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From "Dave Beech (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-218) Add new Target.WriteMode to skip the write and continue pipeline if an output target exists
Date Thu, 13 Jun 2013 06:58:22 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13681966#comment-13681966
] 

Dave Beech commented on CRUNCH-218:
-----------------------------------

Josh - thanks for putting a patch together so quickly! I'll test it out myself this morning
but just to confirm what Gabriel said, the idea is definitely to avoid running MyExpensiveOperation
if the data at 'processed' exists, and in general, to only run MyExpensiveOperation as many
times as strictly necessary. 

At the moment, in order to achieve this in a single pipeline run I might have to put a 'run'
call after the first write:

PCollection<String> words = ...;
    PCollection<String> processedWords = words.parallelDo(new MyExpensiveOperation());
    pipeline.write(processedWords, "processed", WriteMode.SKIP_IF_EXISTS);
    pipeline.run(); // RUN PIPELINE UP TO WRITE OF "processed" 
    PCollection<Pair<String,Long>> counted = processedWords.count();
    pipeline.write(counted, "counted");

I think this would only run MyExpensiveOperation once. But then if I run the same pipeline
again, because "processed" already exists I don't want to run MyExpensiveOperation _at all_


                
> Add new Target.WriteMode to skip the write and continue pipeline if an output target
exists
> -------------------------------------------------------------------------------------------
>
>                 Key: CRUNCH-218
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-218
>             Project: Crunch
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.6.0
>            Reporter: Dave Beech
>            Assignee: Josh Wills
>            Priority: Minor
>         Attachments: CRUNCH-218b.patch, CRUNCH-218.patch
>
>
> Quite often I write pipelines which persist data to the filesystem midway through the
process, and then carry on doing further work. 
> If this intermediate data is already present, I think it would be good if I could set
a write mode which skips over this first half of processing. This way I'd avoid running jobs
unnecessarily and wasting cluster resources regenerating data I already have. 
> Example:
> PCollection<B> inter = pipeline.read(source).parallelDo(something).parallelDo(somethingElse);
> inter.write(At.sequenceFile('output'), WriteMode.SKIP_IF_EXISTS);
> PCollection<C> final = inter.parallelDo(moreWork);
>  

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