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discovery.relabel

In Flow, targets are defined as sets of key-value pairs called labels.

discovery.relabel rewrites the label set of the input targets by applying one or more relabeling rules. If no rules are defined, then the input targets are exported as-is.

The most common use of discovery.relabel is to filter targets or standardize the target label set that is passed to a downstream component. The rule blocks are applied to the label set of each target in order of their appearance in the configuration file. The configured rules can be retrieved by calling the function in the rules export field.

Target labels which start with a double underscore __ are considered internal, and may be removed by other Flow components prior to telemetry collection. To retain any of these labels, use a labelmap action to remove the prefix, or remap them to a different name. Service discovery mechanisms usually group their labels under __meta_*. For example, the discovery.kubernetes component populates a set of __meta_kubernetes_* labels to provide information about the discovered Kubernetes resources. If a relabeling rule needs to store a label value temporarily, for example as the input to a subsequent step, use the __tmp label name prefix, as it is guaranteed to never be used.

Multiple discovery.relabel components can be specified by giving them different labels.

Usage

discovery.relabel "LABEL" {
  targets = TARGET_LIST

  rule {
    ...
  }

  ...
}

Arguments

The following arguments are supported:

NameTypeDescriptionDefaultRequired
targetslist(map(string))Targets to relabelyes

Blocks

The following blocks are supported inside the definition of discovery.relabel:

HierarchyBlockDescriptionRequired
ruleruleRelabeling rules to apply to targets.no

rule block

The rule block contains the definition of any relabeling rules that can be applied to an input metric. If more than one rule block is defined, the transformations are applied in top-down order.

The following arguments can be used to configure a rule. All arguments are optional. Omitted fields take their default values.

NameTypeDescriptionDefaultRequired
source_labelslist(string)The list of labels whose values are to be selected. Their content is concatenated using the separator and matched against regex.no
separatorstringThe separator used to concatenate the values present in source_labels.;no
regexstringA valid RE2 expression with support for parenthesized capture groups. Used to match the extracted value from the combination of the source_label and separator fields or filter labels during the labelkeep/labeldrop/labelmap actions.(.*)no
modulusuintA positive integer used to calculate the modulus of the hashed source label values.no
target_labelstringLabel to which the resulting value will be written to.no
replacementstringThe value against which a regex replace is performed, if the regex matches the extracted value. Supports previously captured groups.$1no
actionstringThe relabeling action to perform.replaceno

Here’s a list of the available actions, along with a brief description of their usage.

  • replace - Matches regex to the concatenated labels. If there’s a match, it replaces the content of the target_label using the contents of the replacement field.
  • keep - Keeps metrics where regex matches the string extracted using the source_labels and separator.
  • drop - Drops metrics where regex matches the string extracted using the source_labels and separator.
  • hashmod - Hashes the concatenated labels, calculates its modulo modulus and writes the result to the target_label.
  • labelmap - Matches regex against all label names. Any labels that match are renamed according to the contents of the replacement field.
  • labeldrop - Matches regex against all label names. Any labels that match are removed from the metric’s label set.
  • labelkeep - Matches regex against all label names. Any labels that don’t match are removed from the metric’s label set.

Finally, note that the regex capture groups can be referred to using either the $CAPTURE_GROUP_NUMBER or ${CAPTURE_GROUP_NUMBER} notation.

Exported fields

The following fields are exported and can be referenced by other components:

NameTypeDescription
outputlist(map(string))The set of targets after applying relabeling.
rulesRelabelRulesThe currently configured relabeling rules.

Component health

discovery.relabel is only reported as unhealthy when given an invalid configuration. In those cases, exported fields retain their last healthy values.

Debug information

discovery.relabel does not expose any component-specific debug information.

Debug metrics

discovery.relabel does not expose any component-specific debug metrics.

Example

Alloy 
discovery.relabel "keep_backend_only" {
  targets = [
    { "__meta_foo" = "foo", "__address__" = "localhost", "instance" = "one",   "app" = "backend"  },
    { "__meta_bar" = "bar", "__address__" = "localhost", "instance" = "two",   "app" = "database" },
    { "__meta_baz" = "baz", "__address__" = "localhost", "instance" = "three", "app" = "frontend" },
  ]

  rule {
    source_labels = ["__address__", "instance"]
    separator     = "/"
    target_label  = "destination"
    action        = "replace"
  }

  rule {
    source_labels = ["app"]
    action        = "keep"
    regex         = "backend"
  }
}