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From magpierre <...@git.apache.org>
Subject [GitHub] drill pull request: Drill 3878: XML support in Apache Drill
Date Thu, 07 Apr 2016 20:58:24 GMT
Github user magpierre commented on a diff in the pull request:

    https://github.com/apache/drill/pull/451#discussion_r58943891
  
    --- Diff: contrib/storage-xml/src/main/java/org/apache/drill/exec/store/easy/xml/XMLRecordReader.java
---
    @@ -0,0 +1,95 @@
    +/**
    + * 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.drill.exec.store.easy.xml;
    +
    +import com.fasterxml.jackson.annotation.JsonInclude;
    +import com.fasterxml.jackson.databind.JsonNode;
    +import com.fasterxml.jackson.databind.ObjectMapper;
    +import org.apache.drill.common.exceptions.ExecutionSetupException;
    +import org.apache.drill.common.exceptions.UserException;
    +import org.apache.drill.common.expression.SchemaPath;
    +import org.apache.drill.exec.exception.OutOfMemoryException;
    +import org.apache.drill.exec.ops.FragmentContext;
    +import org.apache.drill.exec.ops.OperatorContext;
    +import org.apache.drill.exec.physical.impl.OutputMutator;
    +import org.apache.drill.exec.store.dfs.DrillFileSystem;
    +import org.apache.drill.exec.store.easy.json.JSONRecordReader;
    +import org.apache.hadoop.fs.FSDataInputStream;
    +import org.apache.hadoop.fs.Path;
    +import org.xml.sax.SAXException;
    +import javax.xml.parsers.ParserConfigurationException;
    +import javax.xml.parsers.SAXParser;
    +import javax.xml.parsers.SAXParserFactory;
    +import java.io.IOException;
    +import java.util.List;
    +
    +
    +public class XMLRecordReader extends JSONRecordReader {
    +    private static final org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(XMLRecordReader.class);
    +    private XMLSaxParser handler;
    +    private SAXParser xmlParser;
    +    private JsonNode node;
    +
    +    public XMLRecordReader(FragmentContext fragmentContext, String inputPath, DrillFileSystem
fileSystem, List<SchemaPath> columns, XMLFormatPlugin.XMLFormatConfig xmlConfig) throws
OutOfMemoryException {
    +        super(fragmentContext, inputPath, fileSystem, columns);
    +        try {
    +            FSDataInputStream fsStream = fileSystem.open(new Path(inputPath));
    +            SAXParserFactory saxParserFactory = SAXParserFactory.newInstance();
    +            xmlParser = saxParserFactory.newSAXParser();
    +            handler = new XMLSaxParser();
    +            handler.setRemoveNameSpace(!xmlConfig.getKeepPrefix());
    +            xmlParser.parse(fsStream.getWrappedStream(), handler);
    +            ObjectMapper mapper = new ObjectMapper();
    +            node = mapper.valueToTree(handler.getVal());
    --- End diff --
    
    The XML parser is streaming based. Due to SAX which is a stateless parser where each tag
triggers a series of events and then drops the information, it comes with a very little memory
overhead. However, as noted I keep all parsed objects in a stack since I need to link them.
Due to the way the parsing works the root of the document is really getting its children last,
which means that once parsed I have the whole document in memory but in JSON format so I will
at least not keep both a JSON document and an XML document in memory at the same time. I've
been considering storing state in files instead but I don't think it would be performing well
considering I need to revisit objects frequently. If you have any suggestions such as memory
mapped files that could spill down to disk if large, it would be appreciated. 


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