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From Wenchen Fan <cloud0...@gmail.com>
Subject Re: [discuss][data source v2] remove type parameter in DataReader/WriterFactory
Date Thu, 19 Apr 2018 05:20:07 GMT
 First of all, I think we all agree that data source v2 API should at least
support InternalRow and ColumnarBatch. With this assumption, the current
API has 2 problems:

*First problem*: We use mixin traits to add support for different data
formats.

The mixin traits define API to return DataReader/WriterFactory for
different formats. It brings a lot of trouble to streaming, as streaming
has its own factory interface, which we don't want it to extend the batch
factory. This means we need to duplicate the mixin traits for batch and
streaming. Keep in mind that duplicating the traits is also a possible
solution, if there is no better way.

Another possible solution is, remove the mixin traits and put all
"createFactory" method in DataSourceReader/Writer, with a new method to
indicate which "createFactory" method Spark should call. Then the API looks
like

interface DataSourceReader {
  DataFormat dataFormat;

  default List<DataReaderFactory<Row>> createDataReaderFactories() {
    throw new IllegalStateException();
  }

  default List<DataReaderFactory<ColumnarBatch>>
createColumnarBatchDataReaderFactories() {
    throw new IllegalStateException();
  }
}

or to be more friendly to people who don't care about columnar format

interface DataSourceReader {
  default DataFormat dataFormat { return DataFormat.INTERNAL_ROW };

  List<DataReaderFactory<Row>> createDataReaderFactories();

  default List<DataReaderFactory<ColumnarBatch>>
createColumnarBatchDataReaderFactories()
{
    throw new IllegalStateException();
  }
}

This solution still brings some trouble to streaming, as the streaming
specific DataSourceReader needs to re-define all these "createFactory"
methods, but it's much better than duplicating the mixin traits.

*Second problem*: The DataReader/WriterFactory may have a lot of
constructor parameters, it's painful to define different factories with the
same but very long parameter list.
After a closer look, I think this is the major part of the duplicated code.
This is not a strong reason, so it's OK if people don't think it's a
problem. In the meanwhile, I think it might be better to shift the data
format stuff to the factory so that we can support hybrid storage data
source in the future, like I mentioned before.


Finally, we can also consider Joseph's proposal, to remove the type
parameter entirely and get rid of this problem.



On Thu, Apr 19, 2018 at 8:54 AM, Joseph Torres <joseph.torres@databricks.com
> wrote:

> The fundamental difficulty seems to be that there's a spurious
> "round-trip" in the API. Spark inspects the source to determine what type
> it's going to provide, picks an appropriate method according to that type,
> and then calls that method on the source to finally get what it wants.
> Pushing this out of the DataSourceReader doesn't eliminate this problem; it
> just shifts it. We still need an InternalRow method and a ColumnarBatch
> method and possibly Row and UnsafeRow methods too.
>
> I'd propose it would be better to just accept a bit less type safety here,
> and push the problem all the way down to the DataReader. Make
> DataReader.get() return Object, and document that the runtime type had
> better match the type declared in the reader's DataFormat. Then we can get
> rid of the special Row/UnsafeRow/ColumnarBatch methods cluttering up the
> API, and figure out whether to support Row and UnsafeRow independently of
> all our other API decisions. (I didn't think about this until now, but the
> fact that some orthogonal API decisions have to be conditioned on which set
> of row formats we support seems like a code smell.)
>
> On Wed, Apr 18, 2018 at 3:53 PM, Ryan Blue <rblue@netflix.com.invalid>
> wrote:
>
>> Wenchen, can you explain a bit more clearly why this is necessary? The
>> pseudo-code you used doesn’t clearly demonstrate why. Why couldn’t this be
>> handled this with inheritance from an abstract Factory class? Why define
>> all of the createXDataReader methods, but make the DataFormat a field in
>> the factory?
>>
>> A related issue is that I think there’s a strong case that the v2 sources
>> should produce only InternalRow and that Row and UnsafeRow shouldn’t be
>> exposed; see SPARK-23325
>> <https://issues.apache.org/jira/browse/SPARK-23325>. The basic arguments
>> are:
>>
>>    - UnsafeRow is really difficult to produce without using Spark’s
>>    projection methods. If implementations can produce UnsafeRow, then
>>    they can still pass them as InternalRow and the projection Spark adds
>>    would be a no-op. When implementations can’t produce UnsafeRow, then
>>    it is better for Spark to insert the projection to unsafe. An example of a
>>    data format that doesn’t produce unsafe is the built-in Parquet source,
>>    which produces InternalRow and projects before returning the row.
>>    - For Row, I see no good reason to support it in a new interface when
>>    it will just introduce an extra transformation. The argument that Row
>>    is the “public” API doesn’t apply because UnsafeRow is already
>>    exposed through the v2 API.
>>    - Standardizing on InternalRow would remove the need for these
>>    interfaces entirely and simplify what implementers must provide and would
>>    reduce confusion over what to do.
>>
>> Using InternalRow doesn’t cover the case where we want to produce
>> ColumnarBatch instead, so what you’re proposing might still be a good
>> idea. I just think that we can simplify either path.
>> ​
>>
>> On Mon, Apr 16, 2018 at 11:17 PM, Wenchen Fan <cloud0fan@gmail.com>
>> wrote:
>>
>>> 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/do
>>>> cument/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?
>>>>
>>>
>>>
>>
>>
>> --
>> Ryan Blue
>> Software Engineer
>> Netflix
>>
>
>

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