Scrape: Add scrape_failure_log_file to log Scrape Failures

Signed-off-by: Julien <roidelapluie@o11y.eu>

cmd/prometheus/main.go

@@ -755,6 +755,7 @@ func main() {
 	scrapeManager, err := scrape.NewManager(
 		&cfg.scrape,
 		log.With(logger, "component", "scrape manager"),
+		func(s string) (log.Logger, error) { return logging.NewJSONFileLogger(s) },
 		fanoutStorage,
 		prometheus.DefaultRegisterer,
 	)

cmd/prometheus/scrape_failure_log_test.go

@@ -0,0 +1,193 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
+
+	"github.com/prometheus/prometheus/util/testutil"
+)
+
+func TestScrapeFailureLogFile(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode.")
+	}
+
+	// Tracks the number of requests made to the mock server.
+	var requestCount atomic.Int32
+
+	// Starts a server that always returns HTTP 500 errors.
+	mockServerAddress := startGarbageServer(t, &requestCount)
+
+	// Create a temporary directory for Prometheus configuration and logs.
+	tempDir := t.TempDir()
+
+	// Define file paths for the scrape failure log and Prometheus configuration.
+	// Like other files, the scrape failure log file should be relative to the
+	// config file. Therefore, we split the name we put in the file and the full
+	// path used to check the content of the file.
+	scrapeFailureLogFileName := "scrape_failure.log"
+	scrapeFailureLogFile := filepath.Join(tempDir, scrapeFailureLogFileName)
+	promConfigFile := filepath.Join(tempDir, "prometheus.yml")
+
+	// Step 1: Set up an initial Prometheus configuration that globally
+	// specifies a scrape failure log file.
+	promConfig := fmt.Sprintf(`
+global:
+  scrape_interval: 500ms
+  scrape_failure_log_file: %s
+
+scrape_configs:
+  - job_name: 'test_job'
+    static_configs:
+      - targets: ['%s']
+`, scrapeFailureLogFileName, mockServerAddress)
+
+	err := os.WriteFile(promConfigFile, []byte(promConfig), 0o644)
+	require.NoError(t, err, "Failed to write Prometheus configuration file")
+
+	// Start Prometheus with the generated configuration and a random port, enabling the lifecycle API.
+	port := testutil.RandomUnprivilegedPort(t)
+	params := []string{
+		"-test.main",
+		"--config.file=" + promConfigFile,
+		"--storage.tsdb.path=" + filepath.Join(tempDir, "data"),
+		fmt.Sprintf("--web.listen-address=127.0.0.1:%d", port),
+		"--web.enable-lifecycle",
+	}
+	prometheusProcess := exec.Command(promPath, params...)
+	prometheusProcess.Stdout = os.Stdout
+	prometheusProcess.Stderr = os.Stderr
+
+	err = prometheusProcess.Start()
+	require.NoError(t, err, "Failed to start Prometheus")
+	defer prometheusProcess.Process.Kill()
+
+	// Wait until the mock server receives at least two requests from Prometheus.
+	require.Eventually(t, func() bool {
+		return requestCount.Load() >= 2
+	}, 30*time.Second, 500*time.Millisecond, "Expected at least two requests to the mock server")
+
+	// Verify that the scrape failures have been logged to the specified file.
+	content, err := os.ReadFile(scrapeFailureLogFile)
+	require.NoError(t, err, "Failed to read scrape failure log")
+	require.Contains(t, string(content), "server returned HTTP status 500 Internal Server Error", "Expected scrape failure log entry not found")
+
+	// Step 2: Update the Prometheus configuration to remove the scrape failure
+	// log file setting.
+	promConfig = fmt.Sprintf(`
+global:
+  scrape_interval: 1s
+
+scrape_configs:
+  - job_name: 'test_job'
+    static_configs:
+      - targets: ['%s']
+`, mockServerAddress)
+
+	err = os.WriteFile(promConfigFile, []byte(promConfig), 0o644)
+	require.NoError(t, err, "Failed to update Prometheus configuration file")
+
+	// Reload Prometheus with the updated configuration.
+	reloadPrometheus(t, port)
+
+	// Count the number of lines in the scrape failure log file before any
+	// further requests.
+	preReloadLogLineCount := countLinesInFile(scrapeFailureLogFile)
+
+	// Wait for at least two more requests to the mock server to ensure
+	// Prometheus continues scraping.
+	requestsBeforeReload := requestCount.Load()
+	require.Eventually(t, func() bool {
+		return requestCount.Load() >= requestsBeforeReload+2
+	}, 30*time.Second, 500*time.Millisecond, "Expected two more requests to the mock server after configuration reload")
+
+	// Ensure that no new lines were added to the scrape failure log file after
+	// the configuration change.
+	require.Equal(t, preReloadLogLineCount, countLinesInFile(scrapeFailureLogFile), "No new lines should be added to the scrape failure log file after removing the log setting")
+
+	// Step 3: Re-add the scrape failure log file setting, but this time under
+	// scrape_configs, and reload Prometheus.
+	promConfig = fmt.Sprintf(`
+global:
+  scrape_interval: 1s
+
+scrape_configs:
+  - job_name: 'test_job'
+    scrape_failure_log_file: %s
+    static_configs:
+      - targets: ['%s']
+`, scrapeFailureLogFileName, mockServerAddress)
+
+	err = os.WriteFile(promConfigFile, []byte(promConfig), 0o644)
+	require.NoError(t, err, "Failed to update Prometheus configuration file")
+
+	// Reload Prometheus with the updated configuration.
+	reloadPrometheus(t, port)
+
+	// Wait for at least two more requests to the mock server and verify that
+	// new log entries are created.
+	postReloadLogLineCount := countLinesInFile(scrapeFailureLogFile)
+	requestsBeforeReAddingLog := requestCount.Load()
+	require.Eventually(t, func() bool {
+		return requestCount.Load() >= requestsBeforeReAddingLog+2
+	}, 30*time.Second, 500*time.Millisecond, "Expected two additional requests after re-adding the log setting")
+
+	// Confirm that new lines were added to the scrape failure log file.
+	require.Greater(t, countLinesInFile(scrapeFailureLogFile), postReloadLogLineCount, "New lines should be added to the scrape failure log file after re-adding the log setting")
+}
+
+// reloadPrometheus sends a reload request to the Prometheus server to apply
+// updated configurations.
+func reloadPrometheus(t *testing.T, port int) {
+	resp, err := http.Post(fmt.Sprintf("http://127.0.0.1:%d/-/reload", port), "", nil)
+	require.NoError(t, err, "Failed to reload Prometheus")
+	require.Equal(t, http.StatusOK, resp.StatusCode, "Unexpected status code when reloading Prometheus")
+}
+
+// startGarbageServer sets up a mock server that returns a 500 Internal Server Error
+// for all requests. It also increments the request count each time it's hit.
+func startGarbageServer(t *testing.T, requestCount *atomic.Int32) string {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount.Inc()
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	t.Cleanup(server.Close)
+
+	parsedURL, err := url.Parse(server.URL)
+	require.NoError(t, err, "Failed to parse mock server URL")
+
+	return parsedURL.Host
+}
+
+// countLinesInFile counts and returns the number of lines in the specified file.
+func countLinesInFile(filePath string) int {
+	data, err := os.ReadFile(filePath)
+	if err != nil {
+		return 0 // Return 0 if the file doesn't exist or can't be read.
+	}
+	return bytes.Count(data, []byte{'\n'})
+}

config/config.go

@@ -429,6 +429,8 @@ type GlobalConfig struct {
 	RuleQueryOffset model.Duration `yaml:"rule_query_offset,omitempty"`
 	// File to which PromQL queries are logged.
 	QueryLogFile string `yaml:"query_log_file,omitempty"`
+	// File to which scrape failures are logged.
+	ScrapeFailureLogFile string `yaml:"scrape_failure_log_file,omitempty"`
 	// The labels to add to any timeseries that this Prometheus instance scrapes.
 	ExternalLabels labels.Labels `yaml:"external_labels,omitempty"`
 	// An uncompressed response body larger than this many bytes will cause the
@@ -529,6 +531,7 @@ func validateAcceptScrapeProtocols(sps []ScrapeProtocol) error {
 // SetDirectory joins any relative file paths with dir.
 func (c *GlobalConfig) SetDirectory(dir string) {
 	c.QueryLogFile = config.JoinDir(dir, c.QueryLogFile)
+	c.ScrapeFailureLogFile = config.JoinDir(dir, c.ScrapeFailureLogFile)
 }
 
 // UnmarshalYAML implements the yaml.Unmarshaler interface.
@@ -591,6 +594,7 @@ func (c *GlobalConfig) isZero() bool {
 		c.EvaluationInterval == 0 &&
 		c.RuleQueryOffset == 0 &&
 		c.QueryLogFile == "" &&
+		c.ScrapeFailureLogFile == "" &&
 		c.ScrapeProtocols == nil
 }
 
@@ -632,6 +636,8 @@ type ScrapeConfig struct {
 	ScrapeProtocols []ScrapeProtocol `yaml:"scrape_protocols,omitempty"`
 	// Whether to scrape a classic histogram that is also exposed as a native histogram.
 	ScrapeClassicHistograms bool `yaml:"scrape_classic_histograms,omitempty"`
+	// File to which scrape failures are logged.
+	ScrapeFailureLogFile string `yaml:"scrape_failure_log_file,omitempty"`
 	// The HTTP resource path on which to fetch metrics from targets.
 	MetricsPath string `yaml:"metrics_path,omitempty"`
 	// The URL scheme with which to fetch metrics from targets.
@@ -684,6 +690,7 @@ type ScrapeConfig struct {
 func (c *ScrapeConfig) SetDirectory(dir string) {
 	c.ServiceDiscoveryConfigs.SetDirectory(dir)
 	c.HTTPClientConfig.SetDirectory(dir)
+	c.ScrapeFailureLogFile = config.JoinDir(dir, c.ScrapeFailureLogFile)
 }
 
 // UnmarshalYAML implements the yaml.Unmarshaler interface.
@@ -765,6 +772,9 @@ func (c *ScrapeConfig) Validate(globalConfig GlobalConfig) error {
 	if c.KeepDroppedTargets == 0 {
 		c.KeepDroppedTargets = globalConfig.KeepDroppedTargets
 	}
+	if c.ScrapeFailureLogFile == "" {
+		c.ScrapeFailureLogFile = globalConfig.ScrapeFailureLogFile
+	}
 
 	if c.ScrapeProtocols == nil {
 		c.ScrapeProtocols = globalConfig.ScrapeProtocols

config/config_test.go

@@ -78,6 +78,7 @@ const (
 	globLabelNameLengthLimit  = 200
 	globLabelValueLengthLimit = 200
 	globalGoGC                = 42
+	globScrapeFailureLogFile  = "testdata/fail.log"
 )
 
 var expectedConf = &Config{
@@ -85,7 +86,8 @@ var expectedConf = &Config{
 		ScrapeInterval:       model.Duration(15 * time.Second),
 		ScrapeTimeout:        DefaultGlobalConfig.ScrapeTimeout,
 		EvaluationInterval:   model.Duration(30 * time.Second),
-		QueryLogFile:       "",
+		QueryLogFile:         "testdata/query.log",
+		ScrapeFailureLogFile: globScrapeFailureLogFile,
 
 		ExternalLabels: labels.FromStrings("foo", "bar", "monitor", "codelab"),
 
@@ -211,6 +213,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  "testdata/fail_prom.log",
 
 			MetricsPath: DefaultScrapeConfig.MetricsPath,
 			Scheme:      DefaultScrapeConfig.Scheme,
@@ -314,6 +317,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  210,
 			LabelValueLengthLimit: 210,
 			ScrapeProtocols:       []ScrapeProtocol{PrometheusText0_0_4},
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			HTTPClientConfig: config.HTTPClientConfig{
 				BasicAuth: &config.BasicAuth{
@@ -411,6 +415,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -466,6 +471,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath: "/metrics",
 			Scheme:      "http",
@@ -499,6 +505,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -538,6 +545,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath: DefaultScrapeConfig.MetricsPath,
 			Scheme:      DefaultScrapeConfig.Scheme,
@@ -577,6 +585,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -606,6 +615,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -643,6 +653,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -677,6 +688,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -718,6 +730,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -749,6 +762,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -783,6 +797,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -810,6 +825,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -840,6 +856,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      "/federate",
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -870,6 +887,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -900,6 +918,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -927,6 +946,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -962,6 +982,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -996,6 +1017,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1027,6 +1049,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1057,6 +1080,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1091,6 +1115,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1128,6 +1153,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1184,6 +1210,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1211,6 +1238,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			HTTPClientConfig: config.DefaultHTTPClientConfig,
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1249,6 +1277,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			HTTPClientConfig: config.DefaultHTTPClientConfig,
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1293,6 +1322,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1328,6 +1358,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			HTTPClientConfig: config.DefaultHTTPClientConfig,
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
@@ -1357,6 +1388,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,
@@ -1389,6 +1421,7 @@ var expectedConf = &Config{
 			LabelNameLengthLimit:  globLabelNameLengthLimit,
 			LabelValueLengthLimit: globLabelValueLengthLimit,
 			ScrapeProtocols:       DefaultGlobalConfig.ScrapeProtocols,
+			ScrapeFailureLogFile:  globScrapeFailureLogFile,
 
 			MetricsPath:      DefaultScrapeConfig.MetricsPath,
 			Scheme:           DefaultScrapeConfig.Scheme,

config/testdata/conf.good.yml

@@ -8,6 +8,8 @@ global:
   label_limit: 30
   label_name_length_limit: 200
   label_value_length_limit: 200
+  query_log_file: query.log
+  scrape_failure_log_file: fail.log
   # scrape_timeout is set to the global default (10s).
 
   external_labels:
@@ -72,6 +74,7 @@ scrape_configs:
     # metrics_path defaults to '/metrics'
     # scheme defaults to 'http'.
 
+    scrape_failure_log_file: fail_prom.log
     file_sd_configs:
       - files:
           - foo/*.slow.json

docs/configuration/configuration.md

@@ -84,6 +84,10 @@ global:
   # Reloading the configuration will reopen the file.
   [ query_log_file: <string> ]
 
+  # File to which scrape failures are logged.
+  # Reloading the configuration will reopen the file.
+  [ scrape_failure_log_file: <string> ]
+
   # An uncompressed response body larger than this many bytes will cause the
   # scrape to fail. 0 means no limit. Example: 100MB.
   # This is an experimental feature, this behaviour could
@@ -319,6 +323,10 @@ http_headers:
     # Files to read header values from.
     [ files: [<string>, ...] ] ]
 
+# File to which scrape failures are logged.
+# Reloading the configuration will reopen the file.
+[ scrape_failure_log_file: <string> ]
+
 # List of Azure service discovery configurations.
 azure_sd_configs:
   [ - <azure_sd_config> ... ]

scrape/manager.go

@@ -17,6 +17,7 @@ import (
 	"errors"
 	"fmt"
 	"hash/fnv"
+	"io"
 	"reflect"
 	"sync"
 	"time"
@@ -36,7 +37,7 @@ import (
 )
 
 // NewManager is the Manager constructor.
-func NewManager(o *Options, logger log.Logger, app storage.Appendable, registerer prometheus.Registerer) (*Manager, error) {
+func NewManager(o *Options, logger log.Logger, newScrapeFailureLogger func(string) (log.Logger, error), app storage.Appendable, registerer prometheus.Registerer) (*Manager, error) {
 	if o == nil {
 		o = &Options{}
 	}
@@ -53,6 +54,7 @@ func NewManager(o *Options, logger log.Logger, app storage.Appendable, registere
 		append:                 app,
 		opts:                   o,
 		logger:                 logger,
+		newScrapeFailureLogger: newScrapeFailureLogger,
 		scrapeConfigs:          make(map[string]*config.ScrapeConfig),
 		scrapePools:            make(map[string]*scrapePool),
 		graceShut:              make(chan struct{}),
@@ -107,6 +109,8 @@ type Manager struct {
 	mtxScrape              sync.Mutex // Guards the fields below.
 	scrapeConfigs          map[string]*config.ScrapeConfig
 	scrapePools            map[string]*scrapePool
+	newScrapeFailureLogger func(string) (log.Logger, error)
+	scrapeFailureLoggers   map[string]log.Logger
 	targetSets             map[string][]*targetgroup.Group
 	buffers                *pool.Pool
 
@@ -183,6 +187,11 @@ func (m *Manager) reload() {
 				continue
 			}
 			m.scrapePools[setName] = sp
+			if l, ok := m.scrapeFailureLoggers[scrapeConfig.ScrapeFailureLogFile]; ok {
+				sp.SetScrapeFailureLogger(l)
+			} else {
+				level.Error(sp.logger).Log("msg", "No logger found. This is a bug in Prometheus that should be reported upstream.", "scrape_pool", setName)
+			}
 		}
 
 		wg.Add(1)
@@ -238,11 +247,36 @@ func (m *Manager) ApplyConfig(cfg *config.Config) error {
 	}
 
 	c := make(map[string]*config.ScrapeConfig)
+	scrapeFailureLoggers := map[string]log.Logger{
+		"": nil, // Emptying the file name sets the scrape logger to nil.
+	}
 	for _, scfg := range scfgs {
 		c[scfg.JobName] = scfg
+		if _, ok := scrapeFailureLoggers[scfg.ScrapeFailureLogFile]; !ok {
+			// We promise to reopen the file on each reload.
+			var (
+				l   log.Logger
+				err error
+			)
+			if m.newScrapeFailureLogger != nil {
+				if l, err = m.newScrapeFailureLogger(scfg.ScrapeFailureLogFile); err != nil {
+					return err
+				}
+			}
+			scrapeFailureLoggers[scfg.ScrapeFailureLogFile] = l
+		}
 	}
 	m.scrapeConfigs = c
 
+	oldScrapeFailureLoggers := m.scrapeFailureLoggers
+	for _, s := range oldScrapeFailureLoggers {
+		if closer, ok := s.(io.Closer); ok {
+			defer closer.Close()
+		}
+	}
+
+	m.scrapeFailureLoggers = scrapeFailureLoggers
+
 	if err := m.setOffsetSeed(cfg.GlobalConfig.ExternalLabels); err != nil {
 		return err
 	}
@@ -260,6 +294,13 @@ func (m *Manager) ApplyConfig(cfg *config.Config) error {
 				level.Error(m.logger).Log("msg", "error reloading scrape pool", "err", err, "scrape_pool", name)
 				failed = true
 			}
+			fallthrough
+		case ok:
+			if l, ok := m.scrapeFailureLoggers[cfg.ScrapeFailureLogFile]; ok {
+				sp.SetScrapeFailureLogger(l)
+			} else {
+				level.Error(sp.logger).Log("msg", "No logger found. This is a bug in Prometheus that should be reported upstream.", "scrape_pool", name)
+			}
 		}
 	}
 

scrape/manager_test.go

@@ -511,7 +511,7 @@ scrape_configs:
 	)
 
 	opts := Options{}
-	scrapeManager, err := NewManager(&opts, nil, nil, testRegistry)
+	scrapeManager, err := NewManager(&opts, nil, nil, nil, testRegistry)
 	require.NoError(t, err)
 	newLoop := func(scrapeLoopOptions) loop {
 		ch <- struct{}{}
@@ -576,7 +576,7 @@ scrape_configs:
 func TestManagerTargetsUpdates(t *testing.T) {
 	opts := Options{}
 	testRegistry := prometheus.NewRegistry()
-	m, err := NewManager(&opts, nil, nil, testRegistry)
+	m, err := NewManager(&opts, nil, nil, nil, testRegistry)
 	require.NoError(t, err)
 
 	ts := make(chan map[string][]*targetgroup.Group)
@@ -629,7 +629,7 @@ global:
 
 	opts := Options{}
 	testRegistry := prometheus.NewRegistry()
-	scrapeManager, err := NewManager(&opts, nil, nil, testRegistry)
+	scrapeManager, err := NewManager(&opts, nil, nil, nil, testRegistry)
 	require.NoError(t, err)
 
 	// Load the first config.
@@ -706,7 +706,7 @@ scrape_configs:
 	}
 
 	opts := Options{}
-	scrapeManager, err := NewManager(&opts, nil, nil, testRegistry)
+	scrapeManager, err := NewManager(&opts, nil, nil, nil, testRegistry)
 	require.NoError(t, err)
 
 	reload(scrapeManager, cfg1)
@@ -758,6 +758,7 @@ func TestManagerCTZeroIngestion(t *testing.T) {
 					skipOffsetting:                      true,
 				},
 				log.NewLogfmtLogger(os.Stderr),
+				nil,
 				&collectResultAppendable{app},
 				prometheus.NewRegistry(),
 			)
@@ -857,7 +858,7 @@ func TestUnregisterMetrics(t *testing.T) {
 	// Check that all metrics can be unregistered, allowing a second manager to be created.
 	for i := 0; i < 2; i++ {
 		opts := Options{}
-		manager, err := NewManager(&opts, nil, nil, reg)
+		manager, err := NewManager(&opts, nil, nil, nil, reg)
 		require.NotNil(t, manager)
 		require.NoError(t, err)
 		// Unregister all metrics.
@@ -901,6 +902,7 @@ func runManagers(t *testing.T, ctx context.Context) (*discovery.Manager, *Manage
 	scrapeManager, err := NewManager(
 		&Options{DiscoveryReloadInterval: model.Duration(100 * time.Millisecond)},
 		nil,
+		nil,
 		nopAppendable{},
 		prometheus.NewRegistry(),
 	)

scrape/scrape.go

@@ -90,6 +90,9 @@ type scrapePool struct {
 	noDefaultPort bool
 
 	metrics *scrapeMetrics
+
+	scrapeFailureLogger    log.Logger
+	scrapeFailureLoggerMtx sync.RWMutex
 }
 
 type labelLimits struct {
@@ -218,6 +221,27 @@ func (sp *scrapePool) DroppedTargetsCount() int {
 	return sp.droppedTargetsCount
 }
 
+func (sp *scrapePool) SetScrapeFailureLogger(l log.Logger) {
+	sp.scrapeFailureLoggerMtx.Lock()
+	defer sp.scrapeFailureLoggerMtx.Unlock()
+	if l != nil {
+		l = log.With(l, "job_name", sp.config.JobName)
+	}
+	sp.scrapeFailureLogger = l
+
+	sp.targetMtx.Lock()
+	defer sp.targetMtx.Unlock()
+	for _, s := range sp.loops {
+		s.setScrapeFailureLogger(sp.scrapeFailureLogger)
+	}
+}
+
+func (sp *scrapePool) getScrapeFailureLogger() log.Logger {
+	sp.scrapeFailureLoggerMtx.RLock()
+	defer sp.scrapeFailureLoggerMtx.RUnlock()
+	return sp.scrapeFailureLogger
+}
+
 // stop terminates all scrape loops and returns after they all terminated.
 func (sp *scrapePool) stop() {
 	sp.mtx.Lock()
@@ -361,6 +385,7 @@ func (sp *scrapePool) restartLoops(reuseCache bool) {
 			wg.Done()
 
 			newLoop.setForcedError(forcedErr)
+			newLoop.setScrapeFailureLogger(sp.getScrapeFailureLogger())
 			newLoop.run(nil)
 		}(oldLoop, newLoop)
 
@@ -503,6 +528,7 @@ func (sp *scrapePool) sync(targets []*Target) {
 			if err != nil {
 				l.setForcedError(err)
 			}
+			l.setScrapeFailureLogger(sp.scrapeFailureLogger)
 
 			sp.activeTargets[hash] = t
 			sp.loops[hash] = l
@@ -825,6 +851,7 @@ func (s *targetScraper) readResponse(ctx context.Context, resp *http.Response, w
 type loop interface {
 	run(errc chan<- error)
 	setForcedError(err error)
+	setScrapeFailureLogger(log.Logger)
 	stop()
 	getCache() *scrapeCache
 	disableEndOfRunStalenessMarkers()
@@ -840,6 +867,8 @@ type cacheEntry struct {
 type scrapeLoop struct {
 	scraper                  scraper
 	l                        log.Logger
+	scrapeFailureLogger      log.Logger
+	scrapeFailureLoggerMtx   sync.RWMutex
 	cache                    *scrapeCache
 	lastScrapeSize           int
 	buffers                  *pool.Pool
@@ -1223,6 +1252,15 @@ func newScrapeLoop(ctx context.Context,
 	return sl
 }
 
+func (sl *scrapeLoop) setScrapeFailureLogger(l log.Logger) {
+	sl.scrapeFailureLoggerMtx.Lock()
+	defer sl.scrapeFailureLoggerMtx.Unlock()
+	if ts, ok := sl.scraper.(fmt.Stringer); ok && l != nil {
+		l = log.With(l, "target", ts.String())
+	}
+	sl.scrapeFailureLogger = l
+}
+
 func (sl *scrapeLoop) run(errc chan<- error) {
 	if !sl.skipOffsetting {
 		select {
@@ -1366,6 +1404,11 @@ func (sl *scrapeLoop) scrapeAndReport(last, appendTime time.Time, errc chan<- er
 		bytesRead = len(b)
 	} else {
 		level.Debug(sl.l).Log("msg", "Scrape failed", "err", scrapeErr)
+		sl.scrapeFailureLoggerMtx.RLock()
+		if sl.scrapeFailureLogger != nil {
+			sl.scrapeFailureLogger.Log("err", scrapeErr)
+		}
+		sl.scrapeFailureLoggerMtx.RUnlock()
 		if errc != nil {
 			errc <- scrapeErr
 		}

scrape/scrape_test.go

@@ -158,6 +158,9 @@ type testLoop struct {
 	timeout      time.Duration
 }
 
+func (l *testLoop) setScrapeFailureLogger(log.Logger) {
+}
+
 func (l *testLoop) run(errc chan<- error) {
 	if l.runOnce {
 		panic("loop must be started only once")
@@ -3782,7 +3785,7 @@ scrape_configs:
 	s.DB.EnableNativeHistograms()
 	reg := prometheus.NewRegistry()
 
-	mng, err := NewManager(&Options{EnableNativeHistogramsIngestion: true}, nil, s, reg)
+	mng, err := NewManager(&Options{EnableNativeHistogramsIngestion: true}, nil, nil, s, reg)
 	require.NoError(t, err)
 	cfg, err := config.Load(configStr, false, log.NewNopLogger())
 	require.NoError(t, err)