November 2022
Beginner to intermediate
346 pages
11h
Chinese
Content preview from Kafka权威指南（第2版）
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298
｜
第
14
章
➌
在确保有了正确的分区并定义好时间窗口之后，开始聚合操作。
aggregate
方法会将事
件流拆分成相互叠加的时间窗口（每秒出现一个
5
秒
的时间窗口），然后将聚合方法应
用在时间窗口内的所有事件上。这个方法的第一个参数是一个新对象，用于存放聚合结
果，在这个例子中是
TradeStats
。我们用这个对象存放每个时间窗口的统计信息，包括
最低价格、平均价格和交易数量。
➍
我们提供了一个方法对记录进行聚合，这里，
TradeStats
的
add
方法用于更新时间窗口
内的最低价格、交易数量和总价。
➎
14.3
节曾经讲过，基于时间窗口的聚合需要用到保存在本地存储中的状态。聚合方法的
最后一个参数就是本地状态存储的配置信息。
Materialized
是本地存储配置对象，它的
名字是
trade-aggregates
，也可以是任意具有唯一性的名字。
➏
作为状态存储配置的一部分，我们还提供了
Serde
对象，用于序列化和反序列化聚合结
果（
TradeStats
对象）。
➐
聚合结果是一张
表
，包含了股票行情，并使用时间窗口作为键、聚合结果作为值。我们
将表转成事件流。
➑
更新平均价格。现在，聚合结果中包含了总价和交易数量。接下来遍历所有的记录，并
使用现有的统计信息计算出平均价格，然后把它写到输出流里。
➒
最后，将结果写到
stockstats-output
流里。因为结果是窗口操作的一部分，所以我们
创建了一个
WindowedSerde
对象，将结果保存成窗口数据格式，其中包含了窗口时间
戳。窗口大小会作为
Serde ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.