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https://issues.apache.org/jira/browse/LUCENE-1585?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12865692#action_12865692
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Shai Erera commented on LUCENE-1585:
------------------------------------

Small correction - the comment I've made about concurrency issues w/ just the 
local directory is wrong because you can add an index w/ multiple segments and 
if you use CMS and the MP returns several OneMerges, then concurrency issues 
will happen in that case too.

Anyway, the 3x tests pass w/ the patch. How do we proceed from here - can I 
port that patch to trunk or does anybody have more comments?

> Allow to control how payloads are merged
> ----------------------------------------
>
>                 Key: LUCENE-1585
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1585
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Shai Erera
>            Priority: Minor
>             Fix For: 3.1, 4.0
>
>         Attachments: LUCENE-1585_3x.patch, LUCENE-1585_3x.patch, 
> LUCENE-1585_3x.patch, LUCENE-1585_trunk.patch
>
>
> Lucene handles backwards-compatibility of its data structures by
> converting them from the old into the new formats during segment
> merging. 
> Payloads are simply byte arrays in which users can store arbitrary
> data. Applications that use payloads might want to convert the format
> of their payloads in a similar fashion. Otherwise it's not easily
> possible to ever change the encoding of a payload without reindexing.
> So I propose to introduce a PayloadMerger class that the SegmentMerger
> invokes to merge the payloads from multiple segments. Users can then
> implement their own PayloadMerger to convert payloads from an old into
> a new format.
> In the future we need this kind of flexibility also for column-stride
> fields (LUCENE-1231) and flexible indexing codecs.
> In addition to that it would be nice if users could store version
> information in the segments file. E.g. they could store "in segment _2
> the term a:b uses payloads of format x.y".

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