> ## Documentation Index
> Fetch the complete documentation index at: https://docs.chronosphere.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Preview aggregation rule impact

> Use the Shaping Impact Preview to preview the impact of aggregation rules on your system before applying them.

Shaping Impact Preview lets you create new aggregation rules to preview the impact of
a rule on your overall system. This helps prevent breaking changes and ensure the
rules you create operate as expected. A rule created for preview purposes lets you
profile the impact of the rule, but isn't enabled in your system and doesn't affect
your stored data.

Knowing the impact of a rule, and whether it accomplishes the needed task can be
difficult. Shaping impact preview uses the existing
[rollup](/control/shaping/shape-metrics/rules) or
[mapping](/control/shaping/shape-metrics/rules/mapping) aggregation rules to show you
how your rule affects your Chronosphere Observability Platform installation before
applying the rule and changing your data or stored time series.

## Preview a rule's shaping impact

Rules in preview mode display in the
[Aggregation Rules UI](/control/shaping/shape-metrics/reduce-cardinality/aggregation-rules) similar
to rules already in use. Rules in preview mode use a different background color, and
have a **Preview** label next to the rule type in the **Name** column.

When a preview rule is added, but hasn't finished impact calculations, the
[shaping impact](/control/shaping/shape-metrics/reduce-cardinality/aggregation-rules#shaping-impact)
columns display `calculating`. Completed impact calculations display under
**Rule Efficiency**, **Matched Impact** (if you're using a matched writes license),
and **System Persisted Impact**.

**Utility Score** isn't available for rules in preview mode because the **Output metric**
can't be used until the rule is enabled.

## Create a preview rule

Creating a preview rollup or mapping rule requires adding a configuration variable
to the rule creation file called `mode`.

The `mode` variable has two states:

* `preview`: This rule is in a preview state, and doesn't affect data being mapped,
  rolled up, or added to storage.
* `enabled`: This rule is active and affecting incoming data.

If `mode` isn't present, the rule defaults to the `enabled` state.

For mapping or rollup rules created with
[Chronoctl](/tooling/chronoctl), define `mode` in the `spec` section
of the rule definition file.

```text theme={null}
mode: PREVIEW
```

For rollup rules created with [Terraform](/tooling/infrastructure/terraform),
add:

```text theme={null}
# The operational mode for this rule.
mode = preview
```

## Change a rule from preview to enabled

Preview rules use the same definition files as active rules. To change a rule from
preview to enabled:

1. In the definition file, change the value of `mode` from `preview` to `enabled`.
2. Save the file.
3. Apply the updated definition.

The rule begins affecting data, and the **Preview** notice doesn't display in the
Aggregation Rules UI.