After creating an index, you might need to make an extensive change such as adding a new field to every document or combining multiple indices to form a new one. Rather than deleting your index, making the change offline, and then indexing your data all over again, you can use the `reindex` operation.
With the `reindex` operation, you can copy all or a subset of documents that you select through a query to another index. Reindex is a `POST` operation. In its most basic form, you specify a source index and a destination index.
Reindexing can be an expensive operation depending on the size of your source index. We recommend you disable replicas in your destination index by setting `number_of_replicas` to `0` and re-enable them once the reindex process is complete.
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## Reindex all documents
You can copy all documents from one index to another.
You first need to create a destination index with your desired field mappings and settings or you can copy the ones from your source index:
```json
PUT destination
{
"mappings":{
"Add in your desired mappings"
},
"settings":{
"Add in your desired settings"
}
}
```
This `reindex` command copies all the documents from a source index to a destination index:
```json
POST _reindex
{
"source":{
"index":"source"
},
"dest":{
"index":"destination"
}
}
```
If the destination index is not already created, the `reindex` operation creates a new destination index with default configurations.
## Reindex from a remote cluster
You can copy documents from an index in a remote cluster. Use the `remote` option to specify the remote hostname and the required login credentials.
This command reaches out to a remote cluster, logs in with the username and password, and copies all the documents from the source index in that remote cluster to the destination index in your local cluster:
You can combine documents from one or more indices by adding the source indices as a list.
This command copies all documents from two source indices to one destination index:
```json
POST _reindex
{
"source":{
"index":[
"source_1",
"source_2"
]
},
"dest":{
"index":"destination"
}
}
```
Make sure the number of shards for your source and destination indices are the same.
## Reindex only unique documents
You can copy only documents missing from a destination index by setting the `op_type` option to `create`.
In this case, if a document with the same ID already exists, the operation ignores the one from the source index.
To ignore all version conflicts of documents, set the `conflicts` option to `proceed`.
```json
POST _reindex
{
"conflicts":"proceed",
"source":{
"index":"source"
},
"dest":{
"index":"destination",
"op_type":"create"
}
}
```
## Reindex sorted documents
You can copy certain documents after sorting specific fields in the document.
This command copies the last 10 documents based on the `timestamp` field:
```json
POST _reindex
{
"size":10,
"source":{
"index":"source",
"sort":{
"timestamp":"desc"
}
},
"dest":{
"index":"destination"
}
}
```
## Transform documents during reindexing
You can transform your data during the reindexing process using the `script` option.
We recommend Painless for scripting in OpenSearch.
This command runs the source index through a Painless script that increments a `number` field inside an `account` object before copying it to the destination index:
```json
POST _reindex
{
"source":{
"index":"source"
},
"dest":{
"index":"destination"
},
"script":{
"lang":"painless",
"source":"ctx._account.number++"
}
}
```
You can also specify an ingest pipeline to transform your data during the reindexing process.
You would first have to create a pipeline with `processors` defined. You have a number of different `processors` available to use in your ingest pipeline.
Here's a sample ingest pipeline that defines a `split` processor that splits a `text` field based on a `space` separator and stores it in a new `word` field. The `script` processor is a Painless script that finds the length of the `word` field and stores it in a new `word_count` field. The `remove` processor removes the `test` field.
```json
PUT _ingest/pipeline/pipeline-test
{
"description": "Splits the text field into a list. Computes the length of the 'word' field and stores it in a new 'word_count' field. Removes the 'test' field.",
"processors": [
{
"split": {
"field": "text",
"separator": "\\s+",
"target_field": "word"
},
}
{
"script": {
"lang": "painless",
"source": "ctx.word_count = ctx.word.length"
}
},
{
"remove": {
"field": "test"
}
}
]
}
```
After creating a pipeline, you can use the `reindex` operation:
```json
POST _reindex
{
"source": {
"index": "source",
},
"dest": {
"index": "destination",
"pipeline": "pipeline-test"
}
}
```
## Update documents in the current index
To update the data in your current index itself without copying it to a different index, use the `update_by_query` operation.
The `update_by_query` operation is `POST` operation that you can perform on a single index at a time.
```json
POST <index_name>/_update_by_query
```
If you run this command with no parameters, it increments the version number for all documents in the index.
## Source index options
You can specify the following options for your source index:
Option | Valid values | Description | Required
:--- | :--- | :---
`index` | String | The name of the source index. You can provide multiple source indices as a list. | Yes
`max_docs` | Integer | The maximum number of documents to reindex. | No
`query` | Object | The search query to use for the reindex operation. | No
`size` | Integer | The number of documents to reindex. | No
`slice` | String | Specify manual or automatic slicing to parallelize reindexing. | No
`sort` | List | Sort specific fields in the document before reindexing. | No
## Destination index options
You can specify the following options for your destination index:
Option | Valid values | Description | Required
:--- | :--- | :---
`index` | String | The name of the destination index. | Yes
`version_type` | Enum | The version type for the indexing operation. Valid values: internal, external, external_gt, external_gte. | No
