jt2594838 commented on a change in pull request #258: [IOTDB-143]Development of 
URL: https://github.com/apache/incubator-iotdb/pull/258#discussion_r314569883

 File path: 
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+ * 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
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.iotdb.db.engine.merge.manage;
+import static org.apache.iotdb.db.engine.merge.task.MergeTask.MERGE_SUFFIX;
+import java.io.File;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.stream.Collectors;
+import org.apache.iotdb.db.engine.modification.Modification;
+import org.apache.iotdb.db.engine.storagegroup.TsFileResource;
+import org.apache.iotdb.db.query.context.QueryContext;
+import org.apache.iotdb.db.query.reader.IPointReader;
+import org.apache.iotdb.db.utils.MergeUtils;
+import org.apache.iotdb.tsfile.common.conf.TSFileConfig;
+import org.apache.iotdb.tsfile.file.metadata.ChunkMetaData;
+import org.apache.iotdb.tsfile.file.metadata.enums.TSDataType;
+import org.apache.iotdb.tsfile.read.TsFileSequenceReader;
+import org.apache.iotdb.tsfile.read.common.Chunk;
+import org.apache.iotdb.tsfile.read.common.Path;
+import org.apache.iotdb.tsfile.write.chunk.ChunkBuffer;
+import org.apache.iotdb.tsfile.write.chunk.ChunkWriterImpl;
+import org.apache.iotdb.tsfile.write.chunk.IChunkWriter;
+import org.apache.iotdb.tsfile.write.schema.MeasurementSchema;
+import org.apache.iotdb.tsfile.write.writer.RestorableTsFileIOWriter;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+ * MergeResource manages files and caches of readers, writers, 
MeasurementSchemas and
+ * modifications to avoid unnecessary object creations and file openings.
+ */
+public class MergeResource {
+  private static final Logger logger = 
+  private List<TsFileResource> seqFiles;
+  private List<TsFileResource> unseqFiles;
+  private QueryContext mergeContext = new QueryContext();
+  private Map<TsFileResource, TsFileSequenceReader> fileReaderCache;
 Review comment:
   This coupling hardly brings any merit. It may save some time reading 
TsFileMetadata, but it is relatively small compared to the MergeTask itself. 
And the chance you are merging a file that is being queried is not that big 
too, so I prefer to decouple it from other modules.

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