Re: Using Kafka 0.10.x timestamps as a record value in Flink Streaming

Posted by Tzu-Li (Gordon) Tai on
URL: http://deprecated-apache-flink-user-mailing-list-archive.369.s1.nabble.com/Using-Kafka-0-10-x-timestamps-as-a-record-value-in-Flink-Streaming-tp13119p13180.html

Hi Jia,

How exactly do you want to use the Kafka timestamps? Do you want to access them and alter them with new values as the record timestamp? Or do you want to use them for some application logic in your functions?

If its the former, you should be able to do that by using timestamp / watermark extractors. They come with an interface that exposes the current timestamp of the record. For Kafka 0.10, that timestamp would be the Kafka record’s timestamp if it hasn’t been explicitly assigned any other timestamp yet.

If its the latter, then I think currently you have to use custom operators as Robert mentioned.

Custom operators are classes that extend the `AbstractStreamOperator` base class as one of `OneInputStreamOperator` or `TwoInputStreamOperator` interfaces.

You can take a look at the basic `StreamMap` or `StreamFlatMap` classes for an example, which are the underlying operators for the map and flatMap functions.

At the operator level, you’ll have access to a `StreamRecord` in the `processElement` function which wraps the record value (which you get when implementing functions) as well as the internal timestamp that comes with the record.

Cheers,

Gordon

On 17 May 2017 at 4:27:36 AM, Jia Teoh ([hidden email]) wrote:

Hi Robert,

Thanks for the reply. I ended up implementing an extension of the Kafka fetcher and consumer so that the deserialization API can include the timestamp field, which is sufficient for my specific use case. I can share the code if desired but it seems like it's an intentional design decision not to expose the timestamp in the deserialization API.

I noticed you mentioned that I could use a custom operator to access the record event time. Could you elaborate on what you mean by "operator"? I initially thought that referred to DataStream.map/reduce/etc, but none of those functions provide arguments that can be used to extract the embedded timestamp. 

Thanks,

Jia Teoh

On Fri, May 12, 2017 at 9:25 AM, Robert Metzger <[hidden email]> wrote:

Hi Jia,

The Kafka 0.10 connector internally relies on the Kafka09 fetcher, but it is extensible / pluggable so that also the Kafka 0.9 Fetcher can read the event timestamps from Kafka 10.

We don't expose the timestamp through the deserilaization API, because we set it internally in Flink. (there is a "hidden" field with each record containing the event time of the event)

With a custom operator you can access the event time of a record.

On Fri, May 12, 2017 at 3:26 AM, Jia Teoh <[hidden email]> wrote:

Hi,

Is there a way to retrieve the timestamps that Kafka associates with each key-value pair within Flink? I would like to be able to use these as values within my application flow, and defining them before or after Kafka is not acceptable for the use case due to the latency involved in sending or receiving from Kafka.

It seems that Flink supports Kafka event time (link) but after a brief trace it seems that KafkaConsumer010 still relies on the Kafka09Fetcher for iterating through each Kafka record and deserializing it. The KeyedDeserializationSchema api does not seem to have support for including timestamp as additional metadata (just offset, topic, and partition) so something such as JSONKeyValueDeserializationSchema will not return the Kafka-specified timestamp.

For reference, I am using Kafka 0.10.2 and the Flink-Streaming API + Kafka Connector (1.2.1).

Thanks,

Jia Teoh