Thanks for the feedback. I improved the javadoc a bit, do you like it better?
/**
* Perform an action on each record of {@code KStream}.
* This is a stateless record-by-record operation (cf. {@link
#process(ProcessorSupplier, String...)}).
*
* Peek is a non-terminal operation that triggers a side effect (such as
logging or statistics collection)
* and returns an unchanged stream.
*
* Note that since this operation is stateless, it may execute multiple
times for a single record in failure cases.
*
* @param action an action to perform on each record
* @see #process(ProcessorSupplier, String...)
*/
KStream<K, V> peek(final ForeachAction<? super K, ? super V> action);
Updated in-place on the PR.
> On Feb 7, 2017, at 2:19 AM, Michael Noll <mich...@confluent.io> wrote:
>
> Many thanks for the KIP and the PR, Steven!
>
> My opinion, too, is that we should consider including this.
>
> One thing that I would like to see clarified is the difference between the
> proposed peek() and existing functions map() and foreach(), for instance.
> My understanding (see also the Java 8 links below) is that:
>
> - Like `map`, `peek` will return a KStream. This also means that, unlike
> `foreach`, `peek` is not a terminal operation.
> - The main purpose of `peek` is, similar to `foreach`, the *side effects*
> (such as the metrics counter example in the KIP) -- and, on a related note,
> also to express your *intent* to achieve such side effects in the first
> place (which is similar to when to use `foreach` rather than `map`); and
> typically you should not (must not?) modify the underlying stream itself
> (unlike `map`, which is supposed to do exactly that).
>
> For reference, here are the descriptions of peek, map, foreach in Java 8.
> I could have also included links to StackOverflow questions where people
> were confused about when (not) to use peek. ;-)
>
> https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#peek-java.util.function.Consumer-
> https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#map-java.util.function.Function-
> https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html#forEach-java.util.function.Consumer-
>
> Best wishes,
> Michael
>
>
>
>
>
>
> On Tue, Feb 7, 2017 at 10:37 AM, Damian Guy <damian....@gmail.com> wrote:
>
>> Hi Steven,
>> Thanks for the KIP. I think this is a worthy addition to the API.
>>
>> Thanks,
>> Damian
>>
>> On Tue, 7 Feb 2017 at 09:30 Eno Thereska <eno.there...@gmail.com> wrote:
>>
>>> Hi,
>>>
>>> I like the proposal, thank you. I have found it frustrating myself not to
>>> be able to understand simple things, like how many records have been
>>> currently processed. The peek method would allow those kinds of
>> diagnostics
>>> and debugging.
>>>
>>> Gwen, it is possible to do this with the existing functionality like map,
>>> but you'd have to fake the map method. Also, it is not great using map
>> for
>>> things it was not intended for. Having an explicit peek makes it clearer
>> in
>>> my opinion.
>>>
>>> Thanks
>>> Eno
>>>
>>>> On 7 Feb 2017, at 03:20, Gwen Shapira <g...@confluent.io> wrote:
>>>>
>>>> I've read the wiki and am unclear about the proposal. Can you provide
>>>> something like a Javadoc for peek()? What would this method do?
>>>>
>>>> Also, forgive me if I'm missing an important point here, but can't I
>>>> put the println statement in a map()?
>>>>
>>>> On Mon, Feb 6, 2017 at 5:48 PM, Matthias J. Sax <matth...@confluent.io
>>>
>>> wrote:
>>>>> Steven,
>>>>>
>>>>> Thanks for your KIP. I move this discussion to dev mailing list --
>> KIPs
>>>>> need to be discussed there (and can be cc'ed to user list).
>>>>>
>>>>> Can you also add the KIP to the table "KIPs under discussion":
>>>>>
>>>>>
>>> https://cwiki.apache.org/confluence/display/KAFKA/
>> Kafka+Improvement+Proposals#KafkaImprovementProposals-KIPsunderdiscussion
>>>>>
>>>>>
>>>>> Thanks.
>>>>>
>>>>>
>>>>> -Matthias
>>>>>
>>>>> On 2/6/17 3:35 PM, Steven Schlansker wrote:
>>>>>> Hello users@kafka,
>>>>>>
>>>>>> I would like to propose a small KIP on the Streams framework
>>>>>> that simply adds a KStream#peek implementation.
>>>>>>
>>>>>>
>>> https://cwiki.apache.org/confluence/display/KAFKA/KIP-
>> 121%3A+Add+KStream+peek+method
>>>>>> https://issues.apache.org/jira/browse/KAFKA-4720
>>>>>> https://github.com/apache/kafka/pull/2493
>>>>>>
>>>>>> Please consider my contribution and hopefully you all like it and
>>> agree that it should be merged into 0.10.3 :)
>>>>>> If not, be gentle, this is my first KIP!
>>>>>>
>>>>>> Happy Monday,
>>>>>> Steven
>>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> Gwen Shapira
>>>> Product Manager | Confluent
>>>> 650.450.2760 <(650)%20450-2760> | @gwenshap
>>>> Follow us: Twitter | blog
>>>
>>>
>>
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