组合器是否适用于多个映射器的结果？ [英] Does combiner work on results from multiple mappers?

问题描述

如果多个映射器在同一个节点上执行，合并器会合并多个映射器的结果吗？

我无法在文档或书籍中找到答案。我能找到的合并器示例似乎都有所作为，即使它只能聚合来自一个映射器的结果。

解决方案

措辞在雅虎的教程中（在Leonard的回答中提到）似乎是误导性的。

到目前为止，我们为每个地图任务获得一个组合器。这个问题建议的功能仍在开发中： https://issues.apache.org/ jira / browse / MAPREDUCE-4502

If multiple mappers are executed on the same node, will combiner combine the results from multiple mappers?

I can't find the answer for this in documents or books. And combiner examples I can find all seem to make a difference even if it can aggregate results from one mapper only.

解决方案

The wording in the Yahoo's tutorial (mentioned in Leonard's answer) seems to be misleading.

As of now, we get a combiner for every map task. The functionality suggested by this question is under development still: https://issues.apache.org/jira/browse/MAPREDUCE-4502

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