prometheus/docs/disabled_features.md

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---
title: Disabled Features
sort_rank: 10
---
# Disabled Features
Here is a list of features that are disabled by default since they are breaking changes or are considered experimental.
Their behaviour can change in future releases which will be communicated via the [release changelog](https://github.com/prometheus/prometheus/blob/main/CHANGELOG.md).
You can enable them using the `--enable-feature` flag with a comma separated list of features.
They may be enabled by default in future versions.
## `@` Modifier in PromQL
`--enable-feature=promql-at-modifier`
The `@` modifier lets you specify the evaluation time for instant vector selectors,
range vector selectors, and subqueries. More details can be found [here](querying/basics.md#modifier).
## Expand environment variables in external labels
`--enable-feature=expand-external-labels`
Replace `${var}` or `$var` in the [`external_labels`](configuration/configuration.md#configuration-file)
values according to the values of the current environment variables. References
to undefined variables are replaced by the empty string.
## Negative offset in PromQL
This negative offset is disabled by default since it breaks the invariant
that PromQL does not look ahead of the evaluation time for samples.
`--enable-feature=promql-negative-offset`
In contrast to the positive offset modifier, the negative offset modifier lets
one shift a vector selector into the future. An example in which one may want
to use a negative offset is reviewing past data and making temporal comparisons
with more recent data.
More details can be found [here](querying/basics.md#offset-modifier).
## Remote Write Receiver
`--enable-feature=remote-write-receiver`
The remote write receiver allows Prometheus to accept remote write requests from other Prometheus servers. More details can be found [here](storage.md#overview).
## Exemplars Storage
`--enable-feature=exemplar-storage`
[OpenMetrics](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#exemplars) introduces the ability for scrape targets to add exemplars to certain metrics. Exemplars are references to data outside of the MetricSet. A common use case are IDs of program traces.
Exemplar storage is implemented as a fixed size circular buffer that stores exemplars in memory for all series. Enabling this feature will enable the storage of exemplars scraped by Prometheus. The flag `storage.exemplars.exemplars-limit` can be used to control the size of circular buffer by # of exemplars. An exemplar with just a `traceID=<jaeger-trace-id>` uses roughly 100 bytes of memory via the in-memory exemplar storage.