Kafka Streams 应用程序 - 计数和总和聚合 [英] Kafka Streams App - count and sum aggregate

问题描述

我正在尝试从 KGroupedStream 创建一个 KTable 来存储每个键的值的总和.

I'm trying to create a KTable from a KGroupedStream to store the sum of the value for each key.

 final StreamsBuilder builder = new StreamsBuilder();
 final KTable<String, Long> sum = builder.stream("streams-plaintext-input")
            .groupByKey()
            .aggregate(new Initializer<Long>() {
                @Override
                public Long apply() {
                    return Long.MIN_VALUE;
                }
            }, new Aggregator<String, Long, Long>() {
                @Override
                public Long apply(final String key, final Long value,final Long aggregate) {
                    aggregate += value;
                    return aggregate;
                }
            }, Materialized.<String, Long, KeyValueStore<Byte, byte[]>>as("counts-store"));

但我收到错误:

The method aggregate(Initializer<VR>, Aggregator<? super Object,? super Object,VR>, Materialized<Object,VR,KeyValueStore<Bytes,byte[]>>) in the type KGroupedStream<Object,Object> is not applicable for the arguments (new Initializer<Long>(){}, new Aggregator<String,Long,Long>(){}, Materialized<String,Long,KeyValueStore<Byte,byte[]>>)

我见过的所有示例都将 Serde 作为第三个参数传递，但我已经尝试过并得到了一个非常相似的错误(我认为这可能来自旧版本，因为它与当前的签名不匹配)实施?):

All the examples I've seen pass in a Serde as the third argument but I have tried this and get a very similar error (I think this might be from an older version as it does not match with the signature of the current implementation?):

final StreamsBuilder builder = new StreamsBuilder();
    final KTable<String, Long> sum = builder.stream("streams-plaintext-input")
            .groupByKey()
            .aggregate(new Initializer<Long>() {
                @Override
                public Long apply() {
                    return Long.MIN_VALUE;
                }
            }, new Aggregator<String, Long, Long>() {
                @Override
                public Long apply(final String key, final Long value,final Long aggregate) {
                    aggregate += value;
                    return aggregate;
                }
            }, Serdes.Long());

错误:

The method aggregate(Initializer<VR>, Aggregator<? super Object,? super Object,VR>, Materialized<Object,VR,KeyValueStore<Bytes,byte[]>>) in the type KGroupedStream<Object,Object> is not applicable for the arguments (new Initializer<Long>(){}, new Aggregator<String,Long,Long>(){}, Serde<Long>)

我做错了什么?

使用 Kafka 版本:2.1.0

Using Kafka version: 2.1.0

推荐答案

您的代码中存在一些问题:

There are few issues in your code:

1. 对于 Materialized.as 而不是 java.lang.Byte 你应该传递 org.apache.kafka.common.utils.Bytes
2. 你不应该修改 final 变量:aggregate += value;
3. 您必须向 StreamsBuilder::stream 调用添加键和值的类型 (builder.stream("streams-plaintext-input"))

1. For Materialized.as instead java.lang.Byte you should pass org.apache.kafka.common.utils.Bytes
2. You shouldn't modify final variable: aggregate += value;
3. You have to add types of key and value to StreamsBuilder::stream call (builder.<String, Long>stream("streams-plaintext-input"))

修改后应该大致如下:

KTable<String, Long> sum = builder.<String, Long>stream("streams-plaintext-input")
        .groupByKey()
        .aggregate(new Initializer<Long>() {
            @Override
            public Long apply() {
                return Long.MIN_VALUE;
            }
        }, new Aggregator<String, Long, Long>() {
            @Override
            public Long apply(final String key, final Long value,final Long aggregate) {
                return aggregate + value;
            }
        }, Materialized.<String, Long, KeyValueStore<Bytes, byte[]>>as("counts-store"));

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