Drools：获取3个最新事件 [英] Drools: get the 3 most recent Events

问题描述

我正在研究一个小的Drools项目，因为我想了解更多有关使用规则引擎的信息。我有一个名为 Event 的类，该类具有以下字段：

I'm working on a small Drools project, because I want to learn more about using rule engines. I have a class called Event that has the following fields:

• String标签； 可以是任何字符串的标签。


• long millis; 时间戳记。 （实际上，这是从同样在 Event 中的JodaTime LocalDate 字段转换的。）


• int值； 我要说明的值。

• String tag; A tag which can be any string.
• long millis; A timestamp. (Actually, this is converted from a JodaTime LocalDate field which is also in Event.)
• int value; A value that I want reason about.

我将数百个 Event 实例插入我的知识库中，现在我想获取3个最新的带有标记的事件 。我想出了下面的代码，它可以正常工作：

I insert several hundreds of Event instances into my knowledge base, and now I want to get the 3 most recent events that are tagged with "OK". I came up with the following code, which works:

rule "Three most recent events tagged with 'OK'"
when
    $e1 : Event( tag == "OK",
                 $millis1 : millis )
    $e2 : Event( tag == "OK",
                 millis < $millis1, $millis2 : millis )
    $e3 : Event( tag == "OK",
                 millis < $millis2, $millis3 : millis )

    not Event( tag == "OK",
               millis > $millis1 )
    not Event( tag == "OK",
               millis > $millis2 && millis < $millis1 )
    not Event( tag == "OK",
               millis > $millis3 && millis < $millis2 )
then
  # Do something with $e1.value, $e2.value and $e3.value
end

但是我有一种感觉应该是一种更好的方法。这非常冗长，而且不容易重复使用：如果我想获得 value>的五个最新事件怎么办？例如10 ？我最终会粘贴很多代码，但我不想这样做:)。
另外，代码对我来说看起来也不是很美丽。我真的不喜欢重复的 not Event ... 约束，而且我也不想一遍又一遍地重复相同的标记条件。 （此示例是我的真实应用程序的高度简化版本，其中的条件实际上要复杂得多。）

But I have a feeling there should be a better way to do this. This is quite verbose and not easily re-used: what if I want to get the five most recent events with value > 10, for example? I would end up copy-pasting a lot of code, and I don't want to do that :). Also, the code doesn't look very 'beautiful' to me. I don't really like the repeated not Event... constraints, and I also don't like having to repeat the same tag condition over and over. (This example is a heavily simplified version of my real app, in which the condition is actually a lot more complex.)

如何改进此代码？

推荐答案

假设您使用的是STREAM事件处理模式，并且事件在流中排序：

Assuming you are using the STREAM event processing mode and your events are ordered in the stream:

rule "3 most recent events"
when
    accumulate( $e : Event( tag == "OK" ) over window:length(3),
                $events : collectList( $e ) )
then
    // $events is a list that contains your 3 most recent 
    // events by insertion order
end

=====编辑====

===== edit ====

根据下面的评论，这是在Drools 5.4+中实现所需功能的方法：

Based on your comment bellow, here is how to achieve what you want in Drools 5.4+:

declare window LastEvents
    Event() over window:length(3)
end

rule "OK events among the last 3 events"
when
    accumulate( $e : Event( tag == "OK" ) from window LastEvents,
                $events : collectList( $e ) )
then
    // $events is a list that contains the OK events among the last 3 
    // events by insertion order
end

请仔细检查语法，因为我会很认真地执行此操作，但是应该很接近

Just double check the syntax as I am doing this by heart, but it should be close to this.

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