hadoop-hdfs-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Rahul Bhattacharjee <rahul.rec....@gmail.com>
Subject Re: Hadoop sampler related query!
Date Wed, 24 Apr 2013 07:07:12 GMT
Thanks for the response Mahesh. I thought of this , but do not know why is
this limitation.

While sampling to pick up certain records and run our logic over the key
part.Thats why there is a limitation that mappers key must be the same as
mappers output key.

However , I think , if we can run the mapper of the collected sample and do
our analysis over the key of the mappers then it would be great and there
wouldn't be any limitation like what we have today.

Was wondering why its not like this.


On Wed, Apr 24, 2013 at 11:23 AM, Mahesh Balija

> Hi Rahul,
>              The limitation to use InputSampler is, the K and OK (I mean
> Map INKEY and OUTKEY) both should be of same type.
>              Technically because, while collecting the samples (ie.,
> arraylist of keys) in writePartitionFile method it uses the INKEY as the
> key. And for writing the partition file it uses Mapper OutputKEY as the
> KEY.
>              Logically also this is the expected behavior of sampling
> because, while collecting the samples the only source is the input splits
> (INKEY) from which it collects the samples and for generating partition
> file you need to generate based on the Mapper outkey type.
> Best,
> Mahesh Balija,
> CalsoftLabs.
> On Tue, Apr 23, 2013 at 4:12 PM, Rahul Bhattacharjee <
> rahul.rec.dgp@gmail.com> wrote:
>> + mapred dev
>> On Tue, Apr 16, 2013 at 2:19 PM, Rahul Bhattacharjee <
>> rahul.rec.dgp@gmail.com> wrote:
>>> Hi,
>>> I have a question related to Hadoop's input sampler ,which is used for
>>> investigating the data set before hand using random selection , sampling
>>> etc .Mainly used for total sort , used in pig's skewed join implementation
>>> as well.
>>> The question here is -
>>> Mapper<K,V,OK,OV>
>>> K and V are input key and value of the mapper .Essentially coming in
>>> from the input format. OK and OV are output key and value emitted from the
>>> mapper.
>>> Looking at the input sample's code ,it looks like it is creating the
>>> partition based on the input key of the mapper.
>>> I think the partitions should be created considering the output key (OK)
>>> and the output key sort comparator should be used for sorting the samples.
>>> If partitioning is done based on input key and the mapper emits a
>>> different key then the total sort wouldn't hold any good.
>>> Is there is any condition that input sample is to be only used for
>>> mapper<K,V,K,V1>?
>>> Thanks,
>>> Rahul

View raw message