# Reindex legacy backing indices

**POST /_migration/reindex**

Reindex all legacy backing indices for a data stream.
This operation occurs in a persistent task.
The persistent task ID is returned immediately and the reindexing work is completed in that task.

[About reindexing data streams](https://www.elastic.co/docs/reference/elasticsearch/rest-apis/reindex-data-stream)

## Servers
- http://api.example.com: http://api.example.com ()


## Authentication methods
- Api key auth
- Basic auth
- Bearer auth


## Parameters



### Body: application/json (object)

- **mode** (string)
  Reindex mode. Currently only 'upgrade' is supported.
- **source** (object)
  The source index or data stream (only data streams are currently supported).


## Responses
### 200


#### Body: application/json (object)
- **acknowledged** (boolean)
  For a successful response, this value is always true. On failure, an exception is returned instead.


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