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From Ted Yu <yuzhih...@gmail.com>
Subject Re: Hadoop and XML
Date Mon, 19 Jul 2010 16:08:19 GMT
For your initial question on Text.set().
Text.setCapacity() allocates new byte array. Since keepData is false, old
data wouldn't be copied over.

On Mon, Jul 19, 2010 at 8:01 AM, Peter Minearo <
Peter.Minearo@reardencommerce.com> wrote:

> I am already using XmlInputFormat.  The input into the Map phase is not
> the problem.  The problem lays in between the Map and Reduce phase.
>
> BTW - The article is correct.  DO NOT USE StreamXmlRecordReader.
> XmlInputFormat is a lot faster.  From my testing, StreamXmlRecordReader
> took 8 minutes to read a 1 GB XML document; where as, XmlInputFormat was
> under 2 minutes. (Using 2 Core, 8GB machines)
>
>
> -----Original Message-----
> From: Ted Yu [mailto:yuzhihong@gmail.com]
> Sent: Friday, July 16, 2010 9:44 PM
> To: general@hadoop.apache.org
> Subject: Re: Hadoop and XML
>
> From an earlier post:
> http://oobaloo.co.uk/articles/2010/1/20/processing-xml-in-hadoop.html
>
> On Fri, Jul 16, 2010 at 3:07 PM, Peter Minearo <
> Peter.Minearo@reardencommerce.com> wrote:
>
> > Moving the variable to a local variable did not seem to work:
> >
> >
> > </PrivateRateSet>vateRateSet>
> >
> >
> >
> > public void map(Object key, Object value, OutputCollector output,
> > Reporter
> > reporter) throws IOException {
> >                Text valueText = (Text)value;
> >                String valueString = new String(valueText.getBytes(),
> > "UTF-8");
> >                String keyString = getXmlKey(valueString);
> >                 Text returnKeyText = new Text();
> >                Text returnValueText = new Text();
> >                returnKeyText.set(keyString);
> >                returnValueText.set(valueString);
> >                output.collect(returnKeyText, returnValueText); }
> >
> > -----Original Message-----
> > From: Peter Minearo [mailto:Peter.Minearo@Reardencommerce.com]
> > Sent: Fri 7/16/2010 2:51 PM
> > To: general@hadoop.apache.org
> > Subject: RE: Hadoop and XML
> >
> > Whoops....right after I sent it and someone else made a suggestion; I
> > realized what question 2 was about.  I can try that, but wouldn't that
>
> > cause Object bloat?  During the Hadoop training I went through; it was
>
> > mentioned to reuse the returning Key and Value objects to keep the
> > number of Objects created down to a minimum.  Is this not really a
> > valid point?
> >
> >
> >
> > -----Original Message-----
> > From: Peter Minearo [mailto:Peter.Minearo@Reardencommerce.com]
> > Sent: Friday, July 16, 2010 2:44 PM
> > To: general@hadoop.apache.org
> > Subject: RE: Hadoop and XML
> >
> >
> > I am not using multi-threaded Map tasks.  Also, if I understand your
> > second question correctly:
> > "Also can you try creating the output key and values in the map
> > method(method lacal) ?"
> > In the first code snippet I am doing exactly that.
> >
> > Below is the class that runs the Job.
> >
> > public class HadoopJobClient {
> >
> >        private static final Log LOGGER =
> > LogFactory.getLog(Prds.class.getName());
> >
> >        public static void main(String[] args) {
> >                JobConf conf = new JobConf(Prds.class);
> >
> >                conf.set("xmlinput.start", "<PrivateRateSet>");
> >                conf.set("xmlinput.end", "</PrivateRateSet>");
> >
> >                conf.setJobName("PRDS Parse");
> >
> >                conf.setOutputKeyClass(Text.class);
> >                conf.setOutputValueClass(Text.class);
> >
> >                conf.setMapperClass(PrdsMapper.class);
> >                conf.setReducerClass(PrdsReducer.class);
> >
> >                conf.setInputFormat(XmlInputFormat.class);
> >                conf.setOutputFormat(TextOutputFormat.class);
> >
> >                FileInputFormat.setInputPaths(conf, new Path(args[0]));
> >                FileOutputFormat.setOutputPath(conf, new
> > Path(args[1]));
> >
> >                // Run the job
> >                try {
> >                        JobClient.runJob(conf);
> >                } catch (IOException e) {
> >                        LOGGER.error(e.getMessage(), e);
> >                }
> >
> >        }
> >
> >
> > }
> >
> >
> >
> >
> > -----Original Message-----
> > From: Soumya Banerjee [mailto:soumya.sbanerjee@gmail.com]
> > Sent: Fri 7/16/2010 2:29 PM
> > To: general@hadoop.apache.org
> > Subject: Re: Hadoop and XML
> >
> > Hi,
> >
> > Can you please share the code of the job submission client ?
> >
> > Also can you try creating the output key and values in the map
> > method(method
> > lacal) ?
> > Make sure you are not using multi threaded map task configuration.
> >
> > map()
> > {
> > private Text keyText = new Text();
> >  private Text valueText = new Text();
> >
> > //rest of the code
> > }
> >
> > Soumya.
> >
> > On Sat, Jul 17, 2010 at 2:30 AM, Peter Minearo <
> > Peter.Minearo@reardencommerce.com> wrote:
> >
> > > I have an XML file that has sparse data in it.  I am running a
> > > MapReduce Job that reads in an XML file, pulls out a Key from within
>
> > > the XML snippet and then hands back the Key and the XML snippet (as
> > > the Value) to the OutputCollector.  The reason is to sort the file
> > back into order.
> > > Below is the snippet of code.
> > >
> > > public class XmlMapper extends MapReduceBase implements Mapper {
> > >
> > >  private Text keyText = new Text();
> > >  private Text valueText = new Text();
> > >
> > >  @SuppressWarnings("unchecked")
> > >  public void map(Object key, Object value, OutputCollector output,
> > > Reporter reporter) throws IOException {  Text valueText =
> > > (Text)value;
> >
> > > String valueString = new String(valueText.getBytes(), "UTF-8");
> > > String keyString = getXmlKey(valueString);
> > > getKeyText().set(keyString);  getValueText().set(valueString);
> > > output.collect(getKeyText(), getValueText());  }
> > >
> > >
> > >  public Text getKeyText() {
> > >  return keyText;
> > >  }
> > >
> > >
> > >  public void setKeyText(Text keyText) {  this.keyText = keyText;  }
> > >
> > >
> > >  public Text getValueText() {
> > >  return valueText;
> > >  }
> > >
> > >
> > >  public void setValueText(Text valueText) {  this.valueText =
> > > valueText;  }
> > >
> > >
> > >  private String getXmlKey(String value) {
> > >        // Get the Key from the XML in the value.
> > >  }
> > >
> > > }
> > >
> > > The XML snippet from the Value is fine when it is passed into the
> > > map() method.  I am not changing any data either, just pulling out
> > > information for the key.  The problem I am seeing is between the Map
>
> > > phase and the Reduce phase, the XML is getting munged.  For Example:
> > >
> > >  </PrivateRate>
> > >  </PrivateRateSet>te>
> > >
> > > It is my understanding that Hadoop uses the same instance of the Key
>
> > > and Value object when calling the Map method.  What changes is the
> > > data within those instances.  So, I ran an experiment where I do not
>
> > > have different Key or Value Text Objects.  I reuse the ones passed
> > > into the method, like below:
> > >
> > > public class XmlMapper extends MapReduceBase implements Mapper {
> > >
> > >  @SuppressWarnings("unchecked")
> > >  public void map(Object key, Object value, OutputCollector output,
> > > Reporter reporter) throws IOException {  Text keyText = (Text)key;
> > > Text valueText = (Text)value;  String valueString = new
> > > String(valueText.getBytes(), "UTF-8");  String keyString =
> > > getXmlKey(valueString);  keyText.set(keyString);
> > > valueText.set(valueString);  output.collect(keyText, valueText);  }
> > >
> > >
> > >  private String getXmlKey(String value) {
> > >        // Get the Key from the XML in the value.
> > >  }
> > >
> > > }
> > >
> > > What was interesting about this is the fact that the XML was getting
>
> > > munged within the Map Phase.  When I changed over to the code at the
>
> > > top, the Map phase was fine.  However, the Reduce phase picks up the
>
> > > munged XML.  Trying to debug the problem, I came across this method
> > > in
> >
> > > the Text Object:
> > >
> > > public void set(byte[] utf8, int start, int len) {
> > >    setCapacity(len, false);
> > >    System.arraycopy(utf8, start, bytes, 0, len);
> > >    this.length = len;
> > > }
> > >
> > > If the "bytes" array had a length of 1000 and the "utf8" array has a
>
> > > length of 500; doing a System.arraycopy() would only copy the first
> > > 500 from "utf8" to "bytes" but leave the last 500 in "bytes" alone.
> > > Could this be the cause of the XML munging?
> > >
> > > All of this leads me to a few questions:
> > >
> > > 1) Has anyone successfully used XML snippets as the data format
> > > within
> >
> > > a MapReduce job; not just reading from the file but used during the
> > > shuffle?
> > > 2) Is anyone seeing this problem with XML or any other format?
> > > 3) Does anyone know what is going on?
> > > 4) Is this a bug?
> > >
> > >
> > > Thanks,
> > >
> > > Peter
> > >
> > >
> > >
> >
> >
> >
>

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