# Reindex and concurrent writings in the source index

**URL:** <https://discuss.elastic.co/t/reindex-and-concurrent-writings-in-the-source-index/206991>\
**Category:** Elasticsearch\
**Tags:** ilm-index-lifecycle-management\
**Created:** [November 7, 2019, 4:43pm UTC](https://discuss.elastic.co/t/reindex-and-concurrent-writings-in-the-source-index/206991 "2019-11-07T16:43:32Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![lorenzoranucci](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/lorenzoranucci/32/57340_2.png) [@lorenzoranucci](https://discuss.elastic.co/u/lorenzoranucci)\
**Post date:** [November 7, 2019, 4:43pm UTC](https://discuss.elastic.co/t/reindex-and-concurrent-writings-in-the-source-index/206991/1 "2019-11-07T16:43:32Z")

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Hi, I'm trying to change the index mapping and reindex without downtime.

The strategy is:

1. create a _new-index_ with the new mapping
2. reindex the _old-index_ to the _new-index_
3. swap the alias from the _old-index_ to the _new-index_
4. reindex again the _old-index_ to the _new-index_ (to handle eventually lost data)
5. drop the _old-index_

My questions are:

- What happen to the new data written in the source index during the reindex process?
- I really need the step 4?
- Is there a smarter way to do what I want to do?

Thanks

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<div class="post-metadata">

**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [December 5, 2019, 4:43pm UTC](https://discuss.elastic.co/t/reindex-and-concurrent-writings-in-the-source-index/206991/2 "2019-12-05T16:43:38Z")

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