prometheus/docs/feature_flags.md

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Feature flags	11

Feature flags

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.

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.

Expand environment variables in external labels

--enable-feature=expand-external-labels

Replace ${var} or $var in the external_labels 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.

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.

Exemplars storage

--enable-feature=exemplar-storage

OpenMetrics 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. If the exemplar storage is enabled, we will also append the exemplars to WAL for local persistence (for WAL duration).

Memory snapshot on shutdown

--enable-feature=memory-snapshot-on-shutdown

This takes the snapshot of the chunks that are in memory along with the series information when shutting down and stores it on disk. This will reduce the startup time since the memory state can be restored with this snapshot and m-mapped chunks without the need of WAL replay.

Extra scrape metrics

--enable-feature=extra-scrape-metrics

When enabled, for each instance scrape, Prometheus stores a sample in the following additional time series:

• 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.
• scrape_sample_limit. The configured sample_limit for a target. This allows you to measure each target 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).
• 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.

New service discovery manager

--enable-feature=new-service-discovery-manager

When enabled, Prometheus uses a new service discovery manager that does not restart unchanged discoveries upon reloading. This makes reloads faster and reduces pressure on service discoveries' sources.

Users are encouraged to test the new service discovery manager and report any issues upstream.

In future releases, this new service discovery manager will become the default and this feature flag will be ignored.

Prometheus agent

--enable-feature=agent

When enabled, Prometheus runs in agent mode. The agent mode is limited to discovery, scrape and remote write.

This is useful when you do not need to query the Prometheus data locally, but only from a central remote endpoint.