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Scripted Metric Aggregation

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Scripted Metric Aggregation

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A metric aggregation that executes using scripts to provide a metric output.

Example:

POST ledger/_search?size=0
{
    "query" : {
        "match_all" : {}
    },
    "aggs": {
        "profit": {
            "scripted_metric": {
                "init_script" : "state.transactions = []",
                "map_script" : "state.transactions.add(doc.type.value == 'sale' ? doc.amount.value : -1 * doc.amount.value)", 
                "combine_script" : "double profit = 0; for (t in state.transactions) { profit += t } return profit",
                "reduce_script" : "double profit = 0; for (a in states) { profit += a } return profit"
            }
        }
    }
}

map_script is the only required parameter

The above aggregation demonstrates how one would use the script aggregation compute the total profit from sale and cost transactions.

The response for the above aggregation:

{
    "took": 218,
    ...
    "aggregations": {
        "profit": {
            "value": 240.0
        }
   }
}

The above example can also be specified using stored scripts as follows:

POST ledger/_search?size=0
{
    "aggs": {
        "profit": {
            "scripted_metric": {
                "init_script" : {
                    "id": "my_init_script"
                },
                "map_script" : {
                    "id": "my_map_script"
                },
                "combine_script" : {
                    "id": "my_combine_script"
                },
                "params": {
                    "field": "amount" 
                },
                "reduce_script" : {
                    "id": "my_reduce_script"
                }
            }
        }
    }
}

script parameters for init, map and combine scripts must be specified in a global params object so that it can be shared between the scripts.

For more details on specifying scripts see script documentation.

Allowed return types

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Whilst any valid script object can be used within a single script, the scripts must return or store in the state object only the following types:

primitive types
String
Map (containing only keys and values of the types listed here)
Array (containing elements of only the types listed here)

Scope of scripts

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The scripted metric aggregation uses scripts at 4 stages of its execution:

init_script

Executed prior to any collection of documents. Allows the aggregation to set up any initial state.

In the above example, the init_script creates an array transactions in the state object.

map_script

Executed once per document collected. This is the only required script. If no combine_script is specified, the resulting state needs to be stored in the state object.

In the above example, the map_script checks the value of the type field. If the value is sale the value of the amount field is added to the transactions array. If the value of the type field is not sale the negated value of the amount field is added to transactions.

combine_script

Executed once on each shard after document collection is complete. Allows the aggregation to consolidate the state returned from each shard. If a combine_script is not provided the combine phase will return the aggregation variable.

In the above example, the combine_script iterates through all the stored transactions, summing the values in the profit variable and finally returns profit.

reduce_script

Executed once on the coordinating node after all shards have returned their results. The script is provided with access to a variable states which is an array of the result of the combine_script on each shard. If a reduce_script is not provided the reduce phase will return the states variable.

In the above example, the reduce_script iterates through the profit returned by each shard summing the values before returning the final combined profit which will be returned in the response of the aggregation.

Worked Example

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Imagine a situation where you index the following documents into an index with 2 shards:

PUT /transactions/_doc/_bulk?refresh
{"index":{"_id":1}}
{"type": "sale","amount": 80}
{"index":{"_id":2}}
{"type": "cost","amount": 10}
{"index":{"_id":3}}
{"type": "cost","amount": 30}
{"index":{"_id":4}}
{"type": "sale","amount": 130}

Lets say that documents 1 and 3 end up on shard A and documents 2 and 4 end up on shard B. The following is a breakdown of what the aggregation result is at each stage of the example above.

Before init_script

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state is initialized as a new empty object.

"state" : {}

After init_script

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This is run once on each shard before any document collection is performed, and so we will have a copy on each shard:

Shard A

"state" : {
    "transactions" : []
}

Shard B

"state" : {
    "transactions" : []
}

After map_script

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Each shard collects its documents and runs the map_script on each document that is collected:

Shard A

"state" : {
    "transactions" : [ 80, -30 ]
}

Shard B

"state" : {
    "transactions" : [ -10, 130 ]
}

After combine_script

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The combine_script is executed on each shard after document collection is complete and reduces all the transactions down to a single profit figure for each shard (by summing the values in the transactions array) which is passed back to the coordinating node:

Shard A: 50
Shard B: 120

After reduce_script

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The reduce_script receives a states array containing the result of the combine script for each shard:

"states" : [
    50,
    120
]

It reduces the responses for the shards down to a final overall profit figure (by summing the values) and returns this as the result of the aggregation to produce the response:

{
    ...

    "aggregations": {
        "profit": {
            "value": 170
        }
   }
}

Other Parameters

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params

Optional. An object whose contents will be passed as variables to the init_script, map_script and combine_script. This can be useful to allow the user to control the behavior of the aggregation and for storing state between the scripts. If this is not specified, the default is the equivalent of providing:

"params" : {}

Empty Buckets

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If a parent bucket of the scripted metric aggregation does not collect any documents an empty aggregation response will be returned from the shard with a null value. In this case the reduce_script's states variable will contain null as a response from that shard. reduce_script's should therefore expect and deal with null responses from shards.

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