[GitHub] [lucene-solr] s1monw commented on issue #616: LUCENE-8734: Record nextWriteGens in SegmentCommitInfo

[GitBox <gi...@apache.org>]

s1monw commented on issue #616: LUCENE-8734: Record nextWriteGens in SegmentCommitInfo
URL: https://github.com/apache/lucene-solr/pull/616#issuecomment-475606871
 
 
   > Do we actually need to record these generations, ....
   
   That was my first reaction too. Yet, it's not enough in the scenario that the test case shows which I think is realistic in order to roll back to an existing commit point. Let me give you an example:
   
   ```
   iw = new IW(keepAllCommits)
   iw.addDocument(<Id:1>)
   iw. softUpdateDocument(<Id:1>) -> writes _0.dvd
   iw.commit() <commit1, dvGen: -1 // no updates>
   iw. softUpdateDocument(<Id:1>)  -> writes _0_1.dvd
   iw.commit() <commit2, dvGen: 1>
   iw.close();
   iw = new IW(<commit1: dvGen -1>) --> IFD updates SegmentCommitInfo to <dvGen: -1, nextDvWriteGen: 2> and deletes <commit2, dvGen:1>
   iw.close(); // now only one commit is available 
   
   iw = new IW(<commit1: dvGen -1>) --> IFD updates SegmentCommitInfo to <dvGen: -1, nextDvWriteGen: 1> 
   iw. softUpdateDocument(<Id:1>)  -> writes _0_1.dvd and crashes since the file already exists.
   
   ```
   
   I hope this makes sense?
   
   
   
   
   
   
   

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