[[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.