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https://issues.apache.org/jira/browse/CRUNCH-284?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13800589#comment-13800589
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Josh Wills commented on CRUNCH-284:
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Another thought on the read() option-- what if I wrote an intermediate output
at some point in the pipeline (like a checkpoint) that I only wanted subsequent
stages to read once (like w/a SourceTarget). Maybe the option should be a
property on the Source interface instead of the read method, so that:
pipeline.read(From.textFile(path).onlyOnce())
and
pipeline.write(At.textFile(path).onlyOnce(), WriteMode.CHECKPOINT)
would both work. Or is the onlyOnce() in the context of a write sort of strange?
> Optimize for minimal disk i/o rather than the number of stages?
> ---------------------------------------------------------------
>
> Key: CRUNCH-284
> URL: https://issues.apache.org/jira/browse/CRUNCH-284
> Project: Crunch
> Issue Type: Bug
> Reporter: Chao Shi
>
> I have a pipeline as follows:
> PCollection in = pipeline.read(...)
> PCollection part1 = f1(in)
> PCollection part2 = f2(in)
> pipelien.write(part1.groupByKey...)
> pipeline.write(part2.groupByKey...)
> where f1 extracts a small potion from "in" and f2 returns the rest. Crunch
> optimizes the pipeline into two independent MR jobs, both of which fully read
> the input.
> I think the ideal MRs should be a map-only job reads the input and split them
> to two outputs, and then two MRs read them respectively.
> The problem is that Crunch minimizes the number of MR stages, which is
> optimal for most cases, but not optimal in this case.
> What do you think of this folks?
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