vinothchandar commented on a change in pull request #2342:
URL: https://github.com/apache/hudi/pull/2342#discussion_r547154542



##########
File path: 
hudi-common/src/main/java/org/apache/hudi/metadata/AbstractHoodieTableMetadata.java
##########
@@ -0,0 +1,296 @@
+/*
+ * 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.hudi.metadata;
+
+import org.apache.avro.Schema;
+import org.apache.hadoop.fs.FileSystem;
+import org.apache.hudi.avro.HoodieAvroUtils;
+import org.apache.hudi.avro.model.HoodieMetadataRecord;
+import org.apache.hudi.common.config.SerializableConfiguration;
+import org.apache.hudi.common.fs.FSUtils;
+import org.apache.hudi.common.metrics.Registry;
+import org.apache.hudi.common.model.HoodiePartitionMetadata;
+import org.apache.hudi.common.model.HoodieRecord;
+import org.apache.hudi.common.model.HoodieRecordPayload;
+import org.apache.hudi.common.table.HoodieTableMetaClient;
+import org.apache.hudi.common.table.timeline.HoodieInstant;
+import org.apache.hudi.common.util.HoodieTimer;
+import org.apache.hudi.common.util.Option;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.fs.FileStatus;
+import org.apache.hadoop.fs.Path;
+import org.apache.hudi.exception.HoodieMetadataException;
+import org.apache.log4j.LogManager;
+import org.apache.log4j.Logger;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * Interface that supports querying various pieces of metadata about a hudi 
table.
+ */
+public abstract class AbstractHoodieTableMetadata implements 
HoodieTableMetadata {
+
+  private static final Logger LOG = 
LogManager.getLogger(AbstractHoodieTableMetadata.class);
+
+  static final long MAX_MEMORY_SIZE_IN_BYTES = 1024 * 1024 * 1024;
+  static final int BUFFER_SIZE = 10 * 1024 * 1024;
+
+  protected final SerializableConfiguration hadoopConf;
+  protected final Option<HoodieMetadataMetrics> metrics;
+  protected final String datasetBasePath;
+
+  // Directory used for Spillable Map when merging records
+  final String spillableMapDirectory;
+
+  protected boolean enabled;
+  private final boolean validateLookups;
+  private final boolean assumeDatePartitioning;
+
+  private transient HoodieMetadataMergedInstantRecordScanner 
timelineRecordScanner;
+
+  protected AbstractHoodieTableMetadata(Configuration hadoopConf, String 
datasetBasePath, String spillableMapDirectory,
+                                        boolean enabled, boolean 
validateLookups, boolean enableMetrics,
+                                        boolean assumeDatePartitioning) {
+    this.hadoopConf = new SerializableConfiguration(hadoopConf);
+    this.datasetBasePath = datasetBasePath;
+    this.spillableMapDirectory = spillableMapDirectory;
+
+    this.enabled = enabled;
+    this.validateLookups = validateLookups;
+    this.assumeDatePartitioning = assumeDatePartitioning;
+
+    if (enableMetrics) {
+      this.metrics = Option.of(new 
HoodieMetadataMetrics(Registry.getRegistry("HoodieMetadata")));
+    } else {
+      this.metrics = Option.empty();
+    }
+  }
+
+  public static AbstractHoodieTableMetadata create(Configuration conf, String 
datasetBasePath, String spillableMapPath, boolean useFileListingFromMetadata,
+                                                   boolean verifyListings, 
boolean enableMetrics, boolean shouldAssumeDatePartitioning) {
+    return new HoodieBackedTableMetadata(conf, datasetBasePath, 
spillableMapPath, useFileListingFromMetadata, verifyListings,
+        enableMetrics, shouldAssumeDatePartitioning);
+  }
+
+
+  /**
+   * Return the list of files in a partition.
+   *
+   * If the Metadata Table is enabled, the listing is retrieved from the 
stored metadata. Otherwise, the list of
+   * partitions is retrieved directly from the underlying {@code FileSystem}.
+   *
+   * On any errors retrieving the listing from the metadata, defaults to using 
the file system listings.
+   *
+   * @param partitionPath The absolute path of the partition to list
+   */
+
+  public FileStatus[] getAllFilesInPartition(Path partitionPath) throws 
IOException {
+    if (enabled) {
+      try {
+        return fetchAllFilesInPartition(partitionPath);
+      } catch (Exception e) {
+        LOG.error("Failed to retrive files in partition " + partitionPath + " 
from metadata", e);
+      }
+    }
+    return FSUtils.getFs(partitionPath.toString(), 
hadoopConf.get()).listStatus(partitionPath);
+  }
+
+  /**
+   * Return the list of partitions in the dataset.
+   *
+   * If the Metadata Table is enabled, the listing is retrieved from the 
stored metadata. Otherwise, the list of
+   * partitions is retrieved directly from the underlying {@code FileSystem}.
+   *
+   * On any errors retrieving the listing from the metadata, defaults to using 
the file system listings.
+   *
+   */
+  public List<String> getAllPartitionPaths() throws IOException {
+    if (enabled) {
+      try {
+        return fetchAllPartitionPaths();
+      } catch (Exception e) {
+        LOG.error("Failed to retrieve list of partition from metadata", e);
+      }
+    }
+
+    FileSystem fs = FSUtils.getFs(datasetBasePath, hadoopConf.get());
+    return FSUtils.getAllPartitionPaths(fs, datasetBasePath, 
assumeDatePartitioning);
+  }
+
+  /**
+   * Returns a list of all partitions.
+   */
+  /**
+   * Returns a list of all partitions.
+   */
+  protected List<String> fetchAllPartitionPaths() throws IOException {
+    HoodieTimer timer = new HoodieTimer().startTimer();
+    Option<HoodieRecord<HoodieMetadataPayload>> hoodieRecord = 
getMergedRecordByKey(RECORDKEY_PARTITION_LIST);
+    metrics.ifPresent(m -> 
m.updateMetrics(HoodieMetadataMetrics.LOOKUP_PARTITIONS_STR, timer.endTimer()));
+
+    List<String> partitions = Collections.emptyList();
+    if (hoodieRecord.isPresent()) {
+      if (!hoodieRecord.get().getData().getDeletions().isEmpty()) {
+        throw new HoodieMetadataException("Metadata partition list record is 
inconsistent: "
+                + hoodieRecord.get().getData());
+      }
+
+      partitions = hoodieRecord.get().getData().getFilenames();
+      // Partition-less tables have a single empty partition
+      if (partitions.contains(NON_PARTITIONED_NAME)) {
+        partitions.remove(NON_PARTITIONED_NAME);
+        partitions.add("");
+      }
+    }
+
+    if (validateLookups) {
+      // Validate the Metadata Table data by listing the partitions from the 
file system
+      timer.startTimer();
+      HoodieTableMetaClient metaClient = new 
HoodieTableMetaClient(hadoopConf.get(), datasetBasePath);
+      List<String> actualPartitions  = 
FSUtils.getAllPartitionPaths(metaClient.getFs(), datasetBasePath, false);
+      metrics.ifPresent(m -> 
m.updateMetrics(HoodieMetadataMetrics.VALIDATE_PARTITIONS_STR, 
timer.endTimer()));
+
+      Collections.sort(actualPartitions);
+      Collections.sort(partitions);
+      if (!actualPartitions.equals(partitions)) {
+        LOG.error("Validation of metadata partition list failed. Lists do not 
match.");
+        LOG.error("Partitions from metadata: " + 
Arrays.toString(partitions.toArray()));
+        LOG.error("Partitions from file system: " + 
Arrays.toString(actualPartitions.toArray()));
+
+        metrics.ifPresent(m -> 
m.updateMetrics(HoodieMetadataMetrics.VALIDATE_ERRORS_STR, 0));
+      }
+
+      // Return the direct listing as it should be correct
+      partitions = actualPartitions;
+    }
+
+    LOG.info("Listed partitions from metadata: #partitions=" + 
partitions.size());
+    return partitions;
+  }
+
+  /**
+   * Return all the files from the partition.
+   *
+   * @param partitionPath The absolute path of the partition
+   */
+  private FileStatus[] fetchAllFilesInPartition(Path partitionPath) throws 
IOException {
+    String partitionName = FSUtils.getRelativePartitionPath(new 
Path(datasetBasePath), partitionPath);
+    if (partitionName.isEmpty()) {
+      partitionName = NON_PARTITIONED_NAME;
+    }
+
+    HoodieTimer timer = new HoodieTimer().startTimer();
+    Option<HoodieRecord<HoodieMetadataPayload>> hoodieRecord = 
getMergedRecordByKey(partitionName);
+    metrics.ifPresent(m -> 
m.updateMetrics(HoodieMetadataMetrics.LOOKUP_FILES_STR, timer.endTimer()));
+
+    FileStatus[] statuses = {};
+    if (hoodieRecord.isPresent()) {
+      statuses = hoodieRecord.get().getData().getFileStatuses(partitionPath);
+    }
+
+    if (validateLookups) {
+      // Validate the Metadata Table data by listing the partitions from the 
file system
+      timer.startTimer();
+
+      // Ignore partition metadata file
+      HoodieTableMetaClient metaClient = new 
HoodieTableMetaClient(hadoopConf.get(), datasetBasePath);
+      FileStatus[] directStatuses = 
metaClient.getFs().listStatus(partitionPath,
+          p -> 
!p.getName().equals(HoodiePartitionMetadata.HOODIE_PARTITION_METAFILE));
+      metrics.ifPresent(m -> 
m.updateMetrics(HoodieMetadataMetrics.VALIDATE_FILES_STR, timer.endTimer()));
+
+      List<String> directFilenames = Arrays.stream(directStatuses)
+              .map(s -> s.getPath().getName()).sorted()
+              .collect(Collectors.toList());
+
+      List<String> metadataFilenames = Arrays.stream(statuses)
+              .map(s -> s.getPath().getName()).sorted()
+              .collect(Collectors.toList());
+
+      if (!metadataFilenames.equals(directFilenames)) {
+        LOG.error("Validation of metadata file listing for partition " + 
partitionName + " failed.");
+        LOG.error("File list from metadata: " + 
Arrays.toString(metadataFilenames.toArray()));
+        LOG.error("File list from direct listing: " + 
Arrays.toString(directFilenames.toArray()));
+
+        metrics.ifPresent(m -> 
m.updateMetrics(HoodieMetadataMetrics.VALIDATE_ERRORS_STR, 0));
+      }
+
+      // Return the direct listing as it should be correct
+      statuses = directStatuses;
+    }
+
+    LOG.info("Listed file in partition from metadata: partition=" + 
partitionName + ", #files=" + statuses.length);
+    return statuses;
+  }
+
+  /**
+   * Retrieve the merged {@code HoodieRecord} mapped to the given key.
+   *
+   * @param key The key of the record
+   */
+  private Option<HoodieRecord<HoodieMetadataPayload>> 
getMergedRecordByKey(String key) throws IOException {
+    openTimelineScanner();
+
+    Option<HoodieRecord<HoodieMetadataPayload>> metadataHoodieRecord = 
getRecordByKeyFromMetadata(key);
+
+    // Retrieve record from unsynced timeline instants
+    Option<HoodieRecord<HoodieMetadataPayload>> timelineHoodieRecord = 
timelineRecordScanner.getRecordByKey(key);
+    if (timelineHoodieRecord.isPresent()) {
+      if (metadataHoodieRecord.isPresent()) {
+        HoodieRecordPayload mergedPayload = 
timelineHoodieRecord.get().getData().preCombine(metadataHoodieRecord.get().getData());
+        metadataHoodieRecord = Option.of(new 
HoodieRecord(metadataHoodieRecord.get().getKey(), mergedPayload));
+      } else {
+        metadataHoodieRecord = timelineHoodieRecord;

Review comment:
       Sorry, my core concern was that we are reassigning to a variable that 
holds the returned value from a key method call `getRecordByKeyFromMetadata`
   
   I was suggesting something like. 
   
   ```
       Option<HoodieRecord<HoodieMetadataPayload>> metadataHoodieRecord = 
getRecordByKeyFromMetadata(key);
       // Retrieve record from unsynced timeline instants
       Option<HoodieRecord<HoodieMetadataPayload>> timelineHoodieRecord = 
timelineRecordScanner.getRecordByKey(key);
       if (timelineHoodieRecord.isPresent()) {
         if (metadataHoodieRecord.isPresent()) {
           HoodieRecordPayload mergedPayload = 
timelineHoodieRecord.get().getData().preCombine(metadataHoodieRecord.get().getData());
           return Option.of(new 
HoodieRecord(metadataHoodieRecord.get().getKey(), mergedPayload));
         } else {
           return timelineHoodieRecord;
         }
       }
   
       return metadataHoodieRecord;
   ```
   
   if you feel strongly, we can keep as-is. 
   
   
   




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