feat: full payload templating support for webhook notifier

config/notifiers.go

@@ -653,7 +653,8 @@ type WebhookConfig struct {
 
 	// Timeout is the maximum time allowed to invoke the webhook. Setting this to 0
 	// does not impose a timeout.
-	Timeout time.Duration `yaml:"timeout" json:"timeout"`
+	Timeout time.Duration  `yaml:"timeout" json:"timeout"`
+	Payload map[string]any `yaml:"payload,omitempty" json:"payload,omitempty"`
 }
 
 // UnmarshalYAML implements the yaml.Unmarshaler interface.

docs/configuration.md

@@ -1840,6 +1840,15 @@ url_file: <filepath>
 # no timeout should be applied.
 # NOTE: This will have no effect if set higher than the group_interval.
 [ timeout: <duration> | default = 0s ]
+
+# Define custom payload to be sent to the webhook endpoint.
+# USE AT YOUR OWN RISK: This is an advanced configuration option that allows you
+# to define a custom payload using Go templates. Be aware that the Alertmanager does not
+# perform any validation on the resulting payload, and it is your responsibility to
+# ensure that the generated payload is in the desired format expected by the receiving endpoint.
+# The payload has to be valid JSON. You can use the `toJson` function to help with this.
+# THE ALERTMANAGER TEAM WILL NOT PROVIDE ANY SUPPORT FOR ISSUES ARISING FROM THE USE OF THIS OPTION.
+[ payload: { <string>: <tmpl_string>, ... } ]
 ```
 
 The Alertmanager

notify/webhook/webhook.go

@@ -101,6 +101,14 @@ func (n *Notifier) Notify(ctx context.Context, alerts ...*types.Alert) (bool, er
 		return false, err
 	}
 
+	// Override the payload if a custom one is configured.
+	if n.conf.Payload != nil {
+		buf, err = n.renderPayload(msg)
+		if err != nil {
+			return false, fmt.Errorf("failed to render custom payload: %w", err)
+		}
+	}
+
 	var url string
 	var tmplErr error
 	tmpl := notify.TmplText(n.tmpl, data, &tmplErr)
@@ -144,3 +152,28 @@ func (n *Notifier) Notify(ctx context.Context, alerts ...*types.Alert) (bool, er
 	}
 	return shouldRetry, err
 }
+
+func (n *Notifier) renderPayload(
+	data *Message,
+) (bytes.Buffer, error) {
+	var (
+		tmplTextErr  error
+		tmplText     = notify.TmplText(n.tmpl, data.Data, &tmplTextErr)
+		tmplTextFunc = func(tmpl string) (string, error) {
+			return tmplText(tmpl), tmplTextErr
+		}
+	)
+	var err error
+	rendered := make(map[string]any, len(n.conf.Payload))
+	for k, v := range n.conf.Payload {
+		rendered[k], err = template.DeepCopyWithTemplate(v, tmplTextFunc)
+		if err != nil {
+			return bytes.Buffer{}, err
+		}
+	}
+	var buf bytes.Buffer
+	if err := json.NewEncoder(&buf).Encode(rendered); err != nil {
+		return bytes.Buffer{}, err
+	}
+	return buf, nil
+}

notify/webhook/webhook_test.go

@@ -16,10 +16,12 @@ package webhook
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"fmt"
 	"io"
 	"net/http"
 	"net/http/httptest"
+	"net/url"
 	"os"
 	"testing"
 	"time"
@@ -225,3 +227,127 @@ func TestWebhookURLTemplating(t *testing.T) {
 		})
 	}
 }
+
+type roundTripFunc func(req *http.Request) *http.Response
+
+func (f roundTripFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req), nil
+}
+
+// TestWebhookDefaultPayload tests that the default payload sent by the webhook notifier matches
+// the behaviour before introducing templating.
+func TestWebhookDefaultPayload(t *testing.T) {
+	var capturedPayload []byte
+
+	mockTransport := roundTripFunc(func(req *http.Request) *http.Response {
+		var err error
+		capturedPayload, err = io.ReadAll(req.Body)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return &http.Response{
+			StatusCode: http.StatusOK,
+			Body:       http.NoBody,
+		}
+	})
+
+	u, err := url.Parse("http://localhost")
+	require.NoError(t, err)
+
+	conf := &config.WebhookConfig{
+		URL:        config.SecretTemplateURL(u.String()),
+		HTTPConfig: &commoncfg.HTTPClientConfig{},
+	}
+
+	alerts := []*types.Alert{
+		{
+			Alert: model.Alert{
+				Labels:       model.LabelSet{"alertname": "TestAlert"},
+				Annotations:  model.LabelSet{"summary": "Test summary"},
+				StartsAt:     time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC),
+				EndsAt:       time.Date(2020, 1, 1, 1, 0, 0, 0, time.UTC),
+				GeneratorURL: "http://generator.url",
+			},
+		},
+	}
+	tmpl := test.CreateTmpl(t)
+	ctx := notify.WithGroupKey(context.Background(), "{}:{alertname=\"test1\"}")
+	ctx = notify.WithReceiverName(ctx, "test_receiver")
+	data := notify.GetTemplateData(ctx, tmpl, alerts, promslog.NewNopLogger())
+
+	msg := &Message{
+		Version:  "4",
+		Data:     data,
+		GroupKey: "{}:{alertname=\"test1\"}",
+	}
+
+	var buf bytes.Buffer
+	json.NewEncoder(&buf).Encode(msg)
+	n, err := New(conf, tmpl, promslog.NewNopLogger())
+	require.NoError(t, err)
+	n.client.Transport = mockTransport
+	_, err = n.Notify(ctx, alerts...)
+	require.NoError(t, err)
+
+	require.NotEmpty(t, capturedPayload)
+	require.JSONEq(t, buf.String(), string(capturedPayload))
+}
+
+func TestWebhookCustomPayload(t *testing.T) {
+	var capturedPayload []byte
+
+	mockTransport := roundTripFunc(func(req *http.Request) *http.Response {
+		var err error
+		capturedPayload, err = io.ReadAll(req.Body)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return &http.Response{
+			StatusCode: http.StatusOK,
+			Body:       http.NoBody,
+		}
+	})
+
+	u, err := url.Parse("http://localhost")
+	require.NoError(t, err)
+
+	conf := &config.WebhookConfig{
+		URL:        config.SecretTemplateURL(u.String()),
+		HTTPConfig: &commoncfg.HTTPClientConfig{},
+		Payload: map[string]any{
+			"custom":       `some custom content`,
+			"commonLabels": "{{ .CommonLabels  | toJson }}",
+		},
+	}
+
+	alerts := []*types.Alert{
+		{
+			Alert: model.Alert{
+				Labels:       model.LabelSet{"alertname": "TestAlert"},
+				Annotations:  model.LabelSet{"summary": "Test summary"},
+				StartsAt:     time.Date(2020, 1, 1, 0, 0, 0, 0, time.UTC),
+				EndsAt:       time.Date(2020, 1, 1, 1, 0, 0, 0, time.UTC),
+				GeneratorURL: "http://generator.url",
+			},
+		},
+	}
+	tmpl := test.CreateTmpl(t)
+	ctx := notify.WithGroupKey(context.Background(), "{}:{alertname=\"test1\"}")
+	ctx = notify.WithReceiverName(ctx, "test_receiver")
+
+	msg := map[string]any{
+		"custom":       `some custom content`,
+		"commonLabels": map[string]string{"alertname": "TestAlert"},
+	}
+
+	var buf bytes.Buffer
+	json.NewEncoder(&buf).Encode(msg)
+	n, err := New(conf, tmpl, promslog.NewNopLogger())
+	require.NoError(t, err)
+	n.client.Transport = mockTransport
+	_, err = n.Notify(ctx, alerts...)
+	require.NoError(t, err)
+
+	require.NotEmpty(t, capturedPayload)
+	require.JSONEq(t, buf.String(), string(capturedPayload))
+}