> ## 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

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.