prometheus/docs/disabled_features.md
Callum Styan 8fd73b1d28
Add Exemplar Remote Write support (#8296)
* Write exemplars to the WAL and send them over remote write.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Update example for exemplars, print data in a more obvious format.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Add metrics for remote write of exemplars.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Fix incorrect slices passed to send in remote write.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* We need to unregister the new metrics.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Address review comments

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Order of exemplar append vs write exemplar to WAL needs to change.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Several fixes to prevent sending uninitialized or incorrect samples with an exemplar. Fix dropping exemplar for missing series. Add tests for queue_manager sending exemplars

Signed-off-by: Martin Disibio <mdisibio@gmail.com>

* Store both samples and exemplars in the same timeseries buffer to remove the alloc when building final request, keep sub-slices in separate buffers for re-use

Signed-off-by: Martin Disibio <mdisibio@gmail.com>

* Condense sample/exemplar delivery tests to parameterized sub-tests

Signed-off-by: Martin Disibio <mdisibio@gmail.com>

* Rename test methods for clarity now that they also handle exemplars

Signed-off-by: Martin Disibio <mdisibio@gmail.com>

* Rename counter variable. Fix instances where metrics were not updated correctly

Signed-off-by: Martin Disibio <mdisibio@gmail.com>

* Add exemplars to LoadWAL benchmark

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* last exemplars timestamp metric needs to convert value to seconds with
ms precision

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Process exemplar records in a separate go routine when loading the WAL.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Address review comments related to clarifying comments and variable
names. Also refactor sample/exemplar to enqueue prompb types.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Regenerate types proto with comments, update protoc version again.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Put remote write of exemplars behind a feature flag.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Address some of Ganesh's review comments.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Move exemplar remote write feature flag to a config file field.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Address Bartek's review comments.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Don't allocate exemplar buffers in queue_manager if we're not going to
send exemplars over remote write.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Add ValidateExemplar function, validate exemplars when appending to head
and log them all to WAL before adding them to exemplar storage.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Address more reivew comments from Ganesh.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Add exemplar total label length check.

Signed-off-by: Callum Styan <callumstyan@gmail.com>

* Address a few last review comments

Signed-off-by: Callum Styan <callumstyan@gmail.com>

Co-authored-by: Martin Disibio <mdisibio@gmail.com>
2021-05-06 13:53:52 -07:00

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