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From Reynold Xin <r...@databricks.com>
Subject Re: [discuss] Data Source V2 write path
Date Mon, 25 Sep 2017 03:14:54 GMT
Can there be an explicit create function?


On Sun, Sep 24, 2017 at 7:17 PM, Wenchen Fan <cloud0fan@gmail.com> wrote:

> I agree it would be a clean approach if data source is only responsible to
> write into an already-configured table. However, without catalog
> federation, Spark doesn't have an API to ask an external system(like
> Cassandra) to create a table. Currently it's all done by data source write
> API. Data source implementations are responsible to create or insert a
> table according to the save mode.
>
> As a workaround, I think it's acceptable to pass partitioning/bucketing
> information via data source options, and data sources should decide to take
> these informations and create the table, or throw exception if these
> informations don't match the already-configured table.
>
>
> On Fri, Sep 22, 2017 at 9:35 AM, Ryan Blue <rblue@netflix.com> wrote:
>
>> > input data requirement
>>
>> Clustering and sorting within partitions are a good start. We can always
>> add more later when they are needed.
>>
>> The primary use case I'm thinking of for this is partitioning and
>> bucketing. If I'm implementing a partitioned table format, I need to tell
>> Spark to cluster by my partition columns. Should there also be a way to
>> pass those columns separately, since they may not be stored in the same way
>> like partitions are in the current format?
>>
>> On Wed, Sep 20, 2017 at 3:10 AM, Wenchen Fan <cloud0fan@gmail.com> wrote:
>>
>>> Hi all,
>>>
>>> I want to have some discussion about Data Source V2 write path before
>>> starting a voting.
>>>
>>> The Data Source V1 write path asks implementations to write a DataFrame
>>> directly, which is painful:
>>> 1. Exposing upper-level API like DataFrame to Data Source API is not
>>> good for maintenance.
>>> 2. Data sources may need to preprocess the input data before writing,
>>> like cluster/sort the input by some columns. It's better to do the
>>> preprocessing in Spark instead of in the data source.
>>> 3. Data sources need to take care of transaction themselves, which is
>>> hard. And different data sources may come up with a very similar approach
>>> for the transaction, which leads to many duplicated codes.
>>>
>>>
>>> To solve these pain points, I'm proposing a data source writing
>>> framework which is very similar to the reading framework, i.e.,
>>> WriteSupport -> DataSourceV2Writer -> WriteTask -> DataWriter. You can
take
>>> a look at my prototype to see what it looks like:
>>> https://github.com/apache/spark/pull/19269
>>>
>>> There are some other details need further discussion:
>>> 1. *partitioning/bucketing*
>>> Currently only the built-in file-based data sources support them, but
>>> there is nothing stopping us from exposing them to all data sources. One
>>> question is, shall we make them as mix-in interfaces for data source v2
>>> reader/writer, or just encode them into data source options(a
>>> string-to-string map)? Ideally it's more like options, Spark just transfers
>>> these user-given informations to data sources, and doesn't do anything for
>>> it.
>>>
>>> 2. *input data requirement*
>>> Data sources should be able to ask Spark to preprocess the input data,
>>> and this can be a mix-in interface for DataSourceV2Writer. I think we need
>>> to add clustering request and sorting within partitions request, any more?
>>>
>>> 3. *transaction*
>>> I think we can just follow `FileCommitProtocol`, which is the internal
>>> framework Spark uses to guarantee transaction for built-in file-based data
>>> sources. Generally speaking, we need task level and job level commit/abort.
>>> Again you can see more details in my prototype about it:
>>> https://github.com/apache/spark/pull/19269
>>>
>>> 4. *data source table*
>>> This is the trickiest one. In Spark you can create a table which points
>>> to a data source, so you can read/write this data source easily by
>>> referencing the table name. Ideally data source table is just a pointer
>>> which points to a data source with a list of predefined options, to save
>>> users from typing these options again and again for each query.
>>> If that's all, then everything is good, we don't need to add more
>>> interfaces to Data Source V2. However, data source tables provide special
>>> operators like ALTER TABLE SCHEMA, ADD PARTITION, etc., which requires data
>>> sources to have some extra ability.
>>> Currently these special operators only work for built-in file-based data
>>> sources, and I don't think we will extend it in the near future, I propose
>>> to mark them as out of the scope.
>>>
>>>
>>> Any comments are welcome!
>>> Thanks,
>>> Wenchen
>>>
>>
>>
>>
>> --
>> Ryan Blue
>> Software Engineer
>> Netflix
>>
>
>

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