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From "Ray Chiang (JIRA)" <j...@apache.org>
Subject [jira] [Reopened] (MAPREDUCE-6685) LocalDistributedCacheManager can have overlapping filenames
Date Wed, 01 Jun 2016 05:34:12 GMT

     [ https://issues.apache.org/jira/browse/MAPREDUCE-6685?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Ray Chiang reopened MAPREDUCE-6685:
-----------------------------------

> LocalDistributedCacheManager can have overlapping filenames
> -----------------------------------------------------------
>
>                 Key: MAPREDUCE-6685
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6685
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>    Affects Versions: 3.0.0-alpha1
>            Reporter: Ray Chiang
>            Assignee: Ray Chiang
>         Attachments: MAPREDUCE-6685.001.patch, MAPREDUCE-6685.002.patch
>
>
> LocalDistributedCacheManager has this setup:
> bq. AtomicLong uniqueNumberGenerator = new AtomicLong(System.currentTimeMillis());
> to create this temporary filename:
> bq. new FSDownload(localFSFileContext, ugi, conf, new Path(destPath,  Long.toString(uniqueNumberGenerator.incrementAndGet())),
resource);
> when using LocalJobRunner.  When two or more start on the same machine, then it's possible
to end up having the same timestamp or a large enough overlap that two successive timestamps
may not be sufficiently far apart.
> Given the assumptions:
> 1) Assume timestamp is the same. Then the most common starting random seed will be the
same.
> 2) Process ID will very likely be unique, but will likely be close in value.
> 3) Thread ID is not guaranteed to be unique.
> A unique ID based on PID as a seed (in addition to the timestamp) should be a better
unique identifier for temporary filenames.



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