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Re: A question on the Flink "rolling" FoldFunction


Hi Min,

Please feel welcomed in the Flink community. One small remark, dev mailing list is for developers of Flink and all of the issues/discussions that arise in the process (discussing how to implement new feature etc), so user mailing list is the right one to ask questions about using Flink :)

Asking your question, Flink doesn’t guarantee order of the elements and especially not when it involves multiple parallel operators. The order can brake whenever there is some network exchange/transfer between operators/tasks, for example after using `keyBy`, `rebalance` or on the border of changing parallelism. Locally, within one instance of your Fold function, order is preserved. In other words, if you need some elements order to process records, you have to make sure that it’s there, for example:


env.addSrouce(….)
.keyBy(….)
.process(new MyElementsSorter())
.fold(…)    

However keep in mind that sorting records in an infinite data stream is a quite tricky concept unless it’s only a “best effort". Usually you would probably like to group records into a window and process those windows.

Piotrek

On 7 Dec 2018, at 09:12, min.tan@xxxxxxx wrote:

Hi,

 

I am new to Flink. I have a question on this "rolling" fold function.

 

If its parallelism is large than one, does the "rolling" order remains the same? i.e. it is always keep the "1-2-3-4-5" on an increasing sequence.

 

Regards,

 

Min

 

------------------------------------------------------------------- FoldFunction ---------------------------------------------------------------------------------------------------------------

A "rolling" fold on a keyed data stream with an initial value. Combines the current element with the last folded value and emits the new value. 

A fold function that, when applied on the sequence (1,2,3,4,5), emits the sequence "start-1", "start-1-2", "start-1-2-3", ...

DataStream<String> result =
  keyedStream.fold("start", new FoldFunction<Integer, String>() {
    @Override
    public String fold(String current, Integer value) {
        return current + "-" + value;
    }
  });

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
 
From: jincheng sun [mailto:sunjincheng121@xxxxxxxxx] 
Sent: Freitag, 7. Dezember 2018 02:24
To: rakkukumar2707@xxxxxxxxx
Cc: user@xxxxxxxxxxxxxxxx; dev@xxxxxxxxxxxxxxxx
Subject: [External] Re: delay one of the datastream when performing join operation on event-time and watermark
 
Hi Pakesh Kuma,
I think you can using the interval-join, e.g.:
orderStream
    .keyBy(<KeySelector>)
    .intervalJoin(invoiceStream.keyBy(<KeySelector>))
    .between(Time.minutes(-5), Time.minutes(5))
The semantics of interval-join and detailed usage description can refer to https://ci.apache.org/projects/flink/flink-docs-release-1.7/dev/stream/operators/joining.html#interval-join
 
Hope to help you, and any feedback is welcome!
 
Bests,
Jincheng
 
 
Rakesh Kumar <rakkukumar2707@xxxxxxxxx> 2018126日周四 下午7:10写道:
Hi,
I have two data sources one is  for order data and another one is for invoice data, these two data i am pushing into kafka topic in json form. I wanted to delay order data for 5 mins because invoice data comes only after order data is generated. So, for that i have written a flink program which will take these two data from kafka and apply watermarks and delay order data for 5 mins. After applying watermarks on these data, i wanted to join these data based on order_id which is present in both order and invoice data. After Joining i wanted to push it to kafka in different topic.
 
But, i am not able to join these data streams with 5 min delay and i am not able to figure it out.
 
I am attaching my flink program below and it's dependency.

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