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https://issues.apache.org/jira/browse/SOLR-17725?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17941033#comment-17941033
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Andrzej Bialecki commented on SOLR-17725:
-----------------------------------------

The already existing {{REINDEXCOLLECTION}} command supports a similar usecase 
and has similar requirements for source fields that are needed to re-create the 
original {{{}SolrIndexDocument{}}}s. It doesn't work in place but depending on 
the use case this can be viewed as a feature not a bug ;) because it's easier 
to preserve the original data in case of a catastrophic failure.

> Automatically upgrade Solr indexes without needing to reindex from source
> -------------------------------------------------------------------------
>
>                 Key: SOLR-17725
>                 URL: https://issues.apache.org/jira/browse/SOLR-17725
>             Project: Solr
>          Issue Type: Improvement
>            Reporter: Rahul Goswami
>            Priority: Major
>         Attachments: High Level Design.png
>
>
> Today upgrading from Solr version X to X+2 requires complete reingestion of 
> data from source. This comes from Lucene's constraint which only guarantees 
> index compatibility between the version the index was created in and the 
> immediate next version. 
> This reindexing usually comes with added downtime and/or cost. Especially in 
> case of deployments which are in customer environments and not completely in 
> control of the vendor, this proposition of having to completely reindex the 
> data can become a hard sell.
> I, on behalf of my employer, Commvault, have developed a way which achieves 
> this reindexing in-place on the same index. Also, the process automatically 
> keeps "upgrading" the indexes over multiple subsequent Solr upgrades without 
> needing manual intervention. 
> It comes with the following limitations:
> i) All _source_ fields need to be either stored=true or docValues=true. Any 
> copyField destination fields can be stored=false of course, just that the 
> source fields (or more precisely, the source fields you care about 
> preserving) should be either stored or docValues true. 
> ii) The datatype of an existing field in schema.xml shouldn't change upon 
> Solr upgrade. Introducing new fields is fine. 
> For indexes where this limitation is not a problem (it wasn't for us!), the 
> tool can reindex in-place on the same core with zero downtime and 
> legitimately "upgrade" the index. This can remove a lot of operational 
> headaches, especially in environments with hundreds/thousands of very large 
> indexes.



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