Add 'hashmod' relabel action.

This takes the modulus of a hash of some labels.
Combined with a keep relabel action, this allows
for sharding of targets across multiple prometheus
servers.

config/config.go

@@ -455,9 +455,11 @@ const (
 	// Performs a regex replacement.
 	RelabelReplace RelabelAction = "replace"
 	// Drops targets for which the input does not match the regex.
-	RelabelKeep = "keep"
+	RelabelKeep RelabelAction = "keep"
 	// Drops targets for which the input does match the regex.
-	RelabelDrop = "drop"
+	RelabelDrop RelabelAction = "drop"
+	// Sets a label to the modulus of a hash of labels.
+	RelabelHashMod RelabelAction = "hashmod"
 )
 
 // UnmarshalYAML implements the yaml.Unmarshaler interface.
@@ -467,7 +469,7 @@ func (a *RelabelAction) UnmarshalYAML(unmarshal func(interface{}) error) error {
 		return err
 	}
 	switch act := RelabelAction(strings.ToLower(s)); act {
-	case RelabelReplace, RelabelKeep, RelabelDrop:
+	case RelabelReplace, RelabelKeep, RelabelDrop, RelabelHashMod:
 		*a = act
 		return nil
 	}
@@ -482,7 +484,9 @@ type RelabelConfig struct {
 	// Separator is the string between concatenated values from the source labels.
 	Separator string `yaml:"separator,omitempty"`
 	// Regex against which the concatenation is matched.
-	Regex *Regexp `yaml:"regex"`
+	Regex *Regexp `yaml:"regex",omitempty`
+	// Modulus to take of the hash of concatenated values from the source labels.
+	Modulus uint64 `yaml:"modulus,omitempty"`
 	// The label to which the resulting string is written in a replacement.
 	TargetLabel clientmodel.LabelName `yaml:"target_label,omitempty"`
 	// Replacement is the regex replacement pattern to be used.
@@ -501,9 +505,12 @@ func (c *RelabelConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	if err := unmarshal((*plain)(c)); err != nil {
 		return err
 	}
-	if c.Regex == nil {
+	if c.Regex == nil && c.Action != RelabelHashMod {
 		return fmt.Errorf("relabel configuration requires a regular expression")
 	}
+	if c.Modulus == 0 && c.Action == RelabelHashMod {
+		return fmt.Errorf("relabel configuration for hashmod requires non-zero modulus")
+	}
 	return checkOverflow(c.XXX, "relabel_config")
 }
 
@@ -527,8 +534,11 @@ func (re *Regexp) UnmarshalYAML(unmarshal func(interface{}) error) error {
 }
 
 // MarshalYAML implements the yaml.Marshaler interface.
-func (re Regexp) MarshalYAML() (interface{}, error) {
+func (re *Regexp) MarshalYAML() (interface{}, error) {
-	return re.String(), nil
+	if re != nil {
+		return re.String(), nil
+	}
+	return nil, nil
 }
 
 // Duration encapsulates a time.Duration and makes it YAML marshallable.

config/config_test.go

@@ -114,6 +114,19 @@ var expectedConf = &Config{
 					Separator:    ";",
 					Action:       RelabelDrop,
 				},
+				{
+					SourceLabels: clientmodel.LabelNames{"__address__"},
+					TargetLabel:  "__hash",
+					Modulus:      8,
+					Separator:    ";",
+					Action:       RelabelHashMod,
+				},
+				{
+					SourceLabels: clientmodel.LabelNames{"__hash"},
+					Regex:        &Regexp{*regexp.MustCompile("^1$")},
+					Separator:    ";",
+					Action:       RelabelKeep,
+				},
 			},
 			MetricRelabelConfigs: []*RelabelConfig{
 				{
@@ -194,6 +207,9 @@ var expectedErrors = []struct {
 	}, {
 		filename: "regex_missing.bad.yml",
 		errMsg:   "relabel configuration requires a regular expression",
+	}, {
+		filename: "modulus_missing.bad.yml",
+		errMsg:   "relabel configuration for hashmod requires non-zero modulus",
 	}, {
 		filename: "rules.bad.yml",
 		errMsg:   "invalid rule file path",

config/testdata/conf.good.yml

@@ -71,6 +71,13 @@ scrape_configs:
   - source_labels: [job]
     regex:         (.*)some-[regex]$
     action:        drop
+  - source_labels: [__address__]
+    modulus:       8
+    target_label:  __hash
+    action:        hashmod
+  - source_labels: [__hash]
+    regex:         ^1$
+    action:        keep
 
   metric_relabel_configs:
   - source_labels: [__name__]

config/testdata/modulus_missing.bad.yml

@@ -0,0 +1,5 @@
+scrape_configs:
+  - job_name: prometheus
+    relabel_configs:
+      - regex: abcdef
+        action: hashmod

retrieval/relabel.go

@@ -2,6 +2,7 @@ package retrieval
 
 import (
 	"fmt"
+	"hash/fnv"
 	"strings"
 
 	clientmodel "github.com/prometheus/client_golang/model"
@@ -56,6 +57,11 @@ func relabel(labels clientmodel.LabelSet, cfg *config.RelabelConfig) (clientmode
 		} else {
 			labels[cfg.TargetLabel] = clientmodel.LabelValue(res)
 		}
+	case config.RelabelHashMod:
+		hasher := fnv.New64a()
+		hasher.Write([]byte(val))
+		mod := hasher.Sum64() % cfg.Modulus
+		labels[cfg.TargetLabel] = clientmodel.LabelValue(fmt.Sprintf("%d", mod))
 	default:
 		panic(fmt.Errorf("retrieval.relabel: unknown relabel action type %q", cfg.Action))
 	}

retrieval/relabel_test.go

@@ -151,6 +151,28 @@ func TestRelabel(t *testing.T) {
 				"a": "boo",
 			},
 		},
+		{
+			input: clientmodel.LabelSet{
+				"a": "foo",
+				"b": "bar",
+				"c": "baz",
+			},
+			relabel: []*config.RelabelConfig{
+				{
+					SourceLabels: clientmodel.LabelNames{"c"},
+					TargetLabel:  clientmodel.LabelName("d"),
+					Separator:    ";",
+					Action:       config.RelabelHashMod,
+					Modulus:      1000,
+				},
+			},
+			output: clientmodel.LabelSet{
+				"a": "foo",
+				"b": "bar",
+				"c": "baz",
+				"d": "58",
+			},
+		},
 	}
 
 	for i, test := range tests {