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From "Josh Wills (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-128) Allow one stage of an MR pipeline to depend on another target being created
Date Sun, 16 Dec 2012 20:48:12 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-128?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13533509#comment-13533509
] 

Josh Wills commented on CRUNCH-128:
-----------------------------------

No problem Gabriel-- I wish RB would tell me when it fails to diff the patches, I'm sorry
about that.

Re: keeping the method on PCollectionImpl, the tradeoff is between keeping the PCollection
API simple and breaking the circular dependency between lib.* and impl.* (which this change
does as soon as I replace the CrunchRuntimeExceptions with regular RuntimeExceptions), which
is also worthwhile. I've been coming around more and more to Matthias' way of thinking here.
                
> Allow one stage of an MR pipeline to depend on another target being created
> ---------------------------------------------------------------------------
>
>                 Key: CRUNCH-128
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-128
>             Project: Crunch
>          Issue Type: Improvement
>            Reporter: Josh Wills
>         Attachments: CheckpointingIT.java, CRUNCH-128.patch, CRUNCH-128v2.patch
>
>
> There are a couple of problems (e.g., mapside-joins, total orderings, etc.) where we
need to guarantee that one PCollection has been written to the FileSystem before another MapReduce
pipeline that depends on that file is allowed to run. This doesn't fit cleanly into the current
set of abstractions for Crunch, which is why we force pipelines to execute via the run command
to guarantee that the files have been created before the second stage is run.
> We should add the ability for a particular PCollection to require that a SourceTarget
instance has been created before it can be executed, and the planner should incorporate this
information into the MR pipeline planning process.

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