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Scripted Metric Aggregation
editScripted Metric Aggregation
editA 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" } } } }
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 |
For more details on specifying scripts see script documentation.
Allowed return types
editWhilst 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
editThe 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 arraytransactions
in thestate
object. - map_script
-
Executed once per document collected. This is a 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. This is a required script. Allows the aggregation to consolidate the state returned from each shard.
In the above example, the
combine_script
iterates through all the stored transactions, summing the values in theprofit
variable and finally returnsprofit
. - reduce_script
-
Executed once on the coordinating node after all shards have returned their results. This is a required script. The script is provided with access to a variable
states
which is an array of the result of the combine_script on each shard.In the above example, the
reduce_script
iterates through theprofit
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
editImagine a situation where you index the following documents into an index with 2 shards:
PUT /transactions/_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.
After init_script
editThis 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
editEach 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
editThe 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
editThe 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
edit
params |
Optional. An object whose contents will be passed as variables to the "params" : {} |
Empty buckets
editIf 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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