How to Reindex in Elasticsearch: Syntax and Examples

To reindex in Elasticsearch, use the _reindex API which copies documents from a source index to a destination index. You send a POST request to /_reindex with JSON specifying the source and destination indexes.

Syntax

The _reindex API requires a POST request to /_reindex with a JSON body. The JSON must include a source object specifying the source index and a dest object specifying the destination index.

Optionally, you can add a query inside source to filter documents, or use script to modify documents during reindexing.

json

{
  "source": {
    "index": "source_index"
  },
  "dest": {
    "index": "destination_index"
  }
}

Example

This example copies all documents from old_index to new_index. It demonstrates the basic reindex operation without filters or scripts.

json

POST /_reindex
{
  "source": {
    "index": "old_index"
  },
  "dest": {
    "index": "new_index"
  }
}

Output

{
  "took": 123,
  "timed_out": false,
  "total": 1000,
  "updated": 0,
  "created": 1000,
  "deleted": 0,
  "batches": 1,
  "version_conflicts": 0,
  "noops": 0,
  "retries": {
    "bulk": 0,
    "search": 0
  },
  "throttled_millis": 0,
  "requests_per_second": -1.0,
  "throttled_until_millis": 0,
  "failures": []
}

Common Pitfalls

• Forgetting to create the destination index before reindexing can cause errors.
• Not handling version conflicts if documents already exist in the destination index.
• Using _reindex without filters may copy unwanted data.
• Running reindex on large datasets without throttling can overload the cluster.

Always test reindexing on a small dataset first.

json

POST /_reindex
{
  "source": {
    "index": "old_index"
  },
  "dest": {
    "index": "new_index"
  }
}

-- Wrong: Destination index missing

PUT /new_index
{
  "settings": {
    "number_of_shards": 1
  }
}

-- Right: Create destination index before reindexing

Quick Reference

Remember these key points when reindexing:

• Use POST /_reindex with source and dest indexes.
• Create the destination index before reindexing.
• Use query to filter documents if needed.
• Use script to modify documents during reindex.
• Monitor the output for errors and version conflicts.

Key Takeaways

Use the _reindex API with source and destination indexes to copy documents.

Always create the destination index before starting reindexing.

Filter documents with a query to avoid copying unwanted data.

Use scripts to modify documents during reindex if needed.

Check the reindex response for errors and version conflicts.