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From Jonathan Resnick <>
Subject confused about segment merging and commits
Date Wed, 27 Sep 2017 12:58:58 GMT

I am trying to understand how segment merging interacts with commits.

Consider the following timeline of events:

1. IndexWriter is opened on an index.
2. IndexWriter is used to add/update/delete docs, but not yet commit the
3. Activity in step 2 triggers segment merging on a background thread.
4. Commit() is called on IndexWriter while merging in step 3 has not yet

Does the Commit() in step 4 block while the segment merge in step 3
If not, then when is the segment merge in 3 "committed" to the index? (i.e.
at what point would a new IndexReader see the merged segment file?)
Or does segment merging happen entirely independently of commits?

[More context: we are trying to build a backup system that copies the index
files to a backup server after every commit. Initially I thought it would
be sufficient to just keep track of file add/update/deletes since the
previous commit, but if segment merging is happening concurrently then
perhaps it's not so simple?]

More generally, is there any in-depth documentation available describing
how segment merging interacts with commits (even if it's for the Java
version of Lucene)?  My web searches have not turned up much...

Many thanks,

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