[jira] [Created] (LUCENE-8149) Document why NRTCachingDirectory needs to preemptively delete segment files.

["Erick Erickson (JIRA)" <ji...@apache.org>]

Erick Erickson created LUCENE-8149:
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             Summary: Document why NRTCachingDirectory needs to preemptively delete segment files.
                 Key: LUCENE-8149
                 URL: https://issues.apache.org/jira/browse/LUCENE-8149
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Erick Erickson
            Assignee: Erick Erickson


Moving over here from SOLR-11892. After getting through my confusion I've found the following: NRTCachingDirectory.createOutput tries to delete segment files before creating them. We should at least add a bit of commentary as to what the pre-emptive delete is there for since on the surface it's not obvious.

try {
  in.deleteFile(name);
{{ } catch (IOException ioe) {}}
  // This is fine: file may not exist
{{ }}}

If I change to using MMapDirectory or NIOFSDirectory these exceptions are not thrown. What's special about NRTCachingDirectory that it needs this when two of the possible underlying FS implementations apparently do not? Or is this necessary for, say, Windows or file systems than the two I tried? Or is it some interaction between the RAM based segments and segments on disk?




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