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From "LINZ, Arnaud" <AL...@bouyguestelecom.fr>
Subject How to make a generic key for groupBy
Date Thu, 23 Apr 2015 14:25:10 GMT
Hello,

After a quite successful benchmark yesterday (Flink being about twice faster than Spark on
my use cases), I’ve turned instantly from spark-fan to flink-fan – great job, committers!
So I’ve decided to port my existing Spark tools to Flink. Happily, most of the difficulty
was renaming classes, packages and variables with “spark” in them to something more neutral
☺

However there is one easy thing in Spark I’m still wondering how to do in Flink : generic
keys.

I’m trying to make a framework on which my applications are built. That framework thus manipulate
“generic types” representing the data, inheriting from an abstract class with a common
contract, let’s call it “Bean”.

Among other things Bean exposes an abstract method
public Key getKey();

Key being one of my core types used in several java algorithms.

Let’s say I have the class :
public class Framework<T extends Bean> implements Serializable {

public DataSet<T> doCoolStuff(final DataSet<T> inputDataset) {
        // Group lines according to a key
        final UnsortedGrouping<YT> groupe = inputDataset.groupBy(new KeySelector<T,
Key>() {
            @Override
            public Key getKey(T record)  {
                return record.getKey();
            }
        });
             (…)
       }
}

With Spark, a mapToPair works fine because all I have to do is implements correctly hashCode()
and equals() on my Key type.
With Flink, Key is not recognized as a POJO object (well it is not) and that does not work.

I have tried to expose something like public Tuple getKeyAsTuple(); in Key but Flink does
not accept generic Tuples. I’ve tried to parameterize my Tuple but Flink does not know how
to infer
the generic type value.

So I’m wondering what is the best way to implement it.
For now I have exposed something like public String getKeyAsString(); and turned my generic
treatment into :
final UnsortedGrouping<YT> groupe = inputDataset.groupBy(new KeySelector<T, String>()
{
            @Override
            public String getKey(T record)  {
                return record.getKey().getKeyAsString();
            }
        });
But that “ASCII” representation is suboptimal.

I thought of passing a key to tuple conversion lambda upon creation of the Framework class
but that would be boiler-plate code on the user’s end, which I’m not fond of.

So my questions are :

-          Is there a smarter way to do this ?

-          What kind of objects can be passed as a Key ? Is there an Interface to respect
?

-          In the worst case, is byte[]  ok as a Key ? (I can code the serialization on the
framework side…)


Best regards,
Arnaud


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