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From Alexis Sarda <alexis.sa...@gmail.com>
Subject Re: JDBCInputFormat and SplitDataProperties
Date Fri, 10 Aug 2018 10:04:51 GMT
It seems I may have spoken too soon. After executing the job with more
data, I can see the following things in the Flink dashboard:

- The first subtask is a chained DataSource -> GroupCombine. Even with
parallelism set to 24 and a ParameterValuesProvider returning
Array(Array("first"), Array("second")), only 1 thread processed all records.
- The second subtask is a Sorted Group Reduce, and I see two weird things:
  + The first subtask sent 5,923,802 records, yet the second subtask only
received 5,575,154 records?
  + Again, everything was done in a single thread, even though a groupBy
was used.
- The third and final subtask is a sink that saves back to the database.

Does anyone know why parallelism is not being used?


On Thu, Aug 9, 2018 at 11:22 AM Alexis Sarda <alexis.sarda@gmail.com> wrote:

> Hi Fabian,
> Thanks a lot for the help. The scala DataSet, at least in version 1.5.0,
> declares javaSet as private[flink], so I cannot access it directly.
> Nevertheless, I managed to get around it by using the java environment:
> val env = org.apache.flink.api.java.ExecutionEnvironment.
> getExecutionEnvironment
> val inputFormat = getInputFormat(query, dbUrl, properties)
> val outputFormat = getOutputFormat(dbUrl, properties)
> val source = env.createInput(inputFormat)
> val sdp = source.getSplitDataProperties
> sdp.splitsPartitionedBy(0)
> sdp.splitsOrderedBy(Array(1), Array(Order.ASCENDING))
> // transform java DataSet to scala DataSet...
> new DataSet(source.asInstanceOf[org.apache.flink.api.java.DataSet[Row]])
>   .groupBy(0, 1)
>   .combineGroup(groupCombiner)
>   .withForwardedFields("f0->_1")
>   .groupBy(0, 1)
>   .reduceGroup(groupReducer)
>   .withForwardedFields("_1")
>   .output(outputFormat)
> It seems to work well, and the semantic annotation does remove a hash
> partition from the execution plan.
> Regards,
> Alexis.
> On Thu, Aug 9, 2018 at 10:27 AM Fabian Hueske <fhueske@gmail.com> wrote:
>> Hi Alexis,
>> The Scala API does not expose a DataSource object but only a Scala
>> DataSet which wraps the Java object.
>> You can get the SplitDataProperties from the Scala DataSet as follows:
>> val dbData: DataSet[...] = ???
>> val sdp = dbData.javaSet.asInstanceOf[DataSource].getSplitDataProperties
>> So you first have to get the wrapped Java DataSet, cast it to DataSource
>> and then get the properties.
>> It's not very nice, but should work.
>> In order to use SDPs, you should be a bit familiar how physical data
>> properties are propagated and discarded in the optimizer.
>> For example, applying a simple MapFunction removes all properties because
>> the function might have changed the fields on which a DataSet is
>> partitioned or sorted.
>> You can expose the behavior of a function to the optimizer by using
>> Semantic Annotations [1]
>> Some comments on the code and plan you shared:
>> - You might want to add hostname to ORDER BY to have the output grouped
>> by (ts, hostname).
>> - Check the Global and Local data properties in the plan to validate that
>> the SDP were correctly interpreted.
>> - If the data is already correctly partitioned and sorted, you might not
>> need the Combiners. In either case, you properly want to annotate them with
>> Forward Field annoations.
>> The number of source tasks is unrelated to the number of splits. If you
>> have more tasks than splits, some tasks won't process any data.
>> Best, Fabian
>> [1]
>> https://ci.apache.org/projects/flink/flink-docs-release-1.5/dev/batch/#semantic-annotations
>> 2018-08-08 14:10 GMT+02:00 Alexis Sarda <alexis.sarda@gmail.com>:
>>> Hi Fabian,
>>> Thanks for the clarification. I have a few remarks, but let me provide
>>> more concrete information. You can find the query I'm using, the
>>> JDBCInputFormat creation, and the execution plan in this github gist:
>>> https://gist.github.com/asardaes/8331a117210d4e08139c66c86e8c952d
>>> I cannot call getSplitDataProperties because
>>> env.createInput(inputFormat) returns a DataSet, not a DataSource. In the
>>> code, I do this instead:
>>> val javaEnv =
>>> org.apache.flink.api.java.ExecutionEnvironment.getExecutionEnvironment
>>> val dataSource = new DataSource(javaEnv, inputFormat, rowTypeInfo,
>>> "example")
>>> which feels wrong (the constructor doesn't accept a Scala environment).
>>> Is there a better alternative?
>>> I see absolutely no difference in the execution plan whether I use SDP
>>> or not, so therefore the results are indeed the same. Is this expected?
>>> My ParameterValuesProvider specifies 2 splits, yet the execution plan
>>> shows Parallelism=24. Even the source code is a bit ambiguous, considering
>>> that the constructor for GenericInputSplit takes two parameters:
>>> partitionNumber and totalNumberOfPartitions. Should I assume that there are
>>> 2 splits divided into 24 partitions?
>>> Regards,
>>> Alexis.
>>> On Wed, Aug 8, 2018 at 11:57 AM Fabian Hueske <fhueske@gmail.com> wrote:
>>>> Hi Alexis,
>>>> First of all, I think you leverage the partitioning and sorting
>>>> properties of the data returned by the database using SplitDataProperties.
>>>> However, please be aware that SplitDataProperties are a rather
>>>> experimental feature.
>>>> If used without query parameters, the JDBCInputFormat generates a
>>>> single split and queries the database just once. If you want to leverage
>>>> parallelism, you have to specify a query with parameters in the WHERE
>>>> clause to read different parts of the table.
>>>> Note, depending on the configuration of the database, multiple queries
>>>> result in multiple full scans. Hence, it might make sense to have an index
>>>> on the partitioning columns.
>>>> If properly configured, the JDBCInputFormat generates multiple splits
>>>> which are partitioned. Since the partitioning is encoded in the query, it
>>>> is opaque to Flink and must be explicitly declared.
>>>> This can be done with SDPs. The SDP.splitsPartitionedBy() method tells
>>>> Flink that all records with the same value in the partitioning field are
>>>> read from the same split, i.e, the full data is partitioned on the
>>>> attribute across splits.
>>>> The same can be done for ordering if the queries of the JDBCInputFormat
>>>> is specified with an ORDER BY clause.
>>>> Partitioning and grouping are two different things. You can define a
>>>> query that partitions on hostname and orders by hostname and timestamp and
>>>> declare these properties in the SDP.
>>>> You can get a SDP object by calling
>>>> DataSource.getSplitDataProperties(). In your example this would be
>>>> source.getSplitDataProperties().
>>>> Whatever you do, you should carefully check the execution plan
>>>> (ExecutionEnvironment.getExecutionPlan()) using the plan visualizer [1] and
>>>> validate that the result are identical whether you use SDP or not.
>>>> Best, Fabian
>>>> [1] https://flink.apache.org/visualizer/
>>>> 2018-08-07 22:32 GMT+02:00 Alexis Sarda <alexis.sarda@gmail.com>:
>>>>> Hi everyone,
>>>>> I have the following scenario: I have a database table with 3 columns:
>>>>> a host (string), a timestamp, and an integer ID. Conceptually, what I'd
>>>>> like to do is:
>>>>> group by host and timestamp -> based on all the IDs in each group,
>>>>> create a mapping to n new tuples -> for each unique tuple, count how
>>>>> times it appeared across the resulting data
>>>>> Each new tuple has 3 fields: the host, a new ID, and an Integer=1
>>>>> What I'm currently doing is roughly:
>>>>> val input = JDBCInputFormat.buildJDBCInputFormat()...finish()
>>>>> val source = environment.createInput(inut)
>>>>> source.partitionByHash("host",
>>>>> "timestamp").mapPartition(...).groupBy(0, 1).aggregate(SUM, 2)
>>>>> The query given to JDBCInputFormat provides results ordered by host
>>>>> and timestamp, and I was wondering if performance can be improved by
>>>>> specifying this in the code. I've looked at
>>>>> http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Terminology-Split-Group-and-Partition-td11030.html
>>>>> and
>>>>> http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/Fwd-Processing-Sorted-Input-Datasets-td20038.html,
>>>>> but I still have some questions:
>>>>> - If a split is a subset of a partition, what is the meaning of
>>>>> SplitDataProperties#splitsPartitionedBy? The wording makes me thing that
>>>>> split is divided into partitions, meaning that a partition would be a
>>>>> subset of a split.
>>>>> - At which point can I retrieve and adjust a SplitDataProperties
>>>>> instance, if possible at all?
>>>>> - If I wanted a coarser parallelization where each slot gets all the
>>>>> data for the same host, would I have to manually create the sub-groups
>>>>> based on timestamp?
>>>>> Regards,
>>>>> Alexis.

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