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From Stephan Ewen <se...@apache.org>
Subject Re: Working with State example /flink streaming
Date Thu, 26 Nov 2015 16:14:21 GMT
Hi!

In streaming, there is no "end" of the stream when you would emit the final
sum. That's why there are windows.

If you do not want the partial sums, but only the final sum, you need to
define what window in which the sum is computed. At the end of that window,
that value is emitted. The window can be based on time, counts, or other
measures.

Greetings,
Stephan


On Thu, Nov 26, 2015 at 4:07 PM, Lopez, Javier <javier.lopez@zalando.de>
wrote:

> Hi, thanks for the answer. It worked but not in the way we expected. We
> expect to have only one sum per ID and we are getting all the consecutive
> sums, for example:
>
> We expect this: (11,6) but we get this (11,1) (11,3) (11,6) (the initial
> values are ID -> 11, values -> 1,2,3). Here is the code we are using for
> our test:
>
> DataStream<Tuple2<String, Double>> stream = ...;DataStream<Tuple4<String,
Double, Long, Double>> result = stream.keyBy(0).map(new RollingSum());
>
> public static class RollingSum extends RichMapFunction<Tuple2<String, Double>,
Tuple4<String, Double, Long, Double>> {
>
>         // persistent counter
>     	private OperatorState<Double> sum;
>     	private OperatorState<Long> count;
>     	
>
>         @Override
>         public Tuple4<String, Double, Long, Double> map(Tuple2<String, Double>
value1) {
>         	try {
>         		Double newSum = sum.value()+value1.f1;
>         		
> 				sum.update(newSum);
> 				count.update(count.value()+1);
> 				return new Tuple4<String, Double, Long, Double>(value1.f0,sum.value(),count.value(),sum.value()/count.value());
> 			} catch (IOException e) {
> 				// TODO Auto-generated catch block
> 				e.printStackTrace();
> 			}
>
>         	return null;
>
>         }
>
>         @Override
>         public void open(Configuration config) {
>             sum = getRuntimeContext().getKeyValueState("mySum", Double.class, 0D);
>             count = getRuntimeContext().getKeyValueState("myCounter", Long.class, 0L);
>         }
>
>     }
>
>
> We are using a Tuple4 because we want to calculate the sum and the average
> (So our Tuple is ID, SUM, Count, AVG). Do we need to add another step to
> get a single value out of it? or is this the expected behavior.
>
> Thanks again for your help.
>
> On 25 November 2015 at 17:19, Stephan Ewen <sewen@apache.org> wrote:
>
>> Hi Javier!
>>
>> You can solve this both using windows, or using manual state.
>>
>> What is better depends a bit on when you want to have the result (the
>> sum). Do you want a result emitted after each update (or do some other
>> operation with that value) or do you want only the final sum after a
>> certain time?
>>
>> For the second variant, I would use a window, for the first variant, you
>> could use custom state as follows:
>>
>> For each element, you take the current state for the key, add the value
>> to get the new sum. Then you update the state with the new sum and emit the
>> value as well...
>>
>> Java:
>>
>> DataStream<Tuple2<String, Long>> stream = ...;DataStream<Tuple2<String,
Long>> result = stream.keyBy(0).map(new RollingSum());
>>
>>
>> public class RollingSum extends RichMapFunction<Tuple2<String, Long>, Tuple2<String,
Long>> {
>>
>>     private OperatorState<Long> sum;
>>
>>     @Override
>>     public Tuple2<String, Long> map(Tuple2<String, Long> value) {
>>         *long *newSum = sum.value() + value.f1;        sum.update(newSum);
>>         return *new* Tuple2<>(value.f0, newSum);
>>     }
>>
>>     @Override
>>     public void open(Configuration config) {
>>         counter = getRuntimeContext().getKeyValueState("myCounter", Long.class, 0L);
>>     }}
>>
>>
>>
>> In Scala, you can write this briefly as:
>>
>> val stream: DataStream[(String, Int)] = *...*
>> val counts: DataStream[(String, Int)] = stream
>>   .keyBy(_._1)
>>   .mapWithState((in: (String, Int), sum: Option[Int]) => {    *val* newSum = in._2
+ sum.getOrElse(0)
>>     ( (in._1, newSum), Some(newSum) )
>>  }
>>
>>
>> Does that help?
>>
>> Thanks also for pointing out the error in the sample code...
>>
>> Greetings,
>> Stephan
>>
>>
>> On Wed, Nov 25, 2015 at 4:55 PM, Lopez, Javier <javier.lopez@zalando.de>
>> wrote:
>>
>>> Hi,
>>>
>>> We are trying to do a test using States but we have not been able to
>>> achieve our desired result. Basically we have a data stream with data as
>>> [{"id":"11","value":123}] and we want to calculate the sum of all values
>>> grouping by ID. We were able to achieve this using windows but not with
>>>  states. The example that is in the documentation (
>>> https://ci.apache.org/projects/flink/flink-docs-master/apis/streaming_guide.html#working-with-state)
>>> is not very clear and even has some errors, for example:
>>>
>>> public class CounterSum implements RichReduceFunction<Long>
>>>
>>> should be
>>>
>>> public class CounterSum extends RichReduceFunction<Long>
>>>
>>> as RichReduceFuncion is a Class, not an interface.
>>>
>>> We wanted to ask you if you have an example of how to use States with
>>> Flink.
>>>
>>> Thanks in advance for your help.
>>>
>>>
>>>
>>
>>
>

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