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https://issues.apache.org/jira/browse/HADOOP-9639?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13751876#comment-13751876
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Sangjin Lee commented on HADOOP-9639:
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Thanks for the feedback [~knoguchi].
As for the checksum, perhaps I should have used a different term. What I meant
by checksum is specifically a cryptographically strong hash function, not just
any checksum. I agree that a simple non-cryptographic checksum is not able to
provide security or enough protection against collisions. I was envisioning
using an algorithm like SHA-1 or stronger. With SHA-1 validation on localizing,
it would not be practical to tamper with them.
Point taken on the possibility of people corrupting directories and files. The
nature of the shared cache makes it challenging to address this issue.
One idea would be to create a group for the shared cache and give this group
the permissions to modify the shared cache directory and everyone else the
read-only permissions. The implication is that any user who wants to
participate in the shared cache (e.g. map-reduce users) need to be included in
this group for them to use it. It's probably a little safer, but admittedly not
a complete solution...
> truly shared cache for jars (jobjar/libjar)
> -------------------------------------------
>
> Key: HADOOP-9639
> URL: https://issues.apache.org/jira/browse/HADOOP-9639
> Project: Hadoop Common
> Issue Type: New Feature
> Components: filecache
> Affects Versions: 2.0.4-alpha
> Reporter: Sangjin Lee
> Assignee: Sangjin Lee
> Attachments: shared_cache_design.pdf, shared_cache_design_v2.pdf
>
>
> Currently there is the distributed cache that enables you to cache jars and
> files so that attempts from the same job can reuse them. However, sharing is
> limited with the distributed cache because it is normally on a per-job basis.
> On a large cluster, sometimes copying of jobjars and libjars becomes so
> prevalent that it consumes a large portion of the network bandwidth, not to
> speak of defeating the purpose of "bringing compute to where data is". This
> is wasteful because in most cases code doesn't change much across many jobs.
> I'd like to propose and discuss feasibility of introducing a truly shared
> cache so that multiple jobs from multiple users can share and cache jars.
> This JIRA is to open the discussion.
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