# Reindexing a datastream after changed mappings : best practice?

**URL:** <https://discuss.elastic.co/t/reindexing-a-datastream-after-changed-mappings-best-practice/370275>\
**Category:** Elasticsearch\
**Tags:** reindex, datastreams\
**Created:** [November 10, 2024, 1:04am UTC](https://discuss.elastic.co/t/reindexing-a-datastream-after-changed-mappings-best-practice/370275 "2024-11-10T01:04:13Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![gpineda\_dev](https://sea2.discourse-cdn.com/elastic/user_avatar/discuss.elastic.co/gpineda_dev/32/137494_2.png) [@gpineda\_dev](https://discuss.elastic.co/u/gpineda_dev)\
**Post date:** [November 10, 2024, 1:04am UTC](https://discuss.elastic.co/t/reindexing-a-datastream-after-changed-mappings-best-practice/370275/1 "2024-11-10T01:04:13Z")

</div>

_Before writing this post, I started by gathering available information on this platform, official documentation, related articles._

_Examples to reindex basic indices are easy to find but when it comes to datastream, then the way to proceed is not clearly defined._

Here is my context:  
I started a [datastream](https://www.elastic.co/guide/en/elasticsearch/reference/current/data-streams.html) one month ago and it appears yesterday, it entered the [rollover phase](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-rollover-index.html) producing a new index. As expected my new indice is using a newer version of mapping from the [index template](https://www.elastic.co/guide/en/elasticsearch/reference/current/set-up-a-data-stream.html#create-index-template) .. resulting in conflicts within my complete datastream, partially breaking charts, dashboards.

Then I searched for recommendations on how to properly [reindex](https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html) a datastream .. here are the following options:

1. Reindex the complete datastream into a new one (ensuring the new one matches the expected index template)
2. Reindex rollover indices one per one to a new datastream

Both are basically ending with the same outcome but what if I need to guarantee no downtime when I have GBs to recover ?

Let's assume, DS-01 is my current datastream and DS-02 my new one.  
At first [alias](https://www.elastic.co/guide/en/elasticsearch/reference/current/aliases.html) DS-Generic points to DS-01  
During such major reindexation, shall I :

- trigger a rollup on DS-01 to create a (N+1)th new indice
- reindex DS-01 from indice 1 to N where new events will be sent to the newly created N+1 th indice
- link alias DS-Generic to DS-02 (write enabled)
- reindex DS-01 N+1 into DS-02

Or can I simply link DS-Generic to DS-02 so new events will be handled by DS-02 and perform reindexation at the same time ?

As far as i understood, datastreams are appending events to the last indice, so with the second approach I fear order in timestamp will not be preserved resulting in DS-02 having mixed timestamps within the same indice.

In other words, are there any good practices to perform a "clean" reindexation on an active datastream not loosing data ?

Ps: this is my first post on the platform... feel free to ask me rephrasing some parts if needed.  
Thanks for any support and I hope this thread will be helpful to anyone struggling with reindexing datastreams.