java - 在 KafkaStreams 中的聚合数据上使用 SessionWindows (0.11)

Question

我尝试在 Kafka (0.11) 的聚合函数中使用 SessionWindows，但无法理解为什么会出现错误。

这是我的代码片段:

// defining some values:
public static final Integer SESSION_TIMEOUT_MS = 6000000;
public static final String INTOPIC = "input";
public static final String HOST = "host";

// setting up serdes:
final Serializer<JsonNode> jsonSerializer = new JsonSerializer();
final Deserializer<JsonNode> jsonDeserializer = new JsonDeserializer();
final Serde<JsonNode> jsonSerde = Serdes.serdeFrom(jsonSerializer, jsonDeserializer);

// some more code to build up the streams
KStreamBuilder builder = new KStreamBuilder();
KStream<String, JsonNode> dataStream = builder.stream(Serdes.String(), jsonSerde, INTOPIC);

// constructing the initalMessage ObjectNode:
ObjectNode initialMessage = JsonNodeFactory.instance.objectNode();
initialMessage.put("count", 0);
initialMessage.put("endTime", "");

// transforming data to KGroupedStream<String,JsonNode>
KGroupedStream<String, JsonNode> data = dataStream.map((key, value) ->{return new KeyValue<>(value.get(HOST).asText(), value);  }).groupByKey(Serdes.String(), jsonSerde);

// finally aggregate the data usind SessionWindows
KTable<Windowed<String>, JsonNode> aggregatedData = data.aggregate(
            () -> initialMessage,

            (key, incomingMessage, initialMessage) -> countData(incomingMessage, initialMessage),

            SessionWindows.with(SESSION_TIMEOUT_MS),

            jsonSerde, 

            "aggregated-data");

 private static JsonNode countData(JsonNode incomingMessage, JsonNode initialMessage){
 // some dataprocessing
 }

当我改变

 KTable<Windowed<String>,JsonNode>

至

 KTable<String, JsonNode>

并删除

 SessionWindows.with(SESSION_TIMEOUT_MS)

从聚合函数来看，一切正常。

如果我不这样做，Eclipse 会告诉我行

 KTable<Windowed<String>, JsonNode> aggregatedData = data.aggregate( [...])

The method aggregate(Initializer, Aggregator, Windows, Serde, String) in the type KGroupedStream is not applicable for the arguments (() -> {}, ( key, incomingMessage, initialMessage) -> {}, SessionWindows, Serde, String)

对于该行

() -> initialMessage

Type mismatch: cannot convert from ObjectNode to VR

和:

 (key, incomingMessage, initialMessage) -> countData(incomingMessage, initialMessage),

The method countData(JsonNode, JsonNode) in the type DataWindowed is not applicable for the arguments (JsonNode, VR)

我真的不明白，类型在哪里丢失了!任何提示都会很棒!

谢谢:D

Answer 1

我确实需要实现合并:

Merger<? super String, JsonNode>tmpMerger = new MergerClass<String, JsonNode>();

并将其添加到聚合函数中:

KTable<Windowed<String>, JsonNode> aggregatedData = data.aggregate(
        () -> initialMessage,

        (key, incomingMessage, initialMessage) -> countData(incomingMessage, initialMessage),

        tmpMerger,

        SessionWindows.with(SESSION_TIMEOUT_MS),

        jsonSerde, 

        "aggregated-data");