[[docs-delete-by-query]]
== Delete By Query API

experimental[The delete-by-query API is new and should still be considered experimental.  The API may change in ways that are not backwards compatible]

The simplest usage of `_delete_by_query` just performs a deletion on every
document that match a query. Here is the API:

[source,js]
--------------------------------------------------
POST twitter/_delete_by_query
{
  "query": { <1>
    "match": {
      "message": "some message"
    }
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

<1> The query must be passed as a value to the `query` key, in the same
way as the <<search-search,Search API>>. You can also use the `q`
parameter in the same way as the search api.

That will return something like this:

[source,js]
--------------------------------------------------
{
  "took" : 147,
  "timed_out": false,
  "deleted": 119,
  "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": 119,
  "failures" : [ ]
}
--------------------------------------------------
// TESTRESPONSE[s/"took" : 147/"took" : "$body.took"/]

`_delete_by_query` gets a snapshot of the index when it starts and deletes what
it finds using `internal` versioning. That means that you'll get a version
conflict if the document changes between the time when the snapshot was taken
and when the delete request is processed. When the versions match the document
is deleted.

NOTE: Since `internal` versioning does not support the value 0 as a valid
version number, documents with version equal to zero cannot be deleted using
`_delete_by_query` and will fail the request.

During the `_delete_by_query` execution, multiple search requests are sequentially
executed in order to find all the matching documents to delete. Every time a batch
of documents is found, a corresponding bulk request is executed to delete all
these documents. In case a search or bulk request got rejected, `_delete_by_query`
 relies on a default policy to retry rejected requests (up to 10 times, with
 exponential back off). Reaching the maximum retries limit causes the `_delete_by_query`
 to abort and all failures are returned in the `failures` of the response.
 The deletions that have been performed still stick. In other words, the process
 is not rolled back, only aborted. While the first failure causes the abort, all
 failures that are returned by the failing bulk request are returned in the `failures`
 element; therefore it's possible for there to be quite a few failed entities.

If you'd like to count version conflicts rather than cause them to abort then
set `conflicts=proceed` on the url or `"conflicts": "proceed"` in the request body.

Back to the API format, you can limit `_delete_by_query` to a single type. This
will only delete `tweet` documents from the `twitter` index:

[source,js]
--------------------------------------------------
POST twitter/tweet/_delete_by_query?conflicts=proceed
{
  "query": {
    "match_all": {}
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

It's also possible to delete documents of multiple indexes and multiple
types at once, just like the search API:

[source,js]
--------------------------------------------------
POST twitter,blog/tweet,post/_delete_by_query
{
  "query": {
    "match_all": {}
  }
}
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT twitter\nPUT blog\n/]

If you provide `routing` then the routing is copied to the scroll query,
limiting the process to the shards that match that routing value:

[source,js]
--------------------------------------------------
POST twitter/_delete_by_query?routing=1
{
  "query": {
    "range" : {
        "age" : {
           "gte" : 10
        }
    }
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

By default `_delete_by_query` uses scroll batches of 1000. You can change the
batch size with the `scroll_size` URL parameter:

[source,js]
--------------------------------------------------
POST twitter/_delete_by_query?scroll_size=5000
{
  "query": {
    "term": {
      "user": "kimchy"
    }
  }
}
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]


[float]
=== URL Parameters

In addition to the standard parameters like `pretty`, the Delete By Query API
also supports `refresh`, `wait_for_completion`, `wait_for_active_shards`, and `timeout`.

Sending the `refresh` will refresh all shards involved in the delete by query
once the request completes. This is different than the Delete API's `refresh`
parameter which causes just the shard that received the delete request
to be refreshed.

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-delete-by-query-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 request. 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>>.

`requests_per_second` can be set to any positive decimal number (`1.4`, `6`,
`1000`, etc) and throttles the number of requests per second that the delete-by-query
issues or it can be set to `-1` to disabled throttling. The throttling is done
waiting between bulk batches so that it can manipulate the scroll timeout. The
wait time is the difference between the time it took the batch to complete and
the time `requests_per_second * requests_in_the_batch`. Since the batch isn't
broken into multiple bulk requests 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 is `-1`.

[float]
=== Response body

The JSON response looks like this:

[source,js]
--------------------------------------------------
{
  "took" : 639,
  "deleted": 0,
  "batches": 1,
  "version_conflicts": 2,
  "retries": 0,
  "throttled_millis": 0,
  "failures" : [ ]
}
--------------------------------------------------

`took`::

The number of milliseconds from start to end of the whole operation.

`deleted`::

The number of documents that were successfully deleted.

`batches`::

The number of scroll responses pulled back by the the delete by query.

`version_conflicts`::

The number of version conflicts that the delete by query hit.

`retries`::

The number of retries that the delete by query did in response to a full queue.

`throttled_millis`::

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

`failures`::

Array of all indexing failures. If this is non-empty then the request aborted
because of those failures. See `conflicts` for how to prevent version conflicts
from aborting the operation.


[float]
[[docs-delete-by-query-task-api]]
=== Works with the Task API

You can fetch the status of any running delete-by-query requests with the
<<tasks,Task API>>:

[source,js]
--------------------------------------------------
GET _tasks?detailed=true&actions=*/delete/byquery
--------------------------------------------------
// CONSOLE

The responses 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/delete/byquery",
          "status" : {    <1>
            "total" : 6154,
            "updated" : 0,
            "created" : 0,
            "deleted" : 3500,
            "batches" : 36,
            "version_conflicts" : 0,
            "noops" : 0,
            "retries": 0,
            "throttled_millis": 0
          },
          "description" : ""
        }
      }
    }
  }
}
--------------------------------------------------

<1> this object contains the actual status. It is just like the response json
with 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:

[source,js]
--------------------------------------------------
GET /_tasks/taskId:1
--------------------------------------------------
// 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 on it then it'll come back with 
`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-delete-by-query-cancel-task-api]]
=== Works with the Cancel Task API

Any Delete By Query can be canceled using the <<tasks,Task Cancel API>>:

[source,js]
--------------------------------------------------
POST _tasks/task_id:1/_cancel
--------------------------------------------------
// CONSOLE

The `task_id` can be found using the tasks API above.

Cancellation should happen quickly but might take a few seconds. The task status
API above will continue to list the task until it is wakes to cancel itself.


[float]
[[docs-delete-by-query-rethrottle]]
=== Rethrottling

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

[source,js]
--------------------------------------------------
POST _delete_by_query/task_id:1/_rethrottle?requests_per_second=-1
--------------------------------------------------
// CONSOLE

The `task_id` can be found using the tasks API above.

Just like when setting it on the `_delete_by_query` 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-delete-by-query-manual-slice]]
=== Manually slicing

Delete-by-query supports <<sliced-scroll>> allowing you to manually parallelize
the process relatively easily:

[source,js]
----------------------------------------------------------------
POST twitter/_delete_by_query
{
  "slice": {
    "id": 0,
    "max": 2
  },
  "query": {
    "range": {
      "likes": {
        "lt": 10
      }
    }
  }
}
POST twitter/_delete_by_query
{
  "slice": {
    "id": 1,
    "max": 2
  },
  "query": {
    "range": {
      "likes": {
        "lt": 10
      }
    }
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

Which you can verify works with:

[source,js]
----------------------------------------------------------------
GET _refresh
POST twitter/_search?size=0&filter_path=hits.total
{
  "query": {
    "range": {
      "likes": {
        "lt": 10
      }
    }
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[continued]

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

[source,js]
----------------------------------------------------------------
{
  "hits": {
    "total": 0
  }
}
----------------------------------------------------------------
// TESTRESPONSE

[float]
[[docs-delete-by-query-automatic-slice]]
=== Automatic slicing

You can also let delete-by-query automatically parallelize using
<<sliced-scroll>> to slice on `_uid`:

[source,js]
----------------------------------------------------------------
POST twitter/_delete_by_query?refresh&slices=5
{
  "query": {
    "range": {
      "likes": {
        "lt": 10
      }
    }
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[setup:big_twitter]

Which you also can verify works with:

[source,js]
----------------------------------------------------------------
POST twitter/_search?size=0&filter_path=hits.total
{
  "query": {
    "range": {
      "likes": {
        "lt": 10
      }
    }
  }
}
----------------------------------------------------------------
// CONSOLE
// TEST[continued]

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

[source,js]
----------------------------------------------------------------
{
  "hits": {
    "total": 0
  }
}
----------------------------------------------------------------
// TESTRESPONSE

Adding `slices` to `_delete_by_query` 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-delete-by-query-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 cancellation
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
`_delete_by_query`ed.
* Each sub-requests gets a slightly different snapshot of the source index
though these are all taken at approximately the same time.

[float]
[[docs-delete-by-query-picking-slices]]
=== Picking the number of slices

At this point we have a few recommendations around the number of `slices` to
use (the `max` parameter in the slice API if manually parallelizing):

* Don't use large numbers. `500` creates fairly massive CPU thrash.
* It is more efficient from a query performance standpoint to use some multiple
of the number of shards in the source index.
* Using exactly as many shards as are in the source index is the most efficient
from a query performance standpoint.
* Indexing performance should scale linearly across available resources with
the number of `slices`.
* Whether indexing or query performance dominates that process depends on lots
of factors like the documents being reindexed and the cluster doing the
reindexing.
