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From Sean Owen <so...@cloudera.com>
Subject Re: JavaRDD Aggregate initial value - Closure-serialized zero value reasoning?
Date Wed, 18 Feb 2015 10:09:39 GMT
The serializer is created with

val zeroBuffer = SparkEnv.get.serializer.newInstance().serialize(zeroValue)

Which is definitely not the closure serializer and so should respect
what you are setting with spark.serializer.

Maybe you can do a quick bit of debugging to see where that assumption
breaks down? like are you sure spark.serializer is set everywhere?

On Wed, Feb 18, 2015 at 4:31 AM, Matt Cheah <mcheah@palantir.com> wrote:
> Hi everyone,
>
> I was using JavaPairRDD’s combineByKey() to compute all of my aggregations
> before, since I assumed that every aggregation required a key. However, I
> realized I could do my analysis using JavaRDD’s aggregate() instead and not
> use a key.
>
> I have set spark.serializer to use Kryo. As a result, JavaRDD’s combineByKey
> requires that a “createCombiner” function is provided, and the return value
> from that function must be serializable using Kryo. When I switched to using
> rdd.aggregate I assumed that the zero value would also be strictly Kryo
> serialized, as it is a data item and not part of a closure or the
> aggregation functions. However, I got a serialization exception as the
> closure serializer (only valid serializer is the Java serializer) was used
> instead.
>
> I was wondering the following:
>
> What is the rationale for making the zero value be serialized using the
> closure serializer? This isn’t part of the closure, but is an initial data
> item.
> Would it make sense for us to perhaps write a version of rdd.aggregate()
> that takes a function as a parameter, that generates the zero value? This
> would be more intuitive to be serialized using the closure serializer.
>
> I believe aggregateByKey is also affected.
>
> Thanks,
>
> -Matt Cheah

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