[[docs-reindex]]
== Reindex API

IMPORTANT: Reindex requires <<mapping-source-field,`_source`>> to be enabled for
all documents in the source index.

IMPORTANT: Reindex does not attempt to set up the destination index.  It does
not copy the settings of the source index.  You should set up the destination
index prior to running a `_reindex` action, including setting up mappings, shard
counts, replicas, etc.

The most basic form of `_reindex` just copies documents from one index to another.
This will copy documents from the `twitter` index into the `new_twitter` index:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

That will return something like this:

[source,js]
--------------------------------------------------
{
  "took" : 147,
  "timed_out": false,
  "created": 120,
  "updated": 0,
  "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,
  "total": 120,
  "failures" : [ ]
}
--------------------------------------------------
// TESTRESPONSE[s/"took" : 147/"took" : "$body.took"/]

Just like <<docs-update-by-query,`_update_by_query`>>, `_reindex` gets a
snapshot of the source index but its target must be a **different** index so
version conflicts are unlikely. The `dest` element can be configured like the
index API to control optimistic concurrency control. Just leaving out
`version_type` (as above) or setting it to `internal` will cause Elasticsearch
to blindly dump documents into the target, overwriting any that happen to have
the same type and id:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter",
    "version_type": "internal"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

Setting `version_type` to `external` will cause Elasticsearch to preserve the
`version` from the source, create any documents that are missing, and update
any documents that have an older version in the destination index than they do
in the source index:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter",
    "version_type": "external"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

Settings `op_type` to `create` will cause `_reindex` to only create missing
documents in the target index. All existing documents will cause a version
conflict:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter",
    "op_type": "create"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

By default version conflicts abort the `_reindex` process but you can just
count them by settings `"conflicts": "proceed"` in the request body:

[source,js]
--------------------------------------------------
POST _reindex
{
  "conflicts": "proceed",
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter",
    "op_type": "create"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

You can limit the documents by adding a type to the `source` or by adding a
query. This will only copy tweets made by `kimchy` into `new_twitter`:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter",
    "type": "_doc",
    "query": {
      "term": {
        "user": "kimchy"
      }
    }
  },
  "dest": {
    "index": "new_twitter"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

`index` and `type` in `source` can both be lists, allowing you to copy from
lots of sources in one request. This will copy documents from the `_doc` and
`post` types in the `twitter` and `blog` indices.

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": ["twitter", "blog"],
    "type": ["_doc", "post"]
  },
  "dest": {
    "index": "all_together",
    "type": "_doc"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]
// TEST[s/^/PUT blog\/post\/post1?refresh\n{"test": "foo"}\n/]

NOTE: The Reindex API makes no effort to handle ID collisions so the last
document written will "win" but the order isn't usually predictable so it is
not a good idea to rely on this behavior. Instead, make sure that IDs are unique
using a script.

It's also possible to limit the number of processed documents by setting
`size`. This will only copy a single document from `twitter` to
`new_twitter`:

[source,js]
--------------------------------------------------
POST _reindex
{
  "size": 1,
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

If you want a particular set of documents from the `twitter` index you'll
need to use `sort`. Sorting makes the scroll less efficient but in some contexts
it's worth it. If possible, prefer a more selective query to `size` and `sort`.
This will copy 10000 documents from `twitter` into `new_twitter`:

[source,js]
--------------------------------------------------
POST _reindex
{
  "size": 10000,
  "source": {
    "index": "twitter",
    "sort": { "date": "desc" }
  },
  "dest": {
    "index": "new_twitter"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

The `source` section supports all the elements that are supported in a
<<search-request-body,search request>>. For instance, only a subset of the
fields from the original documents can be reindexed using `source` filtering
as follows:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter",
    "_source": ["user", "_doc"]
  },
  "dest": {
    "index": "new_twitter"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]


Like `_update_by_query`, `_reindex` supports a script that modifies the
document. Unlike `_update_by_query`, the script is allowed to modify the
document's metadata. This example bumps the version of the source document:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter",
    "version_type": "external"
  },
  "script": {
    "source": "if (ctx._source.foo == 'bar') {ctx._version++; ctx._source.remove('foo')}",
    "lang": "painless"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

Just as in `_update_by_query`, you can set `ctx.op` to change the
operation that is executed on the destination index:

`noop`::

Set `ctx.op = "noop"` if your script decides that the document doesn't have
to be indexed in the destination index. This no operation will be reported
in the `noop` counter in the <<docs-reindex-response-body, response body>>.

`delete`::

Set `ctx.op = "delete"` if your script decides that the document must be
 deleted from the destination index. The deletion will be reported in the
 `deleted` counter in the <<docs-reindex-response-body, response body>>.

Setting `ctx.op` to anything else will return an error, as will setting any
other field in `ctx`.

Think of the possibilities! Just be careful; you are able to
change:

 * `_id`
 * `_type`
 * `_index`
 * `_version`
 * `_routing`

Setting `_version` to `null` or clearing it from the `ctx` map is just like not
sending the version in an indexing request; it will cause the document to be
overwritten in the target index regardless of the version on the target or the
version type you use in the `_reindex` request.

By default if `_reindex` sees a document with routing then the routing is
preserved unless it's changed by the script. You can set `routing` on the
`dest` request to change this:

`keep`::

Sets the routing on the bulk request sent for each match to the routing on
the match. This is the default value.

`discard`::

Sets the routing on the bulk request sent for each match to `null`.

`=<some text>`::

Sets the routing on the bulk request sent for each match to all text after
the `=`.

For example, you can use the following request to copy all documents from
the `source` index with the company name `cat` into the `dest` index with
routing set to `cat`.

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "source",
    "query": {
      "match": {
        "company": "cat"
      }
    }
  },
  "dest": {
    "index": "dest",
    "routing": "=cat"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT source\n/]

By default `_reindex` uses scroll batches of 1000. You can change the
batch size with the `size` field in the `source` element:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "source",
    "size": 100
  },
  "dest": {
    "index": "dest",
    "routing": "=cat"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT source\n/]

Reindex can also use the <<ingest>> feature by specifying a
`pipeline` like this:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "source"
  },
  "dest": {
    "index": "dest",
    "pipeline": "some_ingest_pipeline"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT source\n/]

[float]
[[reindex-from-remote]]
=== Reindex from Remote

Reindex supports reindexing from a remote Elasticsearch cluster:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "remote": {
      "host": "http://otherhost:9200",
      "username": "user",
      "password": "pass"
    },
    "index": "source",
    "query": {
      "match": {
        "test": "data"
      }
    }
  },
  "dest": {
    "index": "dest"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:host]
// TEST[s/^/PUT source\n/]
// TEST[s/otherhost:9200",/\${host}"/]
// TEST[s/"username": "user",//]
// TEST[s/"password": "pass"//]

The `host` parameter must contain a scheme, host, port (e.g.
`https://otherhost:9200`) and optional path (e.g. `https://otherhost:9200/proxy`).
The `username` and `password` parameters are optional, and when they are present `_reindex`
will connect to the remote Elasticsearch node using basic auth. Be sure to use `https` when
using basic auth or the password will be sent in plain text.

Remote hosts have to be explicitly whitelisted in elasticsearch.yml using the
`reindex.remote.whitelist` property. It can be set to a comma delimited list
of allowed remote `host` and `port` combinations (e.g.
`otherhost:9200, another:9200, 127.0.10.*:9200, localhost:*`). Scheme is
ignored by the whitelist - only host and port are used, for example:


[source,yaml]
--------------------------------------------------
reindex.remote.whitelist: "otherhost:9200, another:9200, 127.0.10.*:9200, localhost:*"
--------------------------------------------------

The whitelist must be configured on any nodes that will coordinate the reindex.

This feature should work with remote clusters of any version of Elasticsearch
you are likely to find. This should allow you to upgrade from any version of
Elasticsearch to the current version by reindexing from a cluster of the old
version.

To enable queries sent to older versions of Elasticsearch the `query` parameter
is sent directly to the remote host without validation or modification.

NOTE: Reindexing from remote clusters does not support
<<docs-reindex-manual-slice, manual>> or
<<docs-reindex-automatic-slice, automatic slicing>>.

Reindexing from a remote server uses an on-heap buffer that defaults to a
maximum size of 100mb. If the remote index includes very large documents you'll
need to use a smaller batch size. The example below sets the batch size to `10`
which is very, very small.

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "remote": {
      "host": "http://otherhost:9200"
    },
    "index": "source",
    "size": 10,
    "query": {
      "match": {
        "test": "data"
      }
    }
  },
  "dest": {
    "index": "dest"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:host]
// TEST[s/^/PUT source\n/]
// TEST[s/otherhost:9200/\${host}/]

It is also possible to set the socket read timeout on the remote connection
with the `socket_timeout` field and the connection timeout with the
`connect_timeout` field. Both default to 30 seconds. This example
sets the socket read timeout to one minute and the connection timeout to 10
seconds:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "remote": {
      "host": "http://otherhost:9200",
      "socket_timeout": "1m",
      "connect_timeout": "10s"
    },
    "index": "source",
    "query": {
      "match": {
        "test": "data"
      }
    }
  },
  "dest": {
    "index": "dest"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:host]
// TEST[s/^/PUT source\n/]
// TEST[s/otherhost:9200/\${host}/]

[float]
=== URL Parameters

In addition to the standard parameters like `pretty`, the Reindex API also
supports `refresh`, `wait_for_completion`, `wait_for_active_shards`, `timeout`,
`scroll` and `requests_per_second`.

Sending the `refresh` url parameter will cause all indexes to which the request
wrote to be refreshed. This is different than the Index API's `refresh`
parameter which causes just the shard that received the new data to be
refreshed. Also unlike the Index API it does not support `wait_for`.

If the request contains `wait_for_completion=false` then Elasticsearch will
perform some preflight checks, launch the request, and then return a `task`
which can be used with <<docs-reindex-task-api,Tasks APIs>>
to cancel or get the status of the task. Elasticsearch will also create a
record of this task as a document at `.tasks/task/${taskId}`. This is yours
to keep or remove as you see fit. When you are done with it, delete it so
Elasticsearch can reclaim the space it uses.

`wait_for_active_shards` controls how many copies of a shard must be active
before proceeding with the reindexing. See <<index-wait-for-active-shards,here>>
for details. `timeout` controls how long each write request waits for unavailable
shards to become available. Both work exactly how they work in the
<<docs-bulk,Bulk API>>. As `_reindex` uses scroll search, you can also specify
the `scroll` parameter to control how long it keeps the "search context" alive,
(e.g. `?scroll=10m`). The default value is 5 minutes.

`requests_per_second` can be set to any positive decimal number (`1.4`, `6`,
`1000`, etc) and throttles the rate at which `_reindex` issues batches of index
operations by padding each batch with a wait time. The throttling can be
disabled by setting `requests_per_second` to `-1`.

The throttling is done by waiting between batches so that the `scroll` which `_reindex`
uses internally can be given a timeout that takes into account the padding.
The padding time is the difference between the batch size divided by the
`requests_per_second` and the time spent writing. By default the batch size is
`1000`, so if the `requests_per_second` is set to `500`:

[source,txt]
--------------------------------------------------
target_time = 1000 / 500 per second = 2 seconds
wait_time = target_time - write_time = 2 seconds - .5 seconds = 1.5 seconds
--------------------------------------------------

Since the batch is issued as a single `_bulk` request, large batch sizes will
cause Elasticsearch to create many requests and then wait for a while before
starting the next set. This is "bursty" instead of "smooth". The default value is `-1`.

[float]
[[docs-reindex-response-body]]
=== Response body

//////////////////////////
[source,js]
--------------------------------------------------
POST /_reindex?wait_for_completion
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

//////////////////////////

The JSON response looks like this:

[source,js]
--------------------------------------------------
{
  "took": 639,
  "timed_out": false,
  "total": 5,
  "updated": 0,
  "created": 5,
  "deleted": 0,
  "batches": 1,
  "noops": 0,
  "version_conflicts": 2,
  "retries": {
    "bulk": 0,
    "search": 0
  },
  "throttled_millis": 0,
  "requests_per_second": 1,
  "throttled_until_millis": 0,
  "failures": [ ]
}
--------------------------------------------------
// TESTRESPONSE[s/: [0-9]+/: $body.$_path/]

`took`::

The total milliseconds the entire operation took.

`timed_out`::

This flag is set to `true` if any of the requests executed during the
reindex timed out.

`total`::

The number of documents that were successfully processed.

`updated`::

The number of documents that were successfully updated.

`created`::

The number of documents that were successfully created.

`deleted`::

The number of documents that were successfully deleted.

`batches`::

The number of scroll responses pulled back by the reindex.

`noops`::

The number of documents that were ignored because the script used for
the reindex returned a `noop` value for `ctx.op`.

`version_conflicts`::

The number of version conflicts that reindex hit.

`retries`::

The number of retries attempted by reindex. `bulk` is the number of bulk
actions retried and `search` is the number of search actions retried.

`throttled_millis`::

Number of milliseconds the request slept to conform to `requests_per_second`.

`requests_per_second`::

The number of requests per second effectively executed during the reindex.

`throttled_until_millis`::

This field should always be equal to zero in a `_reindex` response. It only
has meaning when using the <<docs-reindex-task-api, Task API>>, where it
indicates the next time (in milliseconds since epoch) a throttled request will be
executed again in order to conform to `requests_per_second`.

`failures`::

Array of failures if there were any unrecoverable errors during the process. If
this is non-empty then the request aborted because of those failures. Reindex
is implemented using batches and any failure causes the entire process to abort
but all failures in the current batch are collected into the array. You can use
the `conflicts` option to prevent reindex from aborting on version conflicts.

[float]
[[docs-reindex-task-api]]
=== Works with the Task API

You can fetch the status of all running reindex requests with the
<<tasks,Task API>>:

[source,js]
--------------------------------------------------
GET _tasks?detailed=true&actions=*reindex
--------------------------------------------------
// CONSOLE
// TEST[skip:No tasks to retrieve]

The response looks like:

[source,js]
--------------------------------------------------
{
  "nodes" : {
    "r1A2WoRbTwKZ516z6NEs5A" : {
      "name" : "r1A2WoR",
      "transport_address" : "127.0.0.1:9300",
      "host" : "127.0.0.1",
      "ip" : "127.0.0.1:9300",
      "attributes" : {
        "testattr" : "test",
        "portsfile" : "true"
      },
      "tasks" : {
        "r1A2WoRbTwKZ516z6NEs5A:36619" : {
          "node" : "r1A2WoRbTwKZ516z6NEs5A",
          "id" : 36619,
          "type" : "transport",
          "action" : "indices:data/write/reindex",
          "status" : {    <1>
            "total" : 6154,
            "updated" : 3500,
            "created" : 0,
            "deleted" : 0,
            "batches" : 4,
            "version_conflicts" : 0,
            "noops" : 0,
            "retries": {
              "bulk": 0,
              "search": 0
            },
            "throttled_millis": 0,
            "requests_per_second": -1,
            "throttled_until_millis": 0
          },
          "description" : "",
          "start_time_in_millis": 1535149899665,
          "running_time_in_nanos": 5926916792,
          "cancellable": true,
          "headers": {}
        }
      }
    }
  }
}
--------------------------------------------------
// TESTRESPONSE
<1> this object contains the actual status. It is identical to the response JSON
except for the important addition of the `total` field. `total` is the total number
of operations that the `_reindex` expects to perform. You can estimate the
progress by adding the `updated`, `created`, and `deleted` fields. The request
will finish when their sum is equal to the `total` field.

With the task id you can look up the task directly. The following example 
retrieves information about the task `r1A2WoRbTwKZ516z6NEs5A:36619`:

[source,js]
--------------------------------------------------
GET /_tasks/r1A2WoRbTwKZ516z6NEs5A:36619
--------------------------------------------------
// CONSOLE
// TEST[catch:missing]

The advantage of this API is that it integrates with `wait_for_completion=false`
to transparently return the status of completed tasks. If the task is completed
and `wait_for_completion=false` was set, it will return a
`results` or an `error` field. The cost of this feature is the document that
`wait_for_completion=false` creates at `.tasks/task/${taskId}`. It is up to
you to delete that document.


[float]
[[docs-reindex-cancel-task-api]]
=== Works with the Cancel Task API

Any Reindex can be canceled using the <<task-cancellation,Task Cancel API>>. For 
example:

[source,js]
--------------------------------------------------
POST _tasks/r1A2WoRbTwKZ516z6NEs5A:36619/_cancel
--------------------------------------------------
// CONSOLE

The task ID can be found using the <<tasks,Tasks API>>.

Cancelation should happen quickly but might take a few seconds. The Tasks
API will continue to list the task until it wakes to cancel itself.


[float]
[[docs-reindex-rethrottle]]
=== Rethrottling

The value of `requests_per_second` can be changed on a running reindex using
the `_rethrottle` API:

[source,js]
--------------------------------------------------
POST _reindex/r1A2WoRbTwKZ516z6NEs5A:36619/_rethrottle?requests_per_second=-1
--------------------------------------------------
// CONSOLE

The task ID can be found using the <<tasks,tasks API>>.

Just like when setting it on the Reindex API, `requests_per_second`
can be either `-1` to disable throttling or any decimal number
like `1.7` or `12` to throttle to that level. Rethrottling that speeds up the
query takes effect immediately but rethrotting that slows down the query will
take effect on after completing the current batch. This prevents scroll
timeouts.

[float]
[[docs-reindex-change-name]]
=== Reindex to change the name of a field

`_reindex` can be used to build a copy of an index with renamed fields. Say you
create an index containing documents that look like this:

[source,js]
--------------------------------------------------
POST test/_doc/1?refresh
{
  "text": "words words",
  "flag": "foo"
}
--------------------------------------------------
// CONSOLE

but you don't like the name `flag` and want to replace it with `tag`.
`_reindex` can create the other index for you:

[source,js]
--------------------------------------------------
POST _reindex
{
  "source": {
    "index": "test"
  },
  "dest": {
    "index": "test2"
  },
  "script": {
    "source": "ctx._source.tag = ctx._source.remove(\"flag\")"
  }
}
--------------------------------------------------
// CONSOLE
// TEST[continued]

Now you can get the new document:

[source,js]
--------------------------------------------------
GET test2/_doc/1
--------------------------------------------------
// CONSOLE
// TEST[continued]

which will return:

[source,js]
--------------------------------------------------
{
  "found": true,
  "_id": "1",
  "_index": "test2",
  "_type": "_doc",
  "_version": 1,
  "_source": {
    "text": "words words",
    "tag": "foo"
  }
}
--------------------------------------------------
// TESTRESPONSE

[float]
[[docs-reindex-slice]]
=== Slicing

Reindex supports <<sliced-scroll>> to parallelize the reindexing process.
This parallelization can improve efficiency and provide a convenient way to
break the request down into smaller parts.

[float]
[[docs-reindex-manual-slice]]
==== Manual slicing
Slice a reindex request manually by providing a slice id and total number of
slices to each request:

[source,js]
----------------------------------------------------------------
POST _reindex
{
  "source": {
    "index": "twitter",
    "slice": {
      "id": 0,
      "max": 2
    }
  },
  "dest": {
    "index": "new_twitter"
  }
}
POST _reindex
{
  "source": {
    "index": "twitter",
    "slice": {
      "id": 1,
      "max": 2
    }
  },
  "dest": {
    "index": "new_twitter"
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

You can verify this works by:

[source,js]
----------------------------------------------------------------
GET _refresh
POST new_twitter/_search?size=0&filter_path=hits.total
----------------------------------------------------------------
// CONSOLE
// TEST[continued]

which results in a sensible `total` like this one:

[source,js]
----------------------------------------------------------------
{
  "hits": {
    "total": 120
  }
}
----------------------------------------------------------------
// TESTRESPONSE

[float]
[[docs-reindex-automatic-slice]]
==== Automatic slicing

You can also let `_reindex` automatically parallelize using <<sliced-scroll>> to
slice on `_uid`. Use `slices` to specify the number of slices to use:

[source,js]
----------------------------------------------------------------
POST _reindex?slices=5&refresh
{
  "source": {
    "index": "twitter"
  },
  "dest": {
    "index": "new_twitter"
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

You can also this verify works by:

[source,js]
----------------------------------------------------------------
POST new_twitter/_search?size=0&filter_path=hits.total
----------------------------------------------------------------
// CONSOLE
// TEST[continued]

which results in a sensible `total` like this one:

[source,js]
----------------------------------------------------------------
{
  "hits": {
    "total": 120
  }
}
----------------------------------------------------------------
// TESTRESPONSE

Setting `slices` to `auto` will let Elasticsearch choose the number of slices
to use. This setting will use one slice per shard, up to a certain limit. If
there are multiple source indices, it will choose the number of slices based
on the index with the smallest number of shards.

Adding `slices` to `_reindex` just automates the manual process used in the
section above, creating sub-requests which means it has some quirks:

* You can see these requests in the <<docs-reindex-task-api,Tasks APIs>>. These
sub-requests are "child" tasks of the task for the request with `slices`.
* Fetching the status of the task for the request with `slices` only contains
the status of completed slices.
* These sub-requests are individually addressable for things like cancelation
and rethrottling.
* Rethrottling the request with `slices` will rethrottle the unfinished
sub-request proportionally.
* Canceling the request with `slices` will cancel each sub-request.
* Due to the nature of `slices` each sub-request won't get a perfectly even
portion of the documents. All documents will be addressed, but some slices may
be larger than others. Expect larger slices to have a more even distribution.
* Parameters like `requests_per_second` and `size` on a request with `slices`
are distributed proportionally to each sub-request. Combine that with the point
above about distribution being uneven and you should conclude that the using
`size` with `slices` might not result in exactly `size` documents being
`_reindex`ed.
* Each sub-request gets a slightly different snapshot of the source index,
though these are all taken at approximately the same time.

[float]
[[docs-reindex-picking-slices]]
===== Picking the number of slices

If slicing automatically, setting `slices` to `auto` will choose a reasonable
number for most indices. If slicing manually or otherwise tuning
automatic slicing, use these guidelines.

Query performance is most efficient when the number of `slices` is equal to the
number of shards in the index. If that number is large (e.g. 500),
choose a lower number as too many `slices` will hurt performance. Setting
`slices` higher than the number of shards generally does not improve efficiency
and adds overhead.

Indexing performance scales linearly across available resources with the
number of slices.

Whether query or indexing performance dominates the runtime depends on the
documents being reindexed and cluster resources.

[float]
=== Reindexing many indices
If you have many indices to reindex it is generally better to reindex them
one at a time rather than using a glob pattern to pick up many indices. That
way you can resume the process if there are any errors by removing the
partially completed index and starting over at that index. It also makes
parallelizing the process fairly simple: split the list of indices to reindex
and run each list in parallel.

One off bash scripts seem to work nicely for this:

[source,bash]
----------------------------------------------------------------
for index in i1 i2 i3 i4 i5; do
  curl -HContent-Type:application/json -XPOST localhost:9200/_reindex?pretty -d'{
    "source": {
      "index": "'$index'"
    },
    "dest": {
      "index": "'$index'-reindexed"
    }
  }'
done
----------------------------------------------------------------
// NOTCONSOLE

[float]
=== Reindex daily indices

Notwithstanding the above advice, you can use `_reindex` in combination with
<<modules-scripting-painless, Painless>> to reindex daily indices to apply
a new template to the existing documents.

Assuming you have indices consisting of documents as follows:

[source,js]
----------------------------------------------------------------
PUT metricbeat-2016.05.30/_doc/1?refresh
{"system.cpu.idle.pct": 0.908}
PUT metricbeat-2016.05.31/_doc/1?refresh
{"system.cpu.idle.pct": 0.105}
----------------------------------------------------------------
// CONSOLE

The new template for the `metricbeat-*` indices is already loaded into Elasticsearch,
but it applies only to the newly created indices. Painless can be used to reindex
the existing documents and apply the new template.

The script below extracts the date from the index name and creates a new index
with `-1` appended. All data from `metricbeat-2016.05.31` will be reindexed
into `metricbeat-2016.05.31-1`.

[source,js]
----------------------------------------------------------------
POST _reindex
{
  "source": {
    "index": "metricbeat-*"
  },
  "dest": {
    "index": "metricbeat"
  },
  "script": {
    "lang": "painless",
    "source": "ctx._index = 'metricbeat-' + (ctx._index.substring('metricbeat-'.length(), ctx._index.length())) + '-1'"
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[continued]

All documents from the previous metricbeat indices can now be found in the `*-1` indices.

[source,js]
----------------------------------------------------------------
GET metricbeat-2016.05.30-1/_doc/1
GET metricbeat-2016.05.31-1/_doc/1
----------------------------------------------------------------
// CONSOLE
// TEST[continued]

The previous method can also be used in conjunction with <<docs-reindex-change-name, change the name of a field>>
to load only the existing data into the new index and rename any fields if needed.

[float]
=== Extracting a random subset of an index

`_reindex` can be used to extract a random subset of an index for testing:

[source,js]
----------------------------------------------------------------
POST _reindex
{
  "size": 10,
  "source": {
    "index": "twitter",
    "query": {
      "function_score" : {
        "query" : { "match_all": {} },
        "random_score" : {}
      }
    },
    "sort": "_score"    <1>
  },
  "dest": {
    "index": "random_twitter"
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

<1> `_reindex` defaults to sorting by `_doc` so `random_score` will not have any
effect unless you override the sort to `_score`.
