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From Ted Yu <yuzhih...@gmail.com>
Subject Re: Hadoop and XML
Date Tue, 20 Jul 2010 16:38:34 GMT
So the correct call should be:
String valueString = new String(valueText.getBytes(), 0,
valueText.getLength(), "UTF-8");

Cheers

On Tue, Jul 20, 2010 at 9:23 AM, Jeff Bean <jwfbean@cloudera.com> wrote:

> data.length is the length of the byte array.
>
> Text.getLength() most likely returns a different value than
> getBytes.length.
>
> Hadoop reuses box class objects like Text, so what it's probably doing is
> writing over the byte array, lengthening it as necessary, and just updating
> a separate length attribute.
>
> Jeff
>
> On Tue, Jul 20, 2010 at 8:56 AM, Ted Yu <yuzhihong@gmail.com> wrote:
>
> > Interesting.
> > String class is able to handle this scenario:
> >
> >  348       public String(byte[] data, String encoding) throws
> > UnsupportedEncodingException {
> >  349           this(data, 0, data.length, encoding);
> >  350       }
> >
> >
> >
> > On Tue, Jul 20, 2010 at 6:01 AM, Jeff Bean <jwfbean@cloudera.com> wrote:
> >
> > > I think the problem is here:
> > >
> > > String valueString = new String(valueText.getBytes(), "UTF-8");
> > >
> > > Javadoc for Text says:
> > >
> > > *getBytes<
> > >
> >
> http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/io/Text.html#getBytes%28%29
> > > >
> > > *()
> > >          Returns the raw bytes; however, only data up to
> > > getLength()<
> > >
> >
> http://hadoop.apache.org/common/docs/current/api/org/apache/hadoop/io/Text.html#getLength%28%29
> > > >is
> > > valid.
> > >
> > > So try getting the length, truncating the byte array at the value
> > returned
> > > by getLength() and THEN converting it to a String.
> > >
> > > Jeff
> > >
> > > On Mon, Jul 19, 2010 at 9:08 AM, Ted Yu <yuzhihong@gmail.com> wrote:
> > >
> > > > 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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