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64 lines
3.1 KiB
Markdown
64 lines
3.1 KiB
Markdown
---
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title: Feature Flags
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sort_rank: 11
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---
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# Feature Flags
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Here is a list of features that are disabled by default since they are breaking changes or are considered experimental.
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Their behaviour can change in future releases which will be communicated via the [release changelog](https://github.com/prometheus/prometheus/blob/main/CHANGELOG.md).
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You can enable them using the `--enable-feature` flag with a comma separated list of features.
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They may be enabled by default in future versions.
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## `@` Modifier in PromQL
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`--enable-feature=promql-at-modifier`
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The `@` modifier lets you specify the evaluation time for instant vector selectors,
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range vector selectors, and subqueries. More details can be found [here](querying/basics.md#modifier).
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## Expand environment variables in external labels
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`--enable-feature=expand-external-labels`
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Replace `${var}` or `$var` in the [`external_labels`](configuration/configuration.md#configuration-file)
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values according to the values of the current environment variables. References
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to undefined variables are replaced by the empty string.
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## Negative offset in PromQL
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This negative offset is disabled by default since it breaks the invariant
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that PromQL does not look ahead of the evaluation time for samples.
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`--enable-feature=promql-negative-offset`
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In contrast to the positive offset modifier, the negative offset modifier lets
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one shift a vector selector into the future. An example in which one may want
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to use a negative offset is reviewing past data and making temporal comparisons
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with more recent data.
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More details can be found [here](querying/basics.md#offset-modifier).
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## Remote Write Receiver
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`--enable-feature=remote-write-receiver`
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The remote write receiver allows Prometheus to accept remote write requests from other Prometheus servers. More details can be found [here](storage.md#overview).
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## Exemplars Storage
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`--enable-feature=exemplar-storage`
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[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.
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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. If the exemplar storage is enabled, we will also append the exemplars to WAL for local persistence (for WAL duration).
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## Memory Snapshot on Shutdown
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`--enable-feature=memory-snapshot-on-shutdown`
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This takes the snapshot of the chunks that are in memory along with the series information when shutting down and stores
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it on disk. This will reduce the startup time since the memory state can be restored with this snapshot and m-mapped
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chunks without the need of WAL replay.
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