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From "Gabriel Reid (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-128) Allow one stage of an MR pipeline to depend on another target being created
Date Wed, 12 Dec 2012 06:15:22 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-128?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13529657#comment-13529657
] 

Gabriel Reid commented on CRUNCH-128:
-------------------------------------

While you're at it, a similar (but also different) scenario is pipelines that read and write
to the same path on the filesystem (typically from different PCollections). 

This will run without crashing as long as the directories are in place, but it is rarely or
never what is really intended, and will give pretty unpredictable behavior. If you're adding
some extra checking for loops, it would be good to handle that situation as well.
                
> 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
>
>
> 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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