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From Aljoscha Krettek <aljos...@apache.org>
Subject Re: streaming join implementation
Date Thu, 14 Apr 2016 16:05:28 GMT
Hi,
right now, Flink does not give you a way to get the the records that where
not joined for a join. You can, however use a co-group operation instead of
a join to figure out which records did not join with records from the other
side and treat them separately.

Let me show an example:

val input1: DataStream[A] = ...
val input2: DataStream[B] = ...

val result = input1.coGroup(input2)
  .where(_.key1)
  .equalTo(_.key2)
  .window(TumblingTimeWindows.of(Time.days(1)))
  .apply(new MyCoGroupFunction)

class MyCoGroupFunction {
  void coGroup(Iterable[A] first, Iterable[B] second, Collector[O] out) {
    if (!first.iterator().hasNext()) {
      // no element from first input matched
      out.collect(<message telling that I only have second input elements>)
    } else if (!second.iterator().hasNext()) {
            out.collect(<message telling that I only have first input
elements>)
    } else {
       // perform the actual join using the two iterables
    }
  }
}

The result will be a stream that contains both join results as well as the
elements telling you that something didn't join. You can process this
stream further by splitting it into different streams of only proper join
results and non-joined elements and so on.

I hope this helps somewhat.

Cheers,
Aljoscha
On Thu, 14 Apr 2016 at 08:55 Balaji Rajagopalan <
balaji.rajagopalan@olacabs.com> wrote:

> Let me give you specific example, say stream1 event1 happened within your
> window 0-5 min with key1, and event2 on stream2 with key2 which could have
> matched with key1 happened at 5:01 outside the join window, so now you will
> have to co-relate the event2 on stream2 with the event1 with stream1 which
> has happened on the previous window, this was the corner case I mentioned
> before. I am not aware if flink can solve this problem for you, that would
> be nice, instead of solving this in application.
>
> On Thu, Apr 14, 2016 at 12:10 PM, Henry Cai <hcai@pinterest.com> wrote:
>
>> Thanks Balaji.  Do you mean you spill the non-matching records after 5
>> minutes into redis?  Does flink give you control on which records is not
>> matching in the current window such that you can copy into a long-term
>> storage?
>>
>>
>>
>> On Wed, Apr 13, 2016 at 11:20 PM, Balaji Rajagopalan <
>> balaji.rajagopalan@olacabs.com> wrote:
>>
>>> You can implement join in flink (which is a inner join) the below
>>> mentioned pseudo code . The below join is for a 5 minute interval, yes will
>>> be some corners cases when the data coming after 5 minutes will be  missed
>>> out in the join window, I actually had solved this problem but storing some
>>> data in redis and wrote correlation logic to take care of the corner cases
>>> that were missed out in the join  window.
>>>
>>> val output: DataStream[(OutputData)] = stream1.join(stream2).where(_.key1).equalTo(_.key2).
>>>   window(TumblingEventTimeWindows.of(Time.of(5, TimeUnit.MINUTE))).apply(new
SomeJoinFunction)
>>>
>>>
>>> On Thu, Apr 14, 2016 at 10:02 AM, Henry Cai <hcai@pinterest.com> wrote:
>>>
>>>> Hi,
>>>>
>>>> We are evaluating different streaming platforms.  For a typical join
>>>> between two streams
>>>>
>>>> select a.*, b.*
>>>> FROM a, b
>>>> ON a.id == b.id
>>>>
>>>> How does flink implement the join?  The matching record from either
>>>> stream can come late, we consider it's a valid join as long as the event
>>>> time for record a and b are in the same day.
>>>>
>>>> I think some streaming platform (e.g. google data flow) will store the
>>>> records from both streams in a K/V lookup store and later do the lookup.
>>>> Is this how flink implement the streaming join?
>>>>
>>>> If we need to store all the records in a state store, that's going to
>>>> be a lots of records for a day.
>>>>
>>>>
>>>
>>
>

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