEX_COLUMNS, 
"").split(","))
+            .map(String::trim)
+            .filter(column -> !column.isEmpty())
+            .distinct()
+            .collect(Collectors.toList());
+    if (!secondaryIndexColumns.isEmpty() && tableVersion != 
HoodieTableVersion.NINE) {
+      throw new IllegalArgumentException(
+          String.format(
+              "%s requires Hudi target table version 9, set %s=9.",
+              SECONDARY_INDEX_COLUMNS, HUDI_TABLE_VERSION));
+    }
+    if (secondaryIndexColumns.size() > 1 && !sparkEngine) {
+      // Hudi builds the index of the first column in the commit and the 
others with its indexer.
+      // In Hudi 1.2.1 the Java engine's 
JavaHoodieMetadataBulkInsertPartitioner writes an index
+      // built from existing files into a single file group, so lookups miss 
most keys.
+      throw new IllegalArgumentException(
+          String.format(
+              "More than one column in %s requires %s=%s.",
+              SECONDARY_INDEX_COLUMNS, EXECUTION_ENGINE, 
EXECUTION_ENGINE_SPARK));
+    }
+    Optional<Integer> recordIndexMinFileGroupCount =
+        parsePositiveInt(targetProperties, RECORD_INDEX_MIN_FILEGROUP_COUNT);
+    Optional<Integer> recordIndexMaxFileGroupCount =
+        parsePositiveInt(targetProperties, RECORD_INDEX_MAX_FILEGROUP_COUNT);
+    if (recordIndexMinFileGroupCount.isPresent() != 
recordIndexMaxFileGroupCount.isPresent()) {
+      throw new IllegalArgumentException(
+          String.format(
+              "%s and %s must be set together.",
+              RECORD_INDEX_MIN_FILEGROUP_COUNT, 
RECORD_INDEX_MAX_FILEGROUP_COUNT));
+    }
+    return new HudiTargetConfig(
+        tableVersion,
+        sparkEngine,
+        secondaryIndexColumns,
+        recordIndexMinFileGroupCount,
+        recordIndexMaxFileGroupCount,
+        parsePositiveInt(targetProperties, SECONDARY_INDEX_PARALLELISM));
+  }
+
+  private static HoodieTableVersion parseTableVersion(Properties properties) {
     HoodieTableVersion tableVersion = DEFAULT_TABLE_VERSION;
-    if (properties != null) {
-      String configured = properties.getProperty(HUDI_TABLE_VERSION);
-      if (configured != null && !configured.trim().isEmpty()) {
-        tableVersion = parseTableVersion(configured.trim());
-      }
+    String configured = properties.getProperty(HUDI_TABLE_VERSION);
+    if (configured != null && !configured.trim().isEmpty()) {
+      tableVersion = parseTableVersion(configured.trim());
     }
     if (tableVersion != HoodieTableVersion.SIX && tableVersion != 
HoodieTableVersion.NINE) {
       throw 
unsupportedTableVersion(String.valueOf(tableVersion.versionCode()), null);
     }
-    return new HudiTargetConfig(tableVersion);
+    return tableVersion;
+  }
+
+  private static boolean parseSparkEngine(Properties properties) {
+    String engine = properties.getProperty(EXECUTION_ENGINE, 
EXECUTION_ENGINE_JAVA).trim();

Review Comment:
   Would defining an enum simplify this function here? 



##########
xtable-core/src/main/java/org/apache/xtable/index/Index.java:
##########
@@ -0,0 +1,66 @@
+/*
+ * 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.xtable.index;
+
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Row;
+
+/**
+ * A secondary index over a table in another format, backed by XTable's Hudi 
conversion. It maps the

Review Comment:
   Remove secondary from the java docs...it's an implementation detail of the 
index backed by hudi not something that needs to be leaked into the interface.



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(

Review Comment:
   There should be utility function to get formatted error message from 
`SyncResult`?  



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =

Review Comment:
   The iceberg data path can be a completely different location hope that's 
handled and we are not simply appending _data? 



##########
xtable-core/src/main/java/org/apache/xtable/hudi/HudiConversionTarget.java:
##########
@@ -482,6 +501,36 @@ private Optional<TableSyncMetadata> getMetadata(
     }
   }
 
+  /**
+   * Registers a definition for every secondary index partition that has none. 
Hudi updates every
+   * secondary index on each commit through its definition, but the commit 
that creates the metadata
+   * table rewrites the column-stats index definition from a stale view of the 
definitions, which
+   * drops the secondary index definition registered in the same commit.
+   */
+  static void restoreSecondaryIndexDefinitions(HoodieTableMetaClient 
metaClient) {

Review Comment:
   All these new private methods for writing and building secondary index have 
bloated HudiConversionTarget, any way we can refactor this to define better 
abstractions? 



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(
+          "Failed to sync the secondary indexes of columns "
+              + indexedColumns
+              + " of table "
+              + tableLocation
+              + ": "
+              + syncResult.getTableFormatSyncStatus());
+    }
+    Set<String> completedPartitions = getCompletedIndexPartitions();
+    List<String> notIndexed =
+        indexedColumns.stream()
+            .filter(
+                column ->
+                    
!completedPartitions.contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + column))
+            .collect(Collectors.toList());
+    if (!notIndexed.isEmpty()) {
+      throw new IllegalStateException(
+          "The secondary indexes of columns "
+              + notIndexed
+              + " of table "
+              + tableLocation
+              + " do not exist. The indexes are built when the index is 
created, and adding a"
+              + " column to an existing index is not supported yet.");
+    }
+    log.info(
+        "Synced the secondary indexes of columns {} of table {}", 
indexedColumns, tableLocation);
+  }
+
+  /**
+   * Returns the metadata table partitions whose build has completed. A 
partition directory can
+   * exist while the build is in flight or after it failed, so the directory 
alone does not show
+   * that the index is usable.
+   */
+  private Set<String> getCompletedIndexPartitions() {

Review Comment:
   Rename to `getIndexedColumns()`? The name of the function shouldn't 
necessarily reveal internal implementation details?  



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(
+          "Failed to sync the secondary indexes of columns "
+              + indexedColumns
+              + " of table "
+              + tableLocation
+              + ": "
+              + syncResult.getTableFormatSyncStatus());
+    }
+    Set<String> completedPartitions = getCompletedIndexPartitions();
+    List<String> notIndexed =
+        indexedColumns.stream()
+            .filter(
+                column ->
+                    
!completedPartitions.contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + column))
+            .collect(Collectors.toList());
+    if (!notIndexed.isEmpty()) {
+      throw new IllegalStateException(
+          "The secondary indexes of columns "
+              + notIndexed
+              + " of table "
+              + tableLocation
+              + " do not exist. The indexes are built when the index is 
created, and adding a"
+              + " column to an existing index is not supported yet.");
+    }
+    log.info(
+        "Synced the secondary indexes of columns {} of table {}", 
indexedColumns, tableLocation);
+  }
+
+  /**
+   * Returns the metadata table partitions whose build has completed. A 
partition directory can
+   * exist while the build is in flight or after it failed, so the directory 
alone does not show
+   * that the index is usable.
+   */
+  private Set<String> getCompletedIndexPartitions() {
+    try {
+      return getMetaClient().getTableConfig().getMetadataPartitions();
+    } catch (TableNotFoundException e) {
+      return Collections.emptySet();
+    }
+  }
+
+  private HoodieTableMetaClient getMetaClient() {
+    HoodieStorage storage =
+        new HoodieHadoopStorage(dataPath, 
javaSparkContext.hadoopConfiguration());
+    return 
HoodieTableMetaClient.builder().setStorage(storage).setBasePath(dataPath).build();
+  }
+
+  private ConversionConfig getConversionConfig(Table icebergTable) {
+    TargetTable targetTable =
+        TargetTable.builder()
+            .name(icebergTable.name())
+            .basePath(dataPath)
+            .formatName(TableFormat.HUDI)
+            .additionalProperties(targetTableProperties)
+            .build();
+    return ConversionConfig.builder()
+        .sourceTable(sourceTable)
+        .targetTables(Collections.singletonList(targetTable))
+        .build();
+  }
+
+  @Override
+  public Dataset<Row> lookup(Table icebergTable, Dataset<Row> keys, String 
columnName) {
+    Preconditions.checkArgument(
+        indexedColumns.contains(columnName),
+        "Column %s is not one of the indexed columns %s",
+        columnName,
+        indexedColumns);
+    StructType resultSchema =
+        new StructType()
+            .add(columnName, DataTypes.StringType, false)
+            .add(FILE_COLUMN, DataTypes.StringType, false)
+            .add(POSITION_COLUMN, DataTypes.LongType, false);
+    // the secondary index stores the values of the indexed column as strings
+    JavaRDD<String> secondaryKeys =
+        keys.where(col(columnName).isNotNull())
+            .select(col(columnName).cast(DataTypes.StringType))
+            .as(Encoders.STRING())
+            .toJavaRDD();
+    HoodieTableMetaClient metaClient = getMetaClient();
+    HoodieStorage storage = metaClient.getStorage();
+    if (!metaClient
+        .getTableConfig()
+        .getMetadataPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName)) {
+      throw new IllegalStateException(
+          "The secondary index of column "
+              + columnName
+              + " of table "
+              + tableLocation
+              + " is not built");
+    }
+    HoodieMetadataConfig metadataConfig =
+        
HoodieMetadataConfig.newBuilder().enable(true).withSecondaryIndexEnabled(true).build();
+    try (HoodieBackedTableMetadata tableMetadata =
+        (HoodieBackedTableMetadata)
+            metaClient
+                .getTableFormat()
+                .getMetadataFactory()
+                .create(engineContext, storage, metadataConfig, dataPath)) {
+      HoodiePairData<String, String> recordKeysBySecondaryKey =
+          tableMetadata.readSecondaryIndexDataTableRecordKeysWithKeys(
+              HoodieJavaRDD.of(secondaryKeys), 
PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+      // capture the field so the closure does not serialize the index instance
+      String dataPath = this.dataPath;
+      JavaRDD<Row> lookupResults =
+          toJavaPairRDD(recordKeysBySecondaryKey)
+              .map(
+                  recordKeyBySecondaryKey ->
+                      toLookupResult(
+                          dataPath, recordKeyBySecondaryKey._1(), 
recordKeyBySecondaryKey._2()));
+      return sparkSession.createDataFrame(lookupResults, resultSchema);
+    }
+  }
+
+  /**
+   * Hudi returns an RDD backed result for keys in an RDD, but a list backed 
result when there are
+   * no keys to look up, for example when every key is null.
+   */
+  private JavaPairRDD<String, String> toJavaPairRDD(HoodiePairData<String, 
String> pairData) {
+    if (pairData instanceof HoodieJavaPairRDD) {
+      return HoodieJavaPairRDD.getJavaPairRDD(pairData);
+    }
+    List<Tuple2<String, String>> pairs =
+        pairData.collectAsList().stream()
+            .map(pair -> new Tuple2<>(pair.getKey(), pair.getValue()))
+            .collect(Collectors.toList());

Review Comment:
   Doesn't this collect everything to the driver and cause OOM? Why do we 
handle this special method of  toJavaPairRDD?  



##########
xtable-core/src/main/java/org/apache/xtable/hudi/HudiTargetConfig.java:
##########
@@ -35,22 +39,130 @@ public class HudiTargetConfig {
    */
   public static final String HUDI_TABLE_VERSION = 
"xtable.hudi.target.table_version";
 
+  /**
+   * Engine that writes the Hudi commit and the metadata table indexes. 
Supported values are {@code
+   * java} (default) and {@code spark}. The Spark engine reuses the active 
Spark session and
+   * distributes the index generation, which is recommended when the record 
index or a secondary
+   * index is enabled because those indexes read every registered data file.
+   */
+  public static final String EXECUTION_ENGINE = 
"xtable.hudi.target.execution.engine";
+
+  public static final String EXECUTION_ENGINE_JAVA = "java";
+  public static final String EXECUTION_ENGINE_SPARK = "spark";

Review Comment:
   An enum would be helpful here? 



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(
+          "Failed to sync the secondary indexes of columns "
+              + indexedColumns
+              + " of table "
+              + tableLocation
+              + ": "
+              + syncResult.getTableFormatSyncStatus());
+    }
+    Set<String> completedPartitions = getCompletedIndexPartitions();
+    List<String> notIndexed =
+        indexedColumns.stream()
+            .filter(
+                column ->
+                    
!completedPartitions.contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + column))
+            .collect(Collectors.toList());
+    if (!notIndexed.isEmpty()) {
+      throw new IllegalStateException(
+          "The secondary indexes of columns "
+              + notIndexed
+              + " of table "
+              + tableLocation
+              + " do not exist. The indexes are built when the index is 
created, and adding a"
+              + " column to an existing index is not supported yet.");
+    }
+    log.info(
+        "Synced the secondary indexes of columns {} of table {}", 
indexedColumns, tableLocation);
+  }
+
+  /**
+   * Returns the metadata table partitions whose build has completed. A 
partition directory can
+   * exist while the build is in flight or after it failed, so the directory 
alone does not show
+   * that the index is usable.
+   */
+  private Set<String> getCompletedIndexPartitions() {
+    try {
+      return getMetaClient().getTableConfig().getMetadataPartitions();
+    } catch (TableNotFoundException e) {
+      return Collections.emptySet();
+    }
+  }
+
+  private HoodieTableMetaClient getMetaClient() {
+    HoodieStorage storage =
+        new HoodieHadoopStorage(dataPath, 
javaSparkContext.hadoopConfiguration());
+    return 
HoodieTableMetaClient.builder().setStorage(storage).setBasePath(dataPath).build();
+  }
+
+  private ConversionConfig getConversionConfig(Table icebergTable) {
+    TargetTable targetTable =
+        TargetTable.builder()
+            .name(icebergTable.name())
+            .basePath(dataPath)
+            .formatName(TableFormat.HUDI)
+            .additionalProperties(targetTableProperties)
+            .build();
+    return ConversionConfig.builder()
+        .sourceTable(sourceTable)
+        .targetTables(Collections.singletonList(targetTable))
+        .build();
+  }
+
+  @Override
+  public Dataset<Row> lookup(Table icebergTable, Dataset<Row> keys, String 
columnName) {
+    Preconditions.checkArgument(
+        indexedColumns.contains(columnName),
+        "Column %s is not one of the indexed columns %s",
+        columnName,
+        indexedColumns);
+    StructType resultSchema =

Review Comment:
   Can resultSchema be defined in the end? 



##########
xtable-core/src/main/java/org/apache/xtable/hudi/HudiConversionTarget.java:
##########
@@ -482,6 +501,36 @@ private Optional<TableSyncMetadata> getMetadata(
     }
   }
 
+  /**
+   * Registers a definition for every secondary index partition that has none. 
Hudi updates every
+   * secondary index on each commit through its definition, but the commit 
that creates the metadata
+   * table rewrites the column-stats index definition from a stale view of the 
definitions, which
+   * drops the secondary index definition registered in the same commit.
+   */
+  static void restoreSecondaryIndexDefinitions(HoodieTableMetaClient 
metaClient) {

Review Comment:
   Is this required to bootstrap indexes for tables where new secondary index 
columns have been added? 



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(
+          "Failed to sync the secondary indexes of columns "
+              + indexedColumns
+              + " of table "
+              + tableLocation
+              + ": "
+              + syncResult.getTableFormatSyncStatus());
+    }
+    Set<String> completedPartitions = getCompletedIndexPartitions();
+    List<String> notIndexed =
+        indexedColumns.stream()
+            .filter(
+                column ->
+                    
!completedPartitions.contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + column))
+            .collect(Collectors.toList());
+    if (!notIndexed.isEmpty()) {
+      throw new IllegalStateException(
+          "The secondary indexes of columns "
+              + notIndexed
+              + " of table "
+              + tableLocation
+              + " do not exist. The indexes are built when the index is 
created, and adding a"
+              + " column to an existing index is not supported yet.");
+    }
+    log.info(
+        "Synced the secondary indexes of columns {} of table {}", 
indexedColumns, tableLocation);
+  }
+
+  /**
+   * Returns the metadata table partitions whose build has completed. A 
partition directory can
+   * exist while the build is in flight or after it failed, so the directory 
alone does not show
+   * that the index is usable.
+   */
+  private Set<String> getCompletedIndexPartitions() {
+    try {
+      return getMetaClient().getTableConfig().getMetadataPartitions();
+    } catch (TableNotFoundException e) {
+      return Collections.emptySet();
+    }
+  }
+
+  private HoodieTableMetaClient getMetaClient() {
+    HoodieStorage storage =
+        new HoodieHadoopStorage(dataPath, 
javaSparkContext.hadoopConfiguration());
+    return 
HoodieTableMetaClient.builder().setStorage(storage).setBasePath(dataPath).build();
+  }
+
+  private ConversionConfig getConversionConfig(Table icebergTable) {
+    TargetTable targetTable =
+        TargetTable.builder()
+            .name(icebergTable.name())
+            .basePath(dataPath)
+            .formatName(TableFormat.HUDI)
+            .additionalProperties(targetTableProperties)
+            .build();
+    return ConversionConfig.builder()
+        .sourceTable(sourceTable)
+        .targetTables(Collections.singletonList(targetTable))
+        .build();
+  }
+
+  @Override
+  public Dataset<Row> lookup(Table icebergTable, Dataset<Row> keys, String 
columnName) {
+    Preconditions.checkArgument(
+        indexedColumns.contains(columnName),
+        "Column %s is not one of the indexed columns %s",
+        columnName,
+        indexedColumns);
+    StructType resultSchema =
+        new StructType()
+            .add(columnName, DataTypes.StringType, false)
+            .add(FILE_COLUMN, DataTypes.StringType, false)
+            .add(POSITION_COLUMN, DataTypes.LongType, false);
+    // the secondary index stores the values of the indexed column as strings
+    JavaRDD<String> secondaryKeys =
+        keys.where(col(columnName).isNotNull())
+            .select(col(columnName).cast(DataTypes.StringType))
+            .as(Encoders.STRING())
+            .toJavaRDD();
+    HoodieTableMetaClient metaClient = getMetaClient();
+    HoodieStorage storage = metaClient.getStorage();
+    if (!metaClient
+        .getTableConfig()
+        .getMetadataPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName)) {
+      throw new IllegalStateException(
+          "The secondary index of column "
+              + columnName
+              + " of table "
+              + tableLocation
+              + " is not built");
+    }
+    HoodieMetadataConfig metadataConfig =

Review Comment:
   Is there any way we can get this from metaClient itself instead of assuming 
and initializing like this? 



##########
xtable-core/src/main/java/org/apache/xtable/iceberg/IcebergPartitionValueConverter.java:
##########
@@ -181,6 +182,28 @@ private InternalPartitionField 
getFromInternalPartitionFieldMap(
                 Collectors.toMap(InternalPartitionField::getTransformType, 
Function.identity())));
   }
 
+  /**
+   * Resolves the position of a partition field in the partition struct by 
name. The struct is

Review Comment:
   Can this be simplified to iceberg' spark reader returns PartitionData 
instead of IndexedRecord? Is there an existing iceberg function that can be 
used here instead? 



##########
xtable-core/src/main/java/org/apache/xtable/index/HudiBackedIcebergSecondaryIndex.java:
##########
@@ -0,0 +1,299 @@
+/*
+ * 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.xtable.index;
+
+import static 
org.apache.hudi.metadata.HoodieTableMetadataUtil.PARTITION_NAME_SECONDARY_INDEX_PREFIX;
+import static org.apache.spark.sql.functions.col;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.Properties;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import lombok.extern.log4j.Log4j2;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.spark.api.java.JavaPairRDD;
+import org.apache.spark.api.java.JavaRDD;
+import org.apache.spark.api.java.JavaSparkContext;
+import org.apache.spark.sql.Dataset;
+import org.apache.spark.sql.Encoders;
+import org.apache.spark.sql.Row;
+import org.apache.spark.sql.RowFactory;
+import org.apache.spark.sql.SparkSession;
+import org.apache.spark.sql.types.DataTypes;
+import org.apache.spark.sql.types.StructType;
+
+import org.apache.hudi.client.common.HoodieSparkEngineContext;
+import org.apache.hudi.common.config.HoodieMetadataConfig;
+import org.apache.hudi.common.data.HoodiePairData;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.HoodieTableVersion;
+import org.apache.hudi.data.HoodieJavaPairRDD;
+import org.apache.hudi.data.HoodieJavaRDD;
+import org.apache.hudi.exception.TableNotFoundException;
+import org.apache.hudi.metadata.HoodieBackedTableMetadata;
+import org.apache.hudi.storage.HoodieStorage;
+import org.apache.hudi.storage.hadoop.HoodieHadoopStorage;
+
+import org.apache.iceberg.Table;
+
+import scala.Tuple2;
+
+import com.google.common.base.Preconditions;
+
+import org.apache.xtable.catalog.TableFormatUtils;
+import org.apache.xtable.conversion.ConversionConfig;
+import org.apache.xtable.conversion.ConversionController;
+import org.apache.xtable.conversion.SourceTable;
+import org.apache.xtable.conversion.TargetTable;
+import org.apache.xtable.hudi.HudiTargetConfig;
+import org.apache.xtable.iceberg.IcebergConversionSourceProvider;
+import org.apache.xtable.model.storage.TableFormat;
+import org.apache.xtable.model.sync.SyncResult;
+import org.apache.xtable.model.sync.SyncStatusCode;
+
+/**
+ * Secondary index for an Iceberg table backed by the Hudi metadata table. The 
index is built by
+ * syncing the Iceberg table to Hudi with the record index and a secondary 
index enabled; the Hudi
+ * metadata lives under {@code <data location>/.hoodie/metadata}. Because the 
Iceberg data files
+ * carry no Hudi record keys, the record key of a row is {@code <file path 
relative to the data
+ * location>_<row position>}, which a lookup resolves back to a file path and 
row position.
+ */
+@Log4j2
+public class HudiBackedIcebergSecondaryIndex implements Index<Table> {
+  private final String tableLocation;
+  private final String dataPath;
+  private final SourceTable sourceTable;
+  private final Properties targetTableProperties;
+  private final List<String> indexedColumns;
+  private final SparkSession sparkSession;
+  private final JavaSparkContext javaSparkContext;
+  private final HoodieSparkEngineContext engineContext;
+
+  /**
+   * @param icebergTable the table to index
+   * @param sourceTable how XTable loads the table for a sync, because a 
loaded table does not
+   *     expose the catalog it came from. A table of a catalog sets its 
catalog config, name and
+   *     namespace, and a table without a catalog config is loaded from its 
location
+   * @param sparkSession the Spark session used to build and query the index
+   * @param targetTableProperties {@link HudiTargetConfig} properties for the 
sync. {@link
+   *     HudiTargetConfig#SECONDARY_INDEX_COLUMNS} lists the columns to index, 
and other properties
+   *     such as the record index file group counts are optional
+   */
+  public HudiBackedIcebergSecondaryIndex(
+      Table icebergTable,
+      SourceTable sourceTable,
+      SparkSession sparkSession,
+      Properties targetTableProperties) {
+    Preconditions.checkArgument(
+        TableFormat.ICEBERG.equals(sourceTable.getFormatName()),
+        "Expected an Iceberg source table but got format %s",
+        sourceTable.getFormatName());
+    this.tableLocation = icebergTable.location();
+    this.dataPath =
+        TableFormatUtils.getTableDataLocation(
+            TableFormat.ICEBERG, tableLocation, icebergTable.properties());
+    // the Hudi table lives under the data location, which Iceberg resolves 
from the table
+    // properties
+    this.sourceTable = sourceTable.toBuilder().dataPath(dataPath).build();
+    this.targetTableProperties = new Properties();
+    this.targetTableProperties.putAll(targetTableProperties);
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.EXECUTION_ENGINE, 
HudiTargetConfig.EXECUTION_ENGINE_SPARK);
+    // the secondary index is only available from Hudi table version 8 onwards
+    this.targetTableProperties.setProperty(
+        HudiTargetConfig.HUDI_TABLE_VERSION, 
String.valueOf(HoodieTableVersion.NINE.versionCode()));
+    this.indexedColumns =
+        
HudiTargetConfig.fromProperties(this.targetTableProperties).getSecondaryIndexColumns();
+    Preconditions.checkArgument(
+        !indexedColumns.isEmpty(),
+        "Set %s to the columns to index",
+        HudiTargetConfig.SECONDARY_INDEX_COLUMNS);
+    this.sparkSession = sparkSession;
+    this.javaSparkContext = 
JavaSparkContext.fromSparkContext(sparkSession.sparkContext());
+    this.engineContext = new HoodieSparkEngineContext(javaSparkContext);
+  }
+
+  @Override
+  public boolean doesIndexExist(String columnName) {
+    return getCompletedIndexPartitions()
+        .contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + columnName);
+  }
+
+  @Override
+  public void syncIndex(Table icebergTable) {
+    Configuration configuration = javaSparkContext.hadoopConfiguration();
+    IcebergConversionSourceProvider sourceProvider = new 
IcebergConversionSourceProvider();
+    sourceProvider.init(configuration);
+    // The sync reports a failed target in its result instead of throwing. It 
returns no result
+    // for the target when the table has no new commits.
+    SyncResult syncResult =
+        new ConversionController(configuration)
+            .sync(getConversionConfig(icebergTable), sourceProvider)
+            .get(TableFormat.HUDI);
+    if (syncResult != null
+        && (syncResult.getTableFormatSyncStatus() == null
+            || syncResult.getTableFormatSyncStatus().getStatusCode() != 
SyncStatusCode.SUCCESS)) {
+      throw new IllegalStateException(
+          "Failed to sync the secondary indexes of columns "
+              + indexedColumns
+              + " of table "
+              + tableLocation
+              + ": "
+              + syncResult.getTableFormatSyncStatus());
+    }
+    Set<String> completedPartitions = getCompletedIndexPartitions();
+    List<String> notIndexed =
+        indexedColumns.stream()
+            .filter(
+                column ->
+                    
!completedPartitions.contains(PARTITION_NAME_SECONDARY_INDEX_PREFIX + column))
+            .collect(Collectors.toList());
+    if (!notIndexed.isEmpty()) {
+      throw new IllegalStateException(
+          "The secondary indexes of columns "
+              + notIndexed
+              + " of table "
+              + tableLocation
+              + " do not exist. The indexes are built when the index is 
created, and adding a"
+              + " column to an existing index is not supported yet.");
+    }

Review Comment:
   Isn't secondary index commit part of the hudi transaction and this kind of 
check is not required? 



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