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https://issues.apache.org/jira/browse/SPARK-9999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14958022#comment-14958022
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Sandy Ryza commented on SPARK-9999:
-----------------------------------
bq. The problem with doing this using a registry (like kryo in RDDs today) is
that then you aren't finding out the object type until you have an example
object from realizing the computation.
My suggestion was that the user would still need to pass the class object, so
this shouldn't be a problem, unless I'm misunderstanding.
Thanks to the pointer to the test suite. So am I to understand correctly that
with Scala implicits magic I can do the following without any additional
boilerplate?
{code}
import <some basic sql stuff>
case class MyClass1(<some fields>)
case class MyClass2(<some fields>)
val ds : Dataset[MyClass1] = ...
val ds2: Dataset[MyClass2] = ds.map(funcThatConvertsFromMyClass1ToMyClass2)
{code}
and in Java, imagining those case classes above were POJOs, we'd be able to
support the following?
{code}
Dataset<MyClass2> ds2 = ds1.map(funcThatConvertsFromMyClass1ToMyClass2,
MyClass2.class);
{code}
If that's the case, then that resolves my concerns above.
Lastly, though, IIUC, it seems like for all the common cases, we could register
an object with the SparkContext that converts from ClassTag to Encoder, and the
RDD API would work. Where does that break down?
> RDD-like API on top of Catalyst/DataFrame
> -----------------------------------------
>
> Key: SPARK-9999
> URL: https://issues.apache.org/jira/browse/SPARK-9999
> Project: Spark
> Issue Type: Story
> Components: SQL
> Reporter: Reynold Xin
> Assignee: Michael Armbrust
>
> The RDD API is very flexible, and as a result harder to optimize its
> execution in some cases. The DataFrame API, on the other hand, is much easier
> to optimize, but lacks some of the nice perks of the RDD API (e.g. harder to
> use UDFs, lack of strong types in Scala/Java).
> The goal of Spark Datasets is to provide an API that allows users to easily
> express transformations on domain objects, while also providing the
> performance and robustness advantages of the Spark SQL execution engine.
> h2. Requirements
> - *Fast* - In most cases, the performance of Datasets should be equal to or
> better than working with RDDs. Encoders should be as fast or faster than
> Kryo and Java serialization, and unnecessary conversion should be avoided.
> - *Typesafe* - Similar to RDDs, objects and functions that operate on those
> objects should provide compile-time safety where possible. When converting
> from data where the schema is not known at compile-time (for example data
> read from an external source such as JSON), the conversion function should
> fail-fast if there is a schema mismatch.
> - *Support for a variety of object models* - Default encoders should be
> provided for a variety of object models: primitive types, case classes,
> tuples, POJOs, JavaBeans, etc. Ideally, objects that follow standard
> conventions, such as Avro SpecificRecords, should also work out of the box.
> - *Java Compatible* - Datasets should provide a single API that works in
> both Scala and Java. Where possible, shared types like Array will be used in
> the API. Where not possible, overloaded functions should be provided for
> both languages. Scala concepts, such as ClassTags should not be required in
> the user-facing API.
> - *Interoperates with DataFrames* - Users should be able to seamlessly
> transition between Datasets and DataFrames, without specifying conversion
> boiler-plate. When names used in the input schema line-up with fields in the
> given class, no extra mapping should be necessary. Libraries like MLlib
> should not need to provide different interfaces for accepting DataFrames and
> Datasets as input.
> For a detailed outline of the complete proposed API:
> [marmbrus/dataset-api|https://github.com/marmbrus/spark/pull/18/files]
> For an initial discussion of the design considerations in this API: [design
> doc|https://docs.google.com/document/d/1ZVaDqOcLm2-NcS0TElmslHLsEIEwqzt0vBvzpLrV6Ik/edit#]
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