[[search-aggregations-reducer-max-bucket-aggregation]]
=== Max Bucket Aggregation

A sibling reducer aggregation which identifies the bucket(s) with the maximum value of a specified metric in a sibling aggregation
and outputs both the value and the key(s) of the bucket(s). The specified metric must be numeric and the sibling aggregation must 
be a multi-bucket aggregation.

==== Syntax

A `max_bucket` aggregation looks like this in isolation:

[source,js]
--------------------------------------------------
{
    "max_bucket": {
        "buckets_path": "the_sum"
    }
}
--------------------------------------------------

.`max_bucket` Parameters
|===
|Parameter Name |Description |Required |Default Value
|`buckets_path` |The path to the buckets we wish to find the maximum for (see <<bucket-path-syntax>> for more
 details) |Required |
 |`gap_policy` |The policy to apply when gaps are found in the data (see <<gap-policy>> for more
 details)|Optional, defaults to `skip` |
 |`format` |format to apply to the output value of this aggregation |Optional, defaults to `null` |
|===

The following snippet calculates the maximum of the total monthly `sales`:

[source,js]
--------------------------------------------------
{
    "aggs" : {
        "sales_per_month" : {
            "date_histogram" : {
                "field" : "date",
                "interval" : "month"
            },
            "aggs": {
                "sales": {
                    "sum": {
                        "field": "price"
                    }
                }
            }
        },
        "max_monthly_sales": {
            "max_bucket": {
                "buckets_paths": "sales_per_month>sales" <1>
            }
        }
    }
}
--------------------------------------------------
<1> `bucket_paths` instructs this max_bucket aggregation that we want the maximum value of the `sales` aggregation in the 
`sales_per_month` date histogram.

And the following may be the response:

[source,js]
--------------------------------------------------
{
   "aggregations": {
      "sales_per_month": {
         "buckets": [
            {
               "key_as_string": "2015/01/01 00:00:00",
               "key": 1420070400000,
               "doc_count": 3,
               "sales": {
                  "value": 550
               }
            },
            {
               "key_as_string": "2015/02/01 00:00:00",
               "key": 1422748800000,
               "doc_count": 2,
               "sales": {
                  "value": 60
               }
            },
            {
               "key_as_string": "2015/03/01 00:00:00",
               "key": 1425168000000,
               "doc_count": 2,
               "sales": {
                  "value": 375
               }
            }
         ]
      },
      "max_monthly_sales": {
          "keys": ["2015/01/01 00:00:00"], <1>
          "value": 550
      }
   }
}
--------------------------------------------------

<1> `keys` is an array of strings since the maximum value may be present in multiple buckets