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From Tathagata Das <tathagata.das1...@gmail.com>
Subject Re: RDD to DStream
Date Thu, 07 Aug 2014 01:32:03 GMT
Hey Aniket,

Great thoughts! I understand the usecase. But as you have realized yourself
it is not trivial to cleanly stream a RDD as a DStream. Since RDD
operations are defined to be scan based, it is not efficient to define RDD
based on slices of data within a partition of another RDD, using pure RDD
transformations. What you have done is a decent, and probably the only
feasible solution, with its limitations.

Also the requirements of converting a batch of data to a stream of data can
be pretty diverse. What rate, what # of events per batch, how many batches,
is it efficient? Hence, it is not trivial to define a good, clean public
API for that. If any one has any thoughts, ideas, etc on this, you are more
than welcome to share them.

TD


On Mon, Aug 4, 2014 at 12:43 AM, Aniket Bhatnagar <
aniket.bhatnagar@gmail.com> wrote:

> The use case for converting RDD into DStream is that I want to simulate a
> stream from an already persisted data for testing analytics. It is trivial
> to create a RDD from any persisted data but not so much for DStream.
> Therefore, my idea to create DStream from RDD. For example, lets say you
> are trying to implement analytics on time series data using Lambda
> architecture. This means you would have to implement the same analytics on
> streaming data (in streaming mode) as well as persisted data (in batch
> mode). The workflow for implementing the anlytics would be to first
> implement it in batch mode using RDD operations and then simulate stream to
> test the analytics in stream mode. The simulated stream should produce the
> elements at a specified rate. So the solution maybe to read data in a RDD,
> split (chunk) it into multiple RDDs with each RDD having the size of
> elements that need to be streamed per time unit and then finally stream
> each RDD using the compute function.
>
> The problem with using QueueInputDStream is that it will stream data as
> per the batch duration specified in the streaming context and one cannot
> specify a custom slide duration. Moreover, the class QueueInputDStream is
> private to streaming package, so I can't really use it/extend it from an
> external package. Also, I could not find a good solution split a RDD into
> equal sized smaller RDDs that can be fed into an extended version of
> QueueInputDStream.
>
> Finally, here is what I came up with:
>
> class RDDExtension[T: ClassTag](rdd: RDD[T]) {
>   def toStream(streamingContext: StreamingContext, chunkSize: Int,
> slideDurationMilli: Option[Long] = None): DStream[T] = {
>     new InputDStream[T](streamingContext) {
>
>       private val iterator = rdd.toLocalIterator // WARNING: each
> partition much fit in RAM of local machine.
>       private val grouped = iterator.grouped(chunkSize)
>
>       override def start(): Unit = {}
>
>       override def stop(): Unit = {}
>
>       override def compute(validTime: Time): Option[RDD[T]] = {
>         if (grouped.hasNext) {
>           Some(rdd.sparkContext.parallelize(grouped.next()))
>         } else {
>           None
>         }
>       }
>
>       override def slideDuration = {
>         slideDurationMilli.map(duration => new Duration(duration)).
>           getOrElse(super.slideDuration)
>       }
>     }
> }
>
> This aims to stream chunkSize elements every slideDurationMilli
> milliseconds (defaults to batch size in streaming context). It's still not
> perfect (for example, the streaming is not precise) but given that this
> will only be used for testing purposes, I don't look for ways to further
> optimize it.
>
> Thanks,
> Aniket
>
>
>
> On 2 August 2014 04:07, Mayur Rustagi <mayur.rustagi@gmail.com> wrote:
>
>> Nice question :)
>> Ideally you should use a queuestream interface to push RDD into a queue &
>> then spark streaming can handle the rest.
>> Though why are you looking to convert RDD to DStream, another workaround
>> folks use is to source DStream from folders & move files that they need
>> reprocessed back into the folder, its a hack but much less headache .
>>
>> Mayur Rustagi
>> Ph: +1 (760) 203 3257
>> http://www.sigmoidanalytics.com
>> @mayur_rustagi <https://twitter.com/mayur_rustagi>
>>
>>
>>
>> On Fri, Aug 1, 2014 at 10:21 AM, Aniket Bhatnagar <
>> aniket.bhatnagar@gmail.com> wrote:
>>
>>> Hi everyone
>>>
>>> I haven't been receiving replies to my queries in the distribution list.
>>> Not pissed but I am actually curious to know if my messages are actually
>>> going through or not. Can someone please confirm that my msgs are getting
>>> delivered via this distribution list?
>>>
>>> Thanks,
>>> Aniket
>>>
>>>
>>> On 1 August 2014 13:55, Aniket Bhatnagar <aniket.bhatnagar@gmail.com>
>>> wrote:
>>>
>>>> Sometimes it is useful to convert a RDD into a DStream for testing
>>>> purposes (generating DStreams from historical data, etc). Is there an easy
>>>> way to do this?
>>>>
>>>> I could come up with the following inefficient way but no sure if there
>>>> is a better way to achieve this. Thoughts?
>>>>
>>>> class RDDExtension[T](rdd: RDD[T]) {
>>>>
>>>>   def chunked(chunkSize: Int): RDD[Seq[T]] = {
>>>>     rdd.mapPartitions(partitionItr => partitionItr.grouped(chunkSize))
>>>>   }
>>>>
>>>>   def skipFirst(): RDD[T] = {
>>>>     rdd.zipWithIndex().filter(tuple => tuple._2 > 0).map(_._1)
>>>>   }
>>>>
>>>>   def toStream(streamingContext: StreamingContext, chunkSize: Int,
>>>> slideDurationMilli: Option[Long] = None): DStream[T] = {
>>>>     new InputDStream[T](streamingContext) {
>>>>
>>>>       @volatile private var currentRDD: RDD[Seq[T]] =
>>>> rdd.chunked(chunkSize)
>>>>
>>>>       override def start(): Unit = {}
>>>>
>>>>       override def stop(): Unit = {}
>>>>
>>>>       override def compute(validTime: Time): Option[RDD[T]] = {
>>>>         val chunk = currentRDD.take(1)
>>>>         currentRDD = currentRDD.skipFirst()
>>>>         Some(rdd.sparkContext.parallelize(chunk))
>>>>       }
>>>>
>>>>       override def slideDuration = {
>>>>         slideDurationMilli.map(duration => new Duration(duration)).
>>>>           getOrElse(super.slideDuration)
>>>>       }
>>>>     }
>>>>
>>>> }
>>>>
>>>
>>>
>>
>

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