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 <
[email protected]> 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
>
>
>
