# Metric Rollup, Supplement

The following supplemental information provides more details of the Kloudfuse implementation for metrics rollup.

For the primary feature description, see [Metrics rollup](https://docs.kloudfuse.com/platform/4.0.2/data-management/rollup/overview/).

## Computation

A **metrics transformer** computes the rollup of each metric series from the Kafka metrics topic (`kf_metrics_topic`) and writes it to the Kafka metrics rollup topic (`kf_metrics_rollup_topic`).

## Storage

A separate new table (`kf_metrics_rollup`) in the Pinot database stores the rolled up data. While the raw metrics table (`kf_metrics`) stores a single value column, the schema for `kf_metrics_rollup` table stores the **sum**, **count**, **min**, **max**, **counter** (last counter value that accounts for resets within the rollup window), and **first** (first value encounter in the bucket). For histogram metrics, the `le` column stores the collapsed counter values for the histogram buckets, and contains first and counter value for each bucket.

## Retention

The `kf_metrics_rollup` table has a different retention time than the raw metric data; the existing `RetentionConfig` for metrics supports specifying retention times for rollup data. If you do not explicitly specify the data retention policy, it defaults to the retention time of raw data.

## Ingester

The **Ingester** computes the expiry, and attaches it to the `TimeSeries` protobuf. The protobuf definition has the additional `expiry_rollup` field that the ingester populates.

## Metrics transformer

The metrics transformer computes the aggregated values: **sum**, **count**, **min**, **max**, **counter** (last counter value that accounts for resets within the rollup window), **first** (first value encounter in the bucket), and **first_ts** of each metric series.

## Decoder

A new **MetricsRollupDecoder** reads from `kf_metrics_rollup_topic` and writes to the `kf_metrics_rollup` table.

## Query Service

The query service automatically selects which table to access for the query.

- If using the rollup feature, the query service routes the query to the correct value column.

For example, for rate, increase, and changes, the query service uses both **counter** and **first** values.

- To handle mixed data, where some (likely older) data has no corresponding rolled up data, the query service accesses the `kf_metrics_rollup` table for the minimum timestamps.

If the query time range contains the minimum timestamp, the query service splits the query into multiple queries. The older data uses the `kf_metrics` table, while newer data uses the rolled up table `kf_metrics_rollup`. Query service then merges the responses before returning the result.