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99 lines
5 KiB
Markdown
99 lines
5 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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## Extra scrape metrics
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`--enable-feature=extra-scrape-metrics`
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When enabled, for each instance scrape, Prometheus stores a sample in the following additional time series:
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- `scrape_timeout_seconds`. The configured `scrape_timeout` for a target. This allows you to measure each target to find out how close they are to timing out with `scrape_duration_seconds / scrape_timeout_seconds`.
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- `scrape_sample_limit`. The configured `sample_limit` for a target. This allows you to measure each target
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to find out how close they are to reaching the limit with `scrape_samples_post_metric_relabeling / scrape_sample_limit`. Note that `scrape_sample_limit` can be zero if there is no limit configured, which means that the query above can return `+Inf` for targets with no limit (as we divide by zero). If you want to query only for targets that do have a sample limit use this query: `scrape_samples_post_metric_relabeling / (scrape_sample_limit > 0)`.
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- `scrape_body_size_bytes`. The uncompressed size of the most recent scrape response, if successful. Scrapes failing because `body_size_limit` is exceeded report `-1`, other scrape failures report `0`.
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## New service discovery manager
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`--enable-feature=new-service-discovery-manager`
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When enabled, Prometheus uses a new service discovery manager that does not
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restart unchanged discoveries upon reloading. This makes reloads faster and reduces
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pressure on service discoveries' sources.
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Users are encouraged to test the new service discovery manager and report any
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issues upstream.
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In future releases, this new service discovery manager will become the default and
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this feature flag will be ignored.
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## Prometheus agent
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`--enable-feature=agent`
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When enabled, Prometheus runs in agent mode. The agent mode is limited to
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discovery, scrape and remote write.
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This is useful when you do not need to query the Prometheus data locally, but
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only from a central [remote endpoint](https://prometheus.io/docs/operating/integrations/#remote-endpoints-and-storage).
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