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From Wenchen Fan <cloud0...@gmail.com>
Subject Re: [discuss] Data Source V2 write path
Date Mon, 25 Sep 2017 02:17:10 GMT
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/apach
>> e/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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