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From Shekhar Sharma <shekhar2...@gmail.com>
Subject Re: Mappers vs. Map tasks
Date Thu, 27 Feb 2014 05:57:29 GMT
Writing a custom input format would be much easier and you would have
better control
You might be tempted to use Jackson lib to do the process json  but that
requires that you need to know your Json data and this assumption would
break if your format of data changes

I would suggest write a custom record reader where you parse the json and
create your own key value pairs
On 27 Feb 2014 09:52, "Sugandha Naolekar" <sugandha.n87@gmail.com> wrote:

> Joao Paulo,
>
> Your suggestion is appreciated. Although, on a side note, what is more
> tedious: Writing a custom InputFormat or changing the code which is
> generating the input splits.?
>
> --
> Thanks & Regards,
> Sugandha Naolekar
>
>
>
>
>
> On Wed, Feb 26, 2014 at 8:03 PM, João Paulo Forny <jpforny@gmail.com>wrote:
>
>> If I understood your problem correctly, you have one huge JSON, which is
>> basically a JSONArray, and you want to process one JSONObject of the array
>> at a time.
>>
>> I have faced the same issue some time ago and instead of changing the
>> input format, I changed the code that was generating this input, to
>> generate lots of JSONObjects, one per line. Hence, using the default
>> TextInputFormat, the map function was getting called with the entire JSON.
>>
>> A JSONArray is not good for a mapreduce input since it has a first [ and
>> a last ] and commas between the JSONs of the array. The array can be
>> represented as the file that the JSONs belong.
>>
>> Of course, this approach works only if you can modify what is generating
>> the input you're talking about.
>>
>>
>> 2014-02-26 8:25 GMT-03:00 Mohammad Tariq <dontariq@gmail.com>:
>>
>> In that case you have to convert your JSON data into seq files first and
>>> then do the processing.
>>>
>>> Warm Regards,
>>> Tariq
>>> cloudfront.blogspot.com
>>>
>>>
>>> On Wed, Feb 26, 2014 at 4:43 PM, Sugandha Naolekar <
>>> sugandha.n87@gmail.com> wrote:
>>>
>>>> Can I use SequenceFileInputFormat to do the same?
>>>>
>>>>  --
>>>> Thanks & Regards,
>>>> Sugandha Naolekar
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Wed, Feb 26, 2014 at 4:38 PM, Mohammad Tariq <dontariq@gmail.com>wrote:
>>>>
>>>>> Since there is no OOTB feature that allows this, you have to write
>>>>> your custom InputFormat to handle JSON data. Alternatively you could
make
>>>>> use of Pig or Hive as they have builtin JSON support.
>>>>>
>>>>> Warm Regards,
>>>>> Tariq
>>>>> cloudfront.blogspot.com
>>>>>
>>>>>
>>>>> On Wed, Feb 26, 2014 at 10:07 AM, Rajesh Nagaraju <
>>>>> rajeshnagaraju@gmail.com> wrote:
>>>>>
>>>>>> 1 simple way is to remove the new line characters so that the default
>>>>>> record reader and default way the block is read will take care of
the input
>>>>>> splits and JSON will not get affected by the removal of NL character
>>>>>>
>>>>>>
>>>>>> On Wed, Feb 26, 2014 at 10:01 AM, Sugandha Naolekar <
>>>>>> sugandha.n87@gmail.com> wrote:
>>>>>>
>>>>>>> Ok. Got it. Now I have a single file which is of 129MB. Thus,
it
>>>>>>> will be split into two blocks. Now, since my file is a json file,
I cannot
>>>>>>> use textinputformat. As, every input split(logical) will be a
single line
>>>>>>> of the json file. Which I dont want. Thus, in this case, can
I write a
>>>>>>> custom input format and a custom record reader so that, every
input
>>>>>>> split(logical) will have only that part of data which I require.
>>>>>>>
>>>>>>> For. e.g:
>>>>>>>
>>>>>>> { "type": "Feature", "properties": { "OSM_NAME": "", "FLAGS":
>>>>>>> 3.000000, "CLAZZ": 42.000000, "ROAD_TYPE": 3.000000, "END_ID":
>>>>>>> 33451.000000, "OSM_META": "", "REVERSE_LE": 217.541279, "X1":
77.552595,
>>>>>>> "OSM_SOURCE": 1520846283.000000, "COST": 0.007058, "OSM_TARGET":
>>>>>>> 1520846293.000000, "X2": 77.554549, "Y2": 12.993056, "CONGESTED_":
>>>>>>> 227.541279, "Y1": 12.993107, "REVERSE_CO": 0.007058, "CONGESTION":
>>>>>>> 10.000000, "OSM_ID": 138697535.000000, "START_ID": 33450.000000,
"KM":
>>>>>>> 0.000000, "LENGTH": 217.541279, "REVERSE__1": 227.541279, "SPEED_IN_K":
>>>>>>> 30.000000, "ROW_FLAG": "F" }, "geometry": { "type": "LineString",
>>>>>>> "coordinates": [ [ 8633115.407361, 1458944.819456 ], [ 8633332.869986,
>>>>>>> 1458938.970140 ] ] } }
>>>>>>> ,
>>>>>>> { "type": "Feature", "properties": { "OSM_NAME": "", "FLAGS":
>>>>>>> 3.000000, "CLAZZ": 32.000000, "ROAD_TYPE": 3.000000, "END_ID":
>>>>>>> 37016.000000, "OSM_META": "", "REVERSE_LE": 156.806535, "X1":
77.538462,
>>>>>>> "OSM_SOURCE": 1037135286.000000, "COST": 0.003052, "OSM_TARGET":
>>>>>>> 1551615728.000000, "X2": 77.537950, "Y2": 12.992099, "CONGESTED_":
>>>>>>> 176.806535, "Y1": 12.993377, "REVERSE_CO": 0.003052, "CONGESTION":
>>>>>>> 20.000000, "OSM_ID": 89417379.000000, "START_ID": 24882.000000,
"KM":
>>>>>>> 0.000000, "LENGTH": 156.806535, "REVERSE__1": 176.806535, "SPEED_IN_K":
>>>>>>> 50.000000, "ROW_FLAG": "F" }, "geometry": { "type": "LineString",
>>>>>>> "coordinates": [ [ 8631542.162393, 1458975.665482 ], [ 8631485.144550,
>>>>>>> 1458829.592709 ] ] } }
>>>>>>>
>>>>>>> *I want here the every input split to consist of entire type
data
>>>>>>> and thus, I can process it accordingly by giving relevant k,V
pairs to the
>>>>>>> map function.*
>>>>>>>
>>>>>>>
>>>>>>> --
>>>>>>> Thanks & Regards,
>>>>>>> Sugandha Naolekar
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On Wed, Feb 26, 2014 at 2:09 AM, Mohammad Tariq <dontariq@gmail.com>wrote:
>>>>>>>
>>>>>>>> Hi Sugandha,
>>>>>>>>
>>>>>>>> Please find my comments embedded below :
>>>>>>>>
>>>>>>>>                   No. of mappers are decided as:
>>>>>>>> Total_File_Size/Max. Block Size. Thus, if the file is smaller
than the
>>>>>>>> block size, only one mapper will be                     
         invoked.
>>>>>>>> Right?
>>>>>>>>                   This is true(but not always). The basic
criteria
>>>>>>>> behind map creation is the logic inside *getSplits* method
of
>>>>>>>> *InputFormat* being used in your                     MR job.
It is
>>>>>>>> the behavior of *file based InputFormats*, typically sub-classes
>>>>>>>> of *FileInputFormat*, to split the input data into splits
based
>>>>>>>>                   on the total size, in bytes, of the input
files. See
>>>>>>>> *this*<http://hadoop.apache.org/docs/current2/api/org/apache/hadoop/mapreduce/InputFormat.html>for
more details. And yes, if the file is smaller than the block size then
>>>>>>>> only 1 mapper will                     be created.
>>>>>>>>
>>>>>>>>                   If yes, it means, the map() will be called
only
>>>>>>>> once. Right? In this case, if there are two datanodes with
a replication
>>>>>>>> factor as 1: only one                               datanode(mapper
>>>>>>>> machine) will perform the task. Right?
>>>>>>>>                   A mapper is called for each split. Don't
get
>>>>>>>> confused with the MR's split and HDFS's block. Both are different(They
may
>>>>>>>> overlap though, as in                     case of FileInputFormat).
HDFS
>>>>>>>> blocks are physical partitioning of your data, while an InputSplit
is just
>>>>>>>> a logical partitioning. If you have a                   
   file which is
>>>>>>>> smaller than the HDFS blocksize then only one split will
be created, hence
>>>>>>>> only 1 mapper will be called. And this will happen on
>>>>>>>> the node where this file resides.
>>>>>>>>
>>>>>>>>                   The map() function is called by all the
>>>>>>>> datanodes/slaves right? If the no. of mappers are more than
the no. of
>>>>>>>> slaves, what happens?
>>>>>>>>                   map() doesn't get called by anybody. It
rather
>>>>>>>> gets created on the node where the chunk of data to be processed
resides. A
>>>>>>>> slave node can run                       multiple mappers
based on the
>>>>>>>> availability of CPU slots.
>>>>>>>>
>>>>>>>>                  One more thing to ask: No. of blocks = no.
of
>>>>>>>> mappers. Thus, those many no. of times the map() function
will be called
>>>>>>>> right?
>>>>>>>>                  No. of blocks = no. of splits = no. of mappers.
A
>>>>>>>> map is called only once per split per node where that split
is present.
>>>>>>>>
>>>>>>>> HTH
>>>>>>>>
>>>>>>>> Warm Regards,
>>>>>>>> Tariq
>>>>>>>> cloudfront.blogspot.com
>>>>>>>>
>>>>>>>>
>>>>>>>> On Tue, Feb 25, 2014 at 3:54 PM, Sugandha Naolekar <
>>>>>>>> sugandha.n87@gmail.com> wrote:
>>>>>>>>
>>>>>>>>> Hi Bertrand,
>>>>>>>>>
>>>>>>>>> As you said, no. of HDFS blocks =  no. of input splits.
But this
>>>>>>>>> is only true when you set isSplittable() as false or
when your input file
>>>>>>>>> size is less than the block size. Also, when it comes
to text files, the
>>>>>>>>> default textinputformat considers each line as one input
split which can be
>>>>>>>>> then read by RecordReader in K,V format.
>>>>>>>>>
>>>>>>>>> Please correct me if I don't make sense.
>>>>>>>>>
>>>>>>>>> --
>>>>>>>>> Thanks & Regards,
>>>>>>>>> Sugandha Naolekar
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Tue, Feb 25, 2014 at 2:07 PM, Bertrand Dechoux <
>>>>>>>>> dechouxb@gmail.com> wrote:
>>>>>>>>>
>>>>>>>>>> The wiki (or Hadoop The Definitive Guide) are good
ressources.
>>>>>>>>>>
>>>>>>>>>> https://www.inkling.com/read/hadoop-definitive-guide-tom-white-3rd/chapter-7/input-formats
>>>>>>>>>>
>>>>>>>>>> Mapper is the name of the abstract class/interface.
It does not
>>>>>>>>>> really make sense to talk about number of mappers.
>>>>>>>>>> A task is a jvm that can be launched only if there
is a free slot
>>>>>>>>>> ie for a given slot, at a given time, there will
be at maximum only a
>>>>>>>>>> single task. During the task, the configured Mapper
will be instantiated.
>>>>>>>>>>
>>>>>>>>>> Always :
>>>>>>>>>> Number of input splits = no. of map tasks
>>>>>>>>>>
>>>>>>>>>> And generally :
>>>>>>>>>> number of hdfs blocks = number of input splits
>>>>>>>>>>
>>>>>>>>>> Regards
>>>>>>>>>>
>>>>>>>>>> Bertrand
>>>>>>>>>>
>>>>>>>>>> PS : I don't know if it is only my client, but avoid
red when
>>>>>>>>>> writting a mail.
>>>>>>>>>>
>>>>>>>>>> On Tue, Feb 25, 2014 at 8:49 AM, Dieter De Witte
<
>>>>>>>>>> drdwitte@gmail.com> wrote:
>>>>>>>>>>
>>>>>>>>>>> Each node has a tasktracker with a number of
map slots. A map
>>>>>>>>>>> slot hosts as mapper. A mapper executes map tasks.
If there are more map
>>>>>>>>>>> tasks than slots obviously there will be multiple
rounds of mapping.
>>>>>>>>>>>
>>>>>>>>>>> The map function is called once for each input
record. A block
>>>>>>>>>>> is typically 64MB and can contain a multitude
of record, therefore a map
>>>>>>>>>>> task = run the map() function on all records
in the block.
>>>>>>>>>>>
>>>>>>>>>>> Number of blocks = no. of map tasks (not mappers)
>>>>>>>>>>>
>>>>>>>>>>> Furthermore you have to make a distinction between
the two
>>>>>>>>>>> layers. You have a layer for computations which
consists of a jobtracker
>>>>>>>>>>> and a set of tasktrackers. The other layer is
responsible for storage. The
>>>>>>>>>>> HDFS has a namenode and a set of datanodes.
>>>>>>>>>>>
>>>>>>>>>>> In mapreduce the code is executed where the data
is. So if a
>>>>>>>>>>> block is in datanode 1, 2 and 3, then the map
task associated with this
>>>>>>>>>>> block will likely be executed on one of those
physical nodes, by
>>>>>>>>>>> tasktracker 1, 2 or 3. But this is not necessary,
thing can be rearranged.
>>>>>>>>>>>
>>>>>>>>>>> Hopefully this gives you a little more insigth.
>>>>>>>>>>>
>>>>>>>>>>> Regards, Dieter
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> 2014-02-25 7:05 GMT+01:00 Sugandha Naolekar <
>>>>>>>>>>> sugandha.n87@gmail.com>:
>>>>>>>>>>>
>>>>>>>>>>>  One more thing to ask: No. of blocks = no. of
mappers. Thus,
>>>>>>>>>>>> those many no. of times the map() function
will be called right?
>>>>>>>>>>>>
>>>>>>>>>>>> --
>>>>>>>>>>>> Thanks & Regards,
>>>>>>>>>>>> Sugandha Naolekar
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> On Tue, Feb 25, 2014 at 11:27 AM, Sugandha
Naolekar <
>>>>>>>>>>>> sugandha.n87@gmail.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> Hello,
>>>>>>>>>>>>>
>>>>>>>>>>>>> As per the various articles I went through
till date, the
>>>>>>>>>>>>> File(s) are split in chunks/blocks. On
the same note, would like to ask few
>>>>>>>>>>>>> things:
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>    1. No. of mappers are decided as:
Total_File_Size/Max.
>>>>>>>>>>>>>    Block Size. Thus, if the file is smaller
than the block size, only one
>>>>>>>>>>>>>    mapper will be invoked. Right?
>>>>>>>>>>>>>    2. If yes, it means, the map() will
be called only once.
>>>>>>>>>>>>>    Right? In this case, if there are
two datanodes with a replication factor
>>>>>>>>>>>>>    as 1: only one datanode(mapper machine)
will perform the task. Right?
>>>>>>>>>>>>>    3. The map() function is called by
all the
>>>>>>>>>>>>>    datanodes/slaves right? If the no.
of mappers are more than the no. of
>>>>>>>>>>>>>    slaves, what happens?
>>>>>>>>>>>>>
>>>>>>>>>>>>> --
>>>>>>>>>>>>> Thanks & Regards,
>>>>>>>>>>>>> Sugandha Naolekar
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

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