Billing and usageControlling Prometheus metrics usageReducing Prometheus metrics usage

Reducing Prometheus metrics usage with relabeling

This guide describes several techniques you can use to reduce your Prometheus metrics usage on Grafana Cloud.

Before applying these techniques, ensure that you’re deduplicating any samples sent from high-availability Prometheus clusters. This will cut your active series count in half. To learn how to do this, please see Sending data from multiple high-availability Prometheus instances.

You can reduce the number of active series sent to Grafana Cloud in two ways:

  • Allowlisting: This involves keeping a set of “important” metrics and labels that you explicitly define, and dropping everything else. To allowlist metrics and labels, you should identify a set of core important metrics and labels that you’d like to keep. To enable allowlisting in Prometheus, use the keep and labelkeep actions with any relabeling configuration.

  • Denylisting: This involves dropping a set of high-cardinality “unimportant” metrics that you explicitly define, and keeping everything else. Denylisting becomes possible once you’ve identified a list of high-cardinality metrics and labels that you’d like to drop. To learn how to discover high-cardinality metrics, please see Analyzing Prometheus metric usage. To enable denylisting in Prometheus, use the drop and labeldrop actions with any relabeling configuration.

Both of these methods are implemented through Prometheus’s metric filtering and relabeling feature, relabel_config. This feature allows you to filter through series labels using regular expressions and keep or drop those that match. You can also manipulate, transform, and rename series labels using relabel_config.

Prom Labs’s Relabeler tool may be helpful when debugging relabel configs. Relabeler allows you to visually confirm the rules implemented by a relabel config.

This guide expects some familiarity with regular expressions. To learn more, please see Regular expression on Wikipedia. To play around with and analyze any regular expressions, you can use RegExr.

Relabel_config syntax

You can filter series using Prometheus’s relabel_config configuration object. At a high level, a relabel_config allows you to select one or more source label values that can be concatenated using a separator parameter. The result can then be matched against using a regex, and an action operation can be performed if a match occurs.

You can perform the following common action operations:

  • keep: Keep a matched target or series, drop all others
  • drop: Drop a matched target or series, keep all others
  • replace: Replace or rename a matched label with a new one defined by the target_label and replacement parameters
  • labelkeep: Match the regex against all label names, drop all labels that don’t match (ignores source_labels and applies to all label names)
  • labeldrop: Match the regex against all label names, drop all labels that match (ignores source_labels and applies to all label names)

For a full list of available actions, please see relabel_config from the Prometheus documentation.

Any relabel_config must have the same general structure:

- source_labels = [source_label_1, source_label_2, ...]
  separator: ;
  action: replace
  regex: (.*)
  replacement: $1

These default values should be modified to suit your relabeling use case.

  • source_labels: Select one or more labels from the available set
  • separator: Concatenate selected label values using this character
  • regex: Match this regular expression on concatenated data
  • action: Execute the specified relabel action
  • replacement: If using one of replace or labelmap, define the replacement value. You can use regex match groups to access data captured by the regex. To learn more about regex match groups, please see this StackOverflow answer.
  • target_label: Assign the extracted and modified label value defined by replacement to this label name.

Parameters that aren’t explicitly set will be filled in using default values. For readability it’s usually best to explicitly define a relabel_config. To learn more about the general format for a relabel_config block, please see relabel_config from the Prometheus docs.

Here’s an example:

- source_labels: [ instance_ip ]
  separator: ;
  action: replace
  regex: (.*)
  replacement: $1
  target_label: host_ip

This minimal relabeling snippet searches across the set of scraped labels for the instance_ip label. If it finds the instance_ip label, it renames this label to host_ip. Since the (.*) regex captures the entire label value, replacement references this capture group, $1, when setting the new target_label. Since we’ve used default regex, replacement, action, and separator values here, they can be omitted for brevity. However, it’s usually best to explicitly define these for readability.

To drop a specific label, select it using source_labels and use a replacement value of "". To bulk drop or keep labels, use the labelkeep and labeldrop actions.

You can use a relabel_config to filter through and relabel:

  • Scrape targets
  • Samples and labels to ingest into Prometheus storage
  • Samples and labels to ship to remote storage

You’ll learn how to do this in the next section.

Relabel_config in a Prometheus configuration file

You can apply a relabel_config to filter and manipulate labels at the following stages of metric collection:

  • Target selection in the relabel_configs section of a scrape_configs job. This allows you to use a relabel_config object to select targets to scrape and relabel metadata created by any service discovery mechanism.
  • Metric selection in the metric_relabel_configs section of a scrape_configs job. This allows you to use a relabel_config object to select labels and series that should be ingested into Prometheus storage.
  • Remote Write in the write_relabel_configs section of a remote_write configuration. This allows you to use a relabel_config to control which labels and series Prometheus ships to remote storage.

This sample configuration file skeleton demonstrates where each of these sections lives in a Prometheus config:

global:
. . .
rule_files:
. . .
scrape_configs:

- job_name: sample_job_1
  kubernetes_sd_configs:
  - . . .
  relabel_configs:
  - source_labels: [. . .]
     . . .
  - source_labels: [. . .]
    . . .
  metric_relabel_configs:
  - source_labels: [. . .]
    . . .
  - source_labels: [. . .]
    . . .

- job_name: sample_job_2
  static_configs:
  - targets: [. . .]
  metric_relabel_configs:
  - source_labels: [. . .]
    . . .

. . .

remote_write:
- url: . . .
  write_relabel_configs:
  - source_labels: [. . .]
    . . .
  - source_labels: [. . .]
    . . .

Use relabel_configs in a given scrape job to select which targets to scrape. This is often useful when fetching sets of targets using a service discovery mechanism like kubernetes_sd_configs, or Kubernetes service discovery. To learn more about Prometheus service discovery features, please see Configuration from the Prometheus docs.

Use metric_relabel_configs in a given scrape job to select which series and labels to keep, and to perform any label replacement operations. This occurs after target selection using relabel_configs.

Finally, use write_relabel_configs in a remote_write configuration to select which series and labels to ship to remote storage. This configuration does not impact any configuration set in metric_relabel_configs or relabel_configs. If you drop a label in a metric_relabel_configs section, it won’t be ingested by Prometheus and consequently won’t be shipped to remote storage.

Scrape target selection using relabel_configs

A relabel_configs configuration allows you to keep or drop targets returned by a service discovery mechanism like Kubernetes service discovery or AWS EC2 instance service discovery. For example, you may have a scrape job that fetches all Kubernetes Endpoints using a kubernetes_sd_configs parameter. By using the following relabel_configs snippet, you can limit scrape targets for this job to those whose Service label corresponds to app=nginx and port name to web:

scrape_configs:
- job_name: kubernetes_nginx
  honor_timestamps: true
  scrape_interval: 30s
  scrape_timeout: 10s
  metrics_path: /metrics
  scheme: http
  kubernetes_sd_configs:
  - role: endpoints
    namespaces:
      names:
      - default
  relabel_configs:
  - source_labels: [__meta_kubernetes_service_label_app]
    regex: nginx
    action: keep
  - source_labels: [__meta_kubernetes_endpoint_port_name]
    regex: web
    action: keep

The initial set of endpoints fetched by kuberentes_sd_configs in the default namespace can be very large depending on the apps you’re running in your cluster. Using the __meta_kubernetes_service_label_app label filter, endpoints whose corresponding services do not have the app=nginx label will be dropped by this scrape job.

Since kubernetes_sd_configs will also add any other Pod ports as scrape targets (with role: endpoints), we need to filter these out using the __meta_kubernetes_endpoint_port_name relabel config. For example, if a Pod backing the Nginx service has two ports, we only scrape the port named web and drop the other.

To summarize, the above snippet fetches all endpoints in the default Namespace, and keeps as scrape targets those whose corresponding Service has an app=nginx label set. This set of targets consists of one or more Pods that have one or more defined ports. We drop all ports that aren’t named web.

Using relabeling at the target selection stage, you can selectively choose which targets and endpoints you want to scrape (or drop) to tune your metric usage.

Metric and label selection using metric_relabel_configs

Relabeling and filtering at this stage modifies or drops samples before Prometheus ingests them locally and ships them to remote storage. This relabeling occurs after target selection. Once Prometheus scrapes a target, metric_relabel_configs allows you to define keep, drop and replace actions to perform on scraped samples:

- job_name: monitoring/kubelet/1
  honor_labels: true
  honor_timestamps: false
  scrape_interval: 30s
  scrape_timeout: 10s
  metrics_path: /metrics/cadvisor
  scheme: https
  kubernetes_sd_configs:
  - role: endpoints
    namespaces:
      names:
      - kube-system
  bearer_token_file: /var/run/secrets/kubernetes.io/serviceaccount/token
  tls_config:
    insecure_skip_verify: true
  relabel_configs:
  - source_labels: [__meta_kubernetes_service_label_k8s_app]
    regex: kubelet
    action: keep
  - source_labels: [__meta_kubernetes_endpoint_port_name]
    regex: https-metrics
    action: keep
. . .
  metric_relabel_configs:
  - source_labels: [__name__]
    regex: container_(network_tcp_usage_total|network_udp_usage_total|tasks_state|cpu_load_average_10s)
    action: drop

This sample piece of configuration instructs Prometheus to first fetch a list of endpoints to scrape using Kubernetes service discovery (kubernetes_sd_configs). Endpoints are limited to the kube-system namespace. Next, using relabel_configs, only Endpoints with the Service Label k8s_app=kubelet are kept. Furthermore, only Endpoints that have https-metrics as a defined port name are kept. This reduced set of targets corresponds to Kubelet https-metrics scrape endpoints.

After scraping these endpoints, Prometheus applies the metric_relabel_configs section, which drops all metrics whose metric name matches the specified regex. You can extract a sample’s metric name using the __name__ meta-label. In this case Prometheus would drop a metric like container_network_tcp_usage_total(. . .). Prometheus keeps all other metrics. You can add additional metric_relabel_configs sections that replace and modify labels here.

metric_relabel_configs are commonly used to relabel and filter samples before ingestion, and limit the amount of data that gets persisted to storage. Using metric_relabel_configs, you can drastically reduce your Prometheus metrics usage by throwing out unneeded samples.

If shipping samples to Grafana Cloud, you also have the option of persisting samples locally, but preventing shipping to remote storage. To do this, use a relabel_config object in the write_relabel_configs subsection of the remote_write section of your Prometheus config. This can be useful when local Prometheus storage is cheap and plentiful, but the set of metrics shipped to remote storage requires judicious curation to avoid excess costs.

Controlling remote write behavior using write_relabel_configs

Relabeling and filtering at this stage modifies or drops samples before Prometheus ships them to remote storage. Using this feature, you can store metrics locally but prevent them from shipping to Grafana Cloud. To learn more about remote_write, please see remote_write from the official Prometheus docs.

Prometheus applies this relabeling and dropping step after performing target selection using relabel_configs and metric selection and relabeling using metric_relabel_configs.

The following snippet of configuration demonstrates an “allowlisting” approach, where the specified metrics are shipped to remote storage, and all others dropped. Recall that these metrics will still get persisted to local storage unless this relabeling configuration takes place in the metric_relabel_configs section of a scrape job.

remote_write:
- url: https://prometheus-us-central1.grafana.net/api/prom/push
  remote_timeout: 30s
  write_relabel_configs:
  - source_labels: [__name__]
    regex: "apiserver_request_total|kubelet_node_config_error|kubelet_runtime_operations_errors_total"
    action: keep
  basic_auth:
    username: <your_remote_endpoint_username_here>
    password: <your_remote_endpoint_password_here>
  queue_config:
    capacity: 500
    max_shards: 1000
    min_shards: 1
    max_samples_per_send: 100
    batch_send_deadline: 5s
    min_backoff: 30ms
    max_backoff: 100ms

This piece of remote_write configuration sets the remote endpoint to which Prometheus will push samples. The write_relabel_configs section defines a keep action for all metrics matching the apiserver_request_total|kubelet_node_config_error|kubelet_runtime_operations_errors_total regex, dropping all others. You can additionally define remote_write-specific relabeling rules here.

Finally, this configures authentication credentials and the remote_write queue. To learn more about remote_write configuration parameters, please see remote_write from the Prometheus docs.

Conclusion

In this guide, we’ve presented an overview of Prometheus’s powerful and flexible relabel_config feature and how you can leverage it to control and reduce your local and Grafana Cloud Prometheus usage.

Choosing which metrics and samples to scrape, store, and ship to Grafana Cloud can seem quite daunting at first. Curated sets of important metrics can be found in Mixins. Mixins are a set of preconfigured dashboards and alerts. The PromQL queries that power these dashboards and alerts reference a core set of “important” observability metrics. There are Mixins for Kubernetes, Consul, Jaeger, and much more. To learn more about them, please see Prometheus Monitoring Mixins. Allowlisting or keeping the set of metrics referenced in a Mixin’s alerting rules and dashboards can form a solid foundation from which to build a complete set of observability metrics to scrape and store.

References