[Solr Wiki] Update of "HowToReindex" by ShawnHeisey

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The "HowToReindex" page has been changed by ShawnHeisey:
https://wiki.apache.org/solr/HowToReindex?action=diff&rev1=10&rev2=11

Comment:
Updated caveats to point to atomic updates.  Added info about the /export handler.

  == Using Solr as a Data Source ==
  Don't do this unless you have no other option.  Solr is not really designed for this role.  Every attempt is made to ensure that Solr is stable, but indexes do get corrupted by unanticipated situations, and by things completely outside developer control.  Solr 4.x and later does have NoSQL features, and SolrCloud goes a long way towards high availability, but absolute data reliability in the face of any problem is difficult to achieve for any software, which is why it's always important to have backups.
  
- <!> Using Solr as a data source to build a new index is only possible if you have 'stored="true"' for every single field in your index except any copyField destinations.  If you have 'stored="false"' on required fields in your schema, you won't be able to recover that data.  It's simply not possible.
+ <!> Using Solr as a data source to build a new index is only possible if your index meets the [[Atomic_Updates#Caveats_and_Limitations|explicit requirements]] for the Atomic Update feature.  If some of your fields don't meet this criteria, you won't be able to recover that data.  It's simply not possible.
  
  If you absolutely must use one Solr index as the data source for another index, and you have stored every field, you have a few possible options.
  
@@ -29, +29 @@

   1. Export the data using Solr queries, then reimport it after making sure it's in the correct format.  You could use XML or CSV for this.  This is not a trivial process.  There is no process or program available from the Solr project for doing this.  Here are some possible ideas:
    1. http://grokbase.com/t/lucene/solr-user/134p562kxs/export-index-and-re-index-xml
    1. http://www.jason-palmer.com/2011/05/how-to-reindex-a-solr-database/
+   1. Recent versions of Solr have added a new [[https://cwiki.apache.org/confluence/display/solr/Exporting+Result+Sets|export capability]] -- the /export handler.  This might prove useful.
  
  == Alternatives when a traditional reindex isn't possible ==
  Sometimes the option of "do your indexing again" is difficult.  Perhaps the original data is very slow to access, or it may be difficult to get in the first place.