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https://issues.apache.org/jira/browse/NIFI-4950?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Michael Moser resolved NIFI-4950.
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Resolution: Fixed
Fix Version/s: 1.6.0
> MergeContent: Defragment can improperly reassemble
> --------------------------------------------------
>
> Key: NIFI-4950
> URL: https://issues.apache.org/jira/browse/NIFI-4950
> Project: Apache NiFi
> Issue Type: Bug
> Components: Extensions
> Affects Versions: 1.5.0
> Reporter: Brandon DeVries
> Assignee: Mark Bean
> Priority: Minor
> Fix For: 1.6.0
>
>
> In Defragment mode, MergeContent can improperly reassemble the pieces of a
> split file. I understand this was previously discussed in NIFI-378, and the
> outcome was to update the documentation for fragment.index [1]:
> {quote} Applicable only if the <Merge Strategy> property is set to
> Defragment. This attribute indicates the order in which the fragments should
> be assembled. This attribute must be present on all FlowFiles when using the
> Defragment Merge Strategy and must be a unique (i.e., unique across all
> FlowFiles that have the same value for the "fragment.identifier" attribute)
> integer between 0 and the value of the fragment.count attribute. If two or
> more FlowFiles have the same value for the "fragment.identifier" attribute
> and the same value for the "fragment.index" attribute, the behavior of this
> Processor is undefined.
> {quote}
> I believe this could (and probably should) be improved upon. Specifically,
> the discussion around NIFI-378 focused on the "improper" use of MergeContent,
> in using the same fragment.identifier to "pair up" files. The situation I've
> encountered isn't really unusual in any way...
> I have a file, being split and sent via PostHTTP to another nifi instance.
> If something "goes wrong", the sending NiFi may not get an acknowledgement of
> success even if the file made it to the receiving NiFi. It then sends the
> segment again. NiFi favors duplication over loss, so this is not unexpected.
> However, I now have a file broken into X fragments arriving on the other
> side as X+1 (or more). The reassembly may work... or both duplicates may be
> chosen, and result in an incorrectly recreated file.
> To satisfy the contract as it exists, you would need to use a DetectDuplicate
> before the MergeContent to filter these out. However, that could potentially
> incur a great of overhead. In contrast, simply checking that there are no
> duplicate fragment id's in a bin should be relatively straightforward. How
> to handle duplicates is a legitimate question... are they ignored, or are
> they discard (if they're actually the same)? If the duplicate id's aren't
> identical, what is the behavior? Personally, I would say if you have actual
> duplicates, drop one and continue with the merge... if you have unequal
> "duplicates", fail the bin. But there's room for discussion there.
> The point is, in this circumstance it is very easy for a user to do a very
> reasonable thing and end up with a corrupt file for reasons that are somewhat
> esoteric. Then, we would need to explain to them why "defragment" doesn't
> actually defragment, but just kind of sorts a bin of matching things. I
> think we can do better than that.
> [1]
> [http://nifi.apache.org/docs/nifi-docs/components/org.apache.nifi/nifi-standard-nar/1.5.0/org.apache.nifi.processors.standard.MergeContent/index.html]
>
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