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From Wenchen Fan <cloud0...@gmail.com>
Subject Re: [discuss][data source v2] remove type parameter in DataReader/WriterFactory
Date Tue, 17 Apr 2018 06:17:41 GMT
Yea definitely not. The only requirement is, the DataReader/WriterFactory
must support at least one DataFormat.

>  how are we going to express capability of the given reader of its
supported format(s), or specific support for each of “real-time data in row
format, and history data in columnar format”?

When DataSourceReader/Writer create factories, the factory must contain
enough information to decide the data format. Let's take ORC as an example.
In OrcReaderFactory, it knows which files to read, and which columns to
output. Since now Spark only support columnar scan for simple types,
OrcReaderFactory will only output ColumnarBatch if the columns to scan are
all simple types.

On Tue, Apr 17, 2018 at 11:38 AM, Felix Cheung <felixcheung_m@hotmail.com>
wrote:

> Is it required for DataReader to support all known DataFormat?
>
> Hopefully, not, as assumed by the ‘throw’ in the interface. Then
> specifically how are we going to express capability of the given reader of
> its supported format(s), or specific support for each of “real-time data in
> row format, and history data in columnar format”?
>
>
> ------------------------------
> *From:* Wenchen Fan <cloud0fan@gmail.com>
> *Sent:* Sunday, April 15, 2018 7:45:01 PM
> *To:* Spark dev list
> *Subject:* [discuss][data source v2] remove type parameter in
> DataReader/WriterFactory
>
> Hi all,
>
> I'd like to propose an API change to the data source v2.
>
> One design goal of data source v2 is API type safety. The FileFormat API
> is a bad example, it asks the implementation to return InternalRow even
> it's actually ColumnarBatch. In data source v2 we add a type parameter to
> DataReader/WriterFactoty and DataReader/Writer, so that data source
> supporting columnar scan returns ColumnarBatch at API level.
>
> However, we met some problems when migrating streaming and file-based data
> source to data source v2.
>
> For the streaming side, we need a variant of DataReader/WriterFactory to
> add streaming specific concept like epoch id and offset. For details please
> see ContinuousDataReaderFactory and https://docs.google.com/document/d/
> 1PJYfb68s2AG7joRWbhrgpEWhrsPqbhyRwUVl9V1wPOE/edit#
>
> But this conflicts with the special format mixin traits like
> SupportsScanColumnarBatch. We have to make the streaming variant of
> DataReader/WriterFactory to extend the original DataReader/WriterFactory,
> and do type cast at runtime, which is unnecessary and violate the type
> safety.
>
> For the file-based data source side, we have a problem with code
> duplication. Let's take ORC data source as an example. To support both
> unsafe row and columnar batch scan, we need something like
>
> // A lot of parameters to carry to the executor side
> class OrcUnsafeRowFactory(...) extends DataReaderFactory[UnsafeRow] {
>   def createDataReader ...
> }
>
> class OrcColumnarBatchFactory(...) extends DataReaderFactory[ColumnarBatch]
> {
>   def createDataReader ...
> }
>
> class OrcDataSourceReader extends DataSourceReader {
>   def createUnsafeRowFactories = ... // logic to prepare the parameters
> and create factories
>
>   def createColumnarBatchFactories = ... // logic to prepare the
> parameters and create factories
> }
>
> You can see that we have duplicated logic for preparing parameters and
> defining the factory.
>
> Here I propose to remove all the special format mixin traits and change
> the factory interface to
>
> public enum DataFormat {
>   ROW,
>   INTERNAL_ROW,
>   UNSAFE_ROW,
>   COLUMNAR_BATCH
> }
>
> interface DataReaderFactory {
>   DataFormat dataFormat;
>
>   default DataReader<Row> createRowDataReader() {
>     throw new IllegalStateException();
>   }
>
>   default DataReader<UnsafeRow> createUnsafeRowDataReader() {
>     throw new IllegalStateException();
>   }
>
>   default DataReader<ColumnarBatch> createColumnarBatchDataReader() {
>     throw new IllegalStateException();
>   }
> }
>
> Spark will look at the dataFormat and decide which create data reader
> method to call.
>
> Now we don't have the problem for the streaming side as these special
> format mixin traits go away. And the ORC data source can also be simplified
> to
>
> class OrcReaderFactory(...) extends DataReaderFactory {
>   def createUnsafeRowReader ...
>
>   def createColumnarBatchReader ...
> }
>
> class OrcDataSourceReader extends DataSourceReader {
>   def createReadFactories = ... // logic to prepare the parameters and
> create factories
> }
>
> We also have a potential benefit of supporting hybrid storage data source,
> which may keep real-time data in row format, and history data in columnar
> format. Then they can make some DataReaderFactory output InternalRow and
> some output ColumnarBatch.
>
> Thoughts?
>

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