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Date: Thu, 6 Nov 2014 10:18:15 +0100
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 <CANMXwW1q37jn6wot5wyit68Ozoac6_upR9=GWRe_xxqe6_vewA@mail.gmail.com>
Subject: Re: PartitionByHash and usage of KeySelector
From: Aljoscha Krettek <aljoscha@apache.org>
To: user@flink.incubator.apache.org
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Hi Stefano,
what to you mean by "creating partitions of each of the partitions"?

The expressions keys can be used to specify fields of objects that
should be used for hashing. So if you had objects of this class in a
DataSet:

class Foo {
  public String bar;
  public Integer baz;
  public Float bat;
}

You could use for example:

input.partitionByHash("baz", "bat")

to perform the partitioning only on those two fields of the objects.

Regards,
Aljoscha

On Thu, Nov 6, 2014 at 9:19 AM, Stefano Bortoli <s.bortoli@gmail.com> wrote:
> Hi all,
>
> I am moving my first steps into becoming an Apache Flink user! I have
> configured and run some simple jobs on a small cluster, and everything
> worked quite fine so far.
>
> What I am trying to do right now is to run a duplication detection task on
> dataset of about 9.5M records. The records are well structured, and
> therefore we can exploit the semantic of attributes to narrow down expensive
> match executions.
>
> My idea is the following:
> 1. partition the dataset according to a macro-parameter written in the
> record. This allows me to get to 7 partitions of different sizes but also
> certainly disjoint. I do that by filtering on a specific type.
> 2. create partitions of each of the partitions created in step 1 based on
> some simple similarity that would reduce the number of expensive function. I
> would like to do that by using partitionByHash and KeySelector.
> 3. compute Cross product for each of the partitions defined in step 2;
> 4. filter each pair of the cross product by applying an expensive boolean
> matching function. Only positive matching duplicates will be retained.
>
> Currently I am working on the step 2, and I have some problems understanding
> how to use the partitionByHash function. The main problem is that I need to
> have a 'rich key' to support partition, and I discovered the ExpressionKeys
> that would allow me to define hash keys with sets of Strings I can collect
> from the record. However, the partitionByHash function does not allow to use
> these objects as the hash must implement comparable.
>
> So, here is my question: how can I partition considering hash keys of more
> than one String?
>
> Is there a better strategy to implement a de-duplication using Flink?
>
>
> thanks a lot for your support.
>
> kind regards,
>
> Stefano Bortoli, PhD
> ENS Technical Director
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