ajantha-bhat commented on a change in pull request #3770:
URL: https://github.com/apache/carbondata/pull/3770#discussion_r435945737



##########
File path: 
sdk/sdk/src/main/java/org/apache/carbondata/sdk/file/PaginationCarbonReader.java
##########
@@ -0,0 +1,303 @@
+/*
+ * 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.carbondata.sdk.file;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.apache.carbondata.common.annotations.InterfaceAudience;
+import org.apache.carbondata.common.annotations.InterfaceStability;
+import org.apache.carbondata.core.cache.CarbonLRUCache;
+import org.apache.carbondata.core.constants.CarbonCommonConstants;
+import org.apache.carbondata.core.indexstore.BlockletDetailInfo;
+import org.apache.carbondata.hadoop.CarbonInputSplit;
+import org.apache.carbondata.sdk.file.cache.BlockletRows;
+
+import org.apache.hadoop.mapreduce.InputSplit;
+
+/**
+ * CarbonData SDK reader with pagination support
+ */
+@InterfaceAudience.User
+@InterfaceStability.Evolving
+public class PaginationCarbonReader<T> extends CarbonReader<T> {
+  // Splits based the file present in the reader path when the reader is built.
+  private List<InputSplit> allBlockletSplits;
+
+  // Rows till the current splits stored as list.
+  private List<Long> rowCountInSplits;
+
+  // Reader builder used to create the pagination reader, used for building 
split level readers.
+  private CarbonReaderBuilder readerBuilder;
+
+  private boolean isClosed;
+
+  // to store the rows of each blocklet in memory based LRU cache.
+  // key: unique blocklet id
+  // value: BlockletRows
+  private CarbonLRUCache cache =
+      new 
CarbonLRUCache(CarbonCommonConstants.CARBON_MAX_PAGINATION_LRU_CACHE_SIZE_IN_MB,
+          
CarbonCommonConstants.CARBON_MAX_PAGINATION_LRU_CACHE_SIZE_IN_MB_DEFAULT);
+
+  /**
+   * Call {@link #builder(String)} to construct an instance
+   */
+
+  PaginationCarbonReader(List<InputSplit> splits, CarbonReaderBuilder 
readerBuilder) {
+    // Initialize super class with no readers.
+    // Based on the splits identified for pagination query, readers will be 
built for the query.
+    super(null);
+    this.allBlockletSplits = splits;
+    this.readerBuilder = readerBuilder;
+    // prepare the mapping.
+    rowCountInSplits = new ArrayList<>(splits.size());
+    long sum = ((CarbonInputSplit) 
splits.get(0)).getDetailInfo().getRowCount();
+    rowCountInSplits.add(sum);
+    for (int i = 1; i < splits.size(); i++) {
+      // prepare a summation array of row counts in each blocklet,
+      // this is used for pruning with pagination vales.
+      // At current index, it contains sum of rows of all the blocklet from 
previous + current.
+      sum += ((CarbonInputSplit) splits.get(i)).getDetailInfo().getRowCount();
+      rowCountInSplits.add(sum);
+    }
+  }
+
+  /**
+   * Pagination query with from and to range.
+   *
+   * @param fromRowNumber must be greater than 0 (as row id starts from 1)
+   *                      and less than or equals to toRowNumber
+   * @param toRowNumber must be greater than 0 (as row id starts from 1)
+   *                and greater than or equals to fromRowNumber
+   *                and should not cross the total rows count
+   * @return array of rows between fromRowNumber and toRowNumber (inclusive)
+   * @throws Exception
+   */
+  public Object[] read(long fromRowNumber, long toRowNumber)
+      throws IOException, InterruptedException {
+    if (isClosed) {
+      throw new RuntimeException("Pagination Reader is closed. please build 
again");
+    }
+    if (fromRowNumber < 1) {
+      throw new IllegalArgumentException("from row id:" + fromRowNumber + " is 
less than 1");
+    }
+    if (fromRowNumber > toRowNumber) {
+      throw new IllegalArgumentException(
+          "from row id:" + fromRowNumber + " is greater than to row id:" + 
toRowNumber);
+    }
+    if (toRowNumber > getTotalRows()) {
+      throw new IllegalArgumentException(
+          "to row id:" + toRowNumber + " is greater than total rows:" + 
getTotalRows());
+    }
+    return getRows(fromRowNumber, toRowNumber);
+  }
+
+  /**
+   * Get total rows in the folder or a list of CarbonData files.
+   * It is based on the snapshot of files taken while building the reader.
+   *
+   * @return total rows from all the files in the reader.
+   */
+  public long getTotalRows() {
+    if (isClosed) {
+      throw new RuntimeException("Pagination Reader is closed. please build 
again");
+    }
+    return rowCountInSplits.get(rowCountInSplits.size() - 1);
+  }
+
+  /**
+   * This interface is for python to call java.
+   * Because python cannot understand java Long object. so send string object.
+   *
+   * Get total rows in the folder or a list of CarbonData files.
+   * It is based on the snapshot of files taken while building the reader.
+   *
+   *
+   * @return total rows from all the files in the reader.
+   */
+  public String getTotalRowsAsString() {
+    if (isClosed) {
+      throw new RuntimeException("Pagination Reader is closed. please build 
again");
+    }
+    return (rowCountInSplits.get(rowCountInSplits.size() - 1)).toString();
+  }
+
+  private static int findBlockletIndex(List<Long> summationArray, Long key) {
+    // summation array is in sorted order, so can use the binary search.
+    int index = Collections.binarySearch(summationArray, key);
+    if (index < 0) {
+      // when key not found, binary search returns negative index [-1 to -N].
+      // which is the possible place where key can be inserted.
+      // with one shifted position. As 0 is also a valid index.
+      // offset the one index shifted and get absolute value of it.
+      index = Math.abs(index + 1);
+    }
+    return index;
+  }
+
+  private Range getBlockletIndexRange(long fromRowNumber, long toRowNumber) {
+    // find the matching blocklet index range by checking with from and to.
+    int upperBound = findBlockletIndex(rowCountInSplits, toRowNumber);
+    // lower bound cannot be more than upper bound, so work on sub list.
+    int lowerBound = findBlockletIndex(rowCountInSplits.subList(0, 
upperBound), fromRowNumber);
+    return new Range(lowerBound, upperBound);
+  }
+
+  private Object[] getRows(long fromRowNumber, long toRowNumber)
+      throws IOException, InterruptedException {
+    int rowCount = 0;
+    Object[] rows = new Object[(int)(toRowNumber - fromRowNumber + 1)];
+    // get the matching split index (blocklets) range for the input range.
+    Range blockletIndexRange = getBlockletIndexRange(fromRowNumber, 
toRowNumber);
+    for (int i = blockletIndexRange.getFrom(); i <= 
blockletIndexRange.getTo(); i++) {
+      String blockletUniqueId = String.valueOf(i);
+      BlockletRows blockletRows;
+      if (cache.get(blockletUniqueId) != null) {
+        blockletRows = (BlockletRows)cache.get(blockletUniqueId);
+      } else {
+        BlockletDetailInfo detailInfo =
+            ((CarbonInputSplit) allBlockletSplits.get(i)).getDetailInfo();
+        int rowCountInBlocklet = detailInfo.getRowCount();
+        Object[] rowsInBlocklet = new Object[rowCountInBlocklet];
+        // read the rows from the blocklet
+        // TODO: read blocklets in multi-thread if there is a performance 
requirement.
+        readerBuilder.setInputSplit(allBlockletSplits.get(i));
+        CarbonReader<Object> carbonReader = readerBuilder.build();
+        int count = 0;
+        while (carbonReader.hasNext()) {
+          rowsInBlocklet[count++] = carbonReader.readNextRow();
+        }
+        carbonReader.close();

Review comment:
       If we need to do something when the exception occurred it make sense to 
enclose it. Now we don't have anything to do.




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