281 lines
8.9 KiB
Plaintext
281 lines
8.9 KiB
Plaintext
[[search-aggregations-metrics-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.
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Example:
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[source,console]
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--------------------------------------------------
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POST ledger/_search?size=0
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{
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"query" : {
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"match_all" : {}
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},
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"aggs": {
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"profit": {
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"scripted_metric": {
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"init_script" : "state.transactions = []", <1>
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"map_script" : "state.transactions.add(doc.type.value == 'sale' ? doc.amount.value : -1 * doc.amount.value)",
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"combine_script" : "double profit = 0; for (t in state.transactions) { profit += t } return profit",
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"reduce_script" : "double profit = 0; for (a in states) { profit += a } return profit"
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}
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}
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}
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}
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--------------------------------------------------
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// TEST[setup:ledger]
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<1> `init_script` is an optional parameter, all other scripts are required.
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The above aggregation demonstrates how one would use the script aggregation compute the total profit from sale and cost transactions.
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The response for the above aggregation:
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[source,console-result]
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--------------------------------------------------
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{
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"took": 218,
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...
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"aggregations": {
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"profit": {
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"value": 240.0
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}
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}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"took": 218/"took": $body.took/]
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// TESTRESPONSE[s/\.\.\./"_shards": $body._shards, "hits": $body.hits, "timed_out": false,/]
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The above example can also be specified using stored scripts as follows:
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[source,console]
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--------------------------------------------------
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POST ledger/_search?size=0
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{
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"aggs": {
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"profit": {
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"scripted_metric": {
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"init_script" : {
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"id": "my_init_script"
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},
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"map_script" : {
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"id": "my_map_script"
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},
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"combine_script" : {
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"id": "my_combine_script"
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},
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"params": {
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"field": "amount" <1>
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},
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"reduce_script" : {
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"id": "my_reduce_script"
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}
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}
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}
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}
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}
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--------------------------------------------------
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// TEST[setup:ledger,stored_scripted_metric_script]
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<1> script parameters for `init`, `map` and `combine` scripts must be specified
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in a global `params` object so that it can be shared between the scripts.
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////
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Verify this response as well but in a hidden block.
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[source,console-result]
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--------------------------------------------------
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{
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"took": 218,
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...
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"aggregations": {
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"profit": {
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"value": 240.0
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}
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}
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}
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--------------------------------------------------
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// TESTRESPONSE[s/"took": 218/"took": $body.took/]
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// TESTRESPONSE[s/\.\.\./"_shards": $body._shards, "hits": $body.hits, "timed_out": false,/]
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////
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For more details on specifying scripts see <<modules-scripting, script documentation>>.
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[[scripted-metric-aggregation-return-types]]
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==== 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:
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* primitive types
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* String
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* Map (containing only keys and values of the types listed here)
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* Array (containing elements of only the types listed here)
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[[scripted-metric-aggregation-scope]]
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==== Scope of scripts
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The scripted metric aggregation uses scripts at 4 stages of its execution:
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init_script:: Executed prior to any collection of documents. Allows the aggregation to set up any initial state.
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+
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In the above example, the `init_script` creates an array `transactions` in the `state` object.
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map_script:: Executed once per document collected. This is a required script. If no combine_script is specified, the resulting state
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needs to be stored in the `state` object.
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+
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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
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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
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to transactions.
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combine_script:: Executed once on each shard after document collection is complete. This is a required script. Allows the aggregation to
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consolidate the state returned from each shard.
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+
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In the above example, the `combine_script` iterates through all the stored transactions, summing the values in the `profit` variable
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and finally returns `profit`.
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reduce_script:: Executed once on the coordinating node after all shards have returned their results. This is a required script. The
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script is provided with access to a variable `states` which is an array of the result of the combine_script on each
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shard.
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+
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In the above example, the `reduce_script` iterates through the `profit` returned by each shard summing the values before returning the
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final combined profit which will be returned in the response of the aggregation.
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[[scripted-metric-aggregation-example]]
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==== Worked example
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Imagine a situation where you index the following documents into an index with 2 shards:
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[source,console]
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--------------------------------------------------
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PUT /transactions/_bulk?refresh
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{"index":{"_id":1}}
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{"type": "sale","amount": 80}
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{"index":{"_id":2}}
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{"type": "cost","amount": 10}
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{"index":{"_id":3}}
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{"type": "cost","amount": 30}
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{"index":{"_id":4}}
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{"type": "sale","amount": 130}
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--------------------------------------------------
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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
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at each stage of the example above.
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===== Before init_script
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`state` is initialized as a new empty object.
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[source,js]
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--------------------------------------------------
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"state" : {}
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--------------------------------------------------
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// NOTCONSOLE
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===== 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:
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Shard A::
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+
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[source,js]
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--------------------------------------------------
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"state" : {
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"transactions" : []
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}
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--------------------------------------------------
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// NOTCONSOLE
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Shard B::
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+
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[source,js]
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--------------------------------------------------
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"state" : {
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"transactions" : []
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}
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--------------------------------------------------
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// NOTCONSOLE
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===== After map_script
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Each shard collects its documents and runs the map_script on each document that is collected:
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Shard A::
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+
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[source,js]
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--------------------------------------------------
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"state" : {
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"transactions" : [ 80, -30 ]
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}
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--------------------------------------------------
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// NOTCONSOLE
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Shard B::
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+
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[source,js]
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--------------------------------------------------
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"state" : {
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"transactions" : [ -10, 130 ]
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}
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--------------------------------------------------
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// NOTCONSOLE
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===== 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
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shard (by summing the values in the transactions array) which is passed back to the coordinating node:
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Shard A:: 50
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Shard B:: 120
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===== After reduce_script
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The reduce_script receives a `states` array containing the result of the combine script for each shard:
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[source,js]
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--------------------------------------------------
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"states" : [
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50,
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120
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]
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--------------------------------------------------
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// NOTCONSOLE
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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
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produce the response:
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[source,js]
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--------------------------------------------------
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{
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...
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"aggregations": {
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"profit": {
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"value": 170
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}
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}
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}
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--------------------------------------------------
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// NOTCONSOLE
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[[scripted-metric-aggregation-parameters]]
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==== Other parameters
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[horizontal]
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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
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useful to allow the user to control the behavior of the aggregation and for storing state between the scripts. If this is not specified,
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the default is the equivalent of providing:
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+
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[source,js]
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--------------------------------------------------
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"params" : {}
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--------------------------------------------------
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// NOTCONSOLE
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[[scripted-metric-aggregation-empty-buckets]]
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==== 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
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shard with a `null` value. In this case the `reduce_script`'s `states` variable will contain `null` as a response from that shard.
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`reduce_script`'s should therefore expect and deal with `null` responses from shards.
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