OpenSearch/docs/painless/painless-contexts/painless-metric-agg-init-co...

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[[painless-metric-agg-init-context]]
=== Metric aggregation initialization context
Use a Painless script to
{es_version}/search-aggregations-metrics-scripted-metric-aggregation.html[initialize]
values for use in a scripted metric aggregation. An initialization script is
run prior to document collection once per shard and is optional as part of the
full metric aggregation.
*Variables*
`params` (`Map`, read-only)::
User-defined parameters passed in as part of the query.
`params['_agg']` (`Map`)::
Empty `Map` used to add values for use in a
<<painless-metric-agg-map-context, map script>>.
*Side Effects*
`params['_agg']` (`Map`)::
Add values to this `Map` to for use in a map. Additional values must
be of the type `Map`, `List`, `String` or primitive.
*Return*
`void`::
No expected return value.
*API*
The standard <<painless-api-reference, Painless API>> is available.