Re: How to achieve exactly once on node failure using Kafka

[Y. Sakamoto]

Thank you for your reply.

Under my understanding, Map / Filter Function operate with "at least once" when 
a failure occurs, and it is necessary to code that it will be saved (overwritten) in 
Elasticsearch with the same ID even if double data comes. Is it correct?
(sorry, I cannot understand how to "write changes to Flink's state to Elastic")

Regards,
Yuichiro


On 2017/02/21 3:56, Stephan Ewen wrote:

Hi!

Exactly-once end-to-end requires sinks that support that kind of behavior 
(typically some form of transactions support).

Kafka currently does not have the mechanisms in place to support exactly-once 
sinks, but the Kafka project is working on that feature.

For ElasticSearch, it is also not simply possible (because of missing transactions), but you can use Flink's state as the "authorative" state (it is exactly once) and then write changes to Flink's state to Elastic. That way the writes to ElasticSearch become "idempotent", which means duplicates simple make no additional changes.

Hope that helps!

Stephan




On Mon, Feb 20, 2017 at 5:53 PM, Y. Sakamoto <phonypian...@gmail.com 
<mailto:phonypian...@gmail.com>> wrote:

    Hi,
    I'm using Flink 1.2.0 and try to do "exactly once" data transfer
    from Kafka to Elasticsearch, but I cannot.
    (Scala 2.11, Kafka 0.10, without YARN)

    There are 2 Flink TaskManager nodes, and when processing
    with 2 parallelism, shutdown one of them (simulating node failure).

    Using flink-connector-kafka, I wrote following code:

       StreamExecutionEnvironment env = StreamExecutionEnvironment
             .getExecutionEnvironment();
       env.enableCheckpointing(1000L);
       env.setParallelism(2);

       Properties kafkaProp = new Properties();
       kafkaProp.setProperty("bootstrap.servers", "192.168.97.42:9092 
<http://192.168.97.42:9092>");
       kafkaProp.setProperty("zookeeper.connect", "192.168.97.42:2181 
<http://192.168.97.42:2181>");
       kafkaProp.setProperty("group.id <http://group.id>", "id");

       DataStream<String> stream = env.addSource(new FlinkKafkaConsumer010<>(
             "topic", new SimpleStringSchema(), kafkaProp));

    I found duplicated data transfer on map function.
    Data from the checkpoint before node failure seems duplicated.

    Is there any way to achieve "exactly once" on failure?


    Thanks.
    Yuichiro

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