break: collect metrics on demand

As per https://prometheus.io/docs/instrumenting/writing_exporters/#scheduling:

> Metrics should only be pulled from the application when Prometheus scrapes them,
> exporters should not perform scrapes based on their own timers.
> That is, all scrapes should be synchronous.

3 files changed, 30 insertions, 42 deletions

diff --git a/main.go b/main.go
index 919fe24..2e0e2cc 100644
--- a/main.go
+++ b/main.go
@@ -13,7 +13,6 @@ import (
 	"regexp"
 	"slices"
 	"strings"
-	"time"
 
 	"github.com/alecthomas/kong"
 	"github.com/prometheus/client_golang/prometheus"
@@ -21,10 +20,9 @@ import (
 )
 
 type Args struct {
-	Port     int           `help:"Port to listen on" short:"p" default:"8888" env:"PORT"`
-	Repos    StringMap     `placeholder:"{\"name\": \"path\", ...}" required:"" help:"Repositories to be grepped" env:"REPOS"`
-	Patterns StringMap     `placeholder:"{\"name\": \"regex\", ...}" required:"" help:"Patterns to be grepped for" env:"PATTERNS"`
-	Interval time.Duration `help:"How often the repos should be grepped (units: s,m,h for seconds, minutes, hours)" default:"10m" env:"INTERVAL"`
+	Port     int       `help:"Port to listen on" short:"p" default:"8888" env:"PORT"`
+	Repos    StringMap `placeholder:"{\"name\": \"path\", ...}" required:"" help:"Repositories to be grepped" env:"REPOS"`
+	Patterns StringMap `placeholder:"{\"name\": \"regex\", ...}" required:"" help:"Patterns to be grepped for" env:"PATTERNS"`
 }
 
 type StringMap map[string]string
@@ -46,31 +44,20 @@ func main() {
 			"Only the HEAD of the repositories will be grepped.",
 	))
 
-	http.Handle("/metrics", promhttp.Handler())
-
 	te, err := newGitGrepExporter(args.Repos, args.Patterns, prometheus.DefaultRegisterer)
 	ctx.FatalIfErrorf(err)
 
-	go func() {
-		ticker := time.NewTicker(args.Interval)
-		defer ticker.Stop()
-
-		// execute immediately on start
-		te.grepRepos()
-
-		for range ticker.C {
-			te.grepRepos()
-		}
-	}()
+	prometheus.MustRegister(te)
+	http.Handle("/metrics", promhttp.Handler())
 
-	slog.Info("starting", "repos", args.Repos, "patterns", args.Patterns, "interval", args.Interval)
+	slog.Info("starting", "repos", args.Repos, "patterns", args.Patterns)
 	slog.Info(fmt.Sprintf("serving metrics at http://localhost:%d/metrics", args.Port))
 	http.ListenAndServe(fmt.Sprintf(":%d", args.Port), nil)
 }
 
 type GitGrepExporter struct {
 	repos           map[string]string
-	gauges          map[string]*prometheus.GaugeVec
+	descs           map[string]*prometheus.Desc
 	regexes         map[string]*regexp.Regexp
 	combinedPattern string
 }
@@ -83,30 +70,38 @@ func newGitGrepExporter(repos map[string]string, patterns map[string]string, reg
 		return nil, fmt.Errorf("no patterns specified")
 	}
 
-	gauges := map[string]*prometheus.GaugeVec{}
+	descs := map[string]*prometheus.Desc{}
 	regexes := map[string]*regexp.Regexp{}
 
 	for name, pattern := range patterns {
-		gauges[name] = prometheus.NewGaugeVec(prometheus.GaugeOpts{
-			Name: fmt.Sprintf("%s_lines", name),
-		}, []string{"repo"})
-		reg.MustRegister(gauges[name])
+		descs[name] = prometheus.NewDesc(
+			fmt.Sprintf("%s_lines", name),
+			"",
+			[]string{"repo"},
+			nil,
+		)
 		regexes[name] = regexp.MustCompile(pattern)
 	}
 
-	return &GitGrepExporter{repos, gauges, regexes, strings.Join(slices.Collect(maps.Values(patterns)), "|")}, nil
+	return &GitGrepExporter{repos, descs, regexes, strings.Join(slices.Collect(maps.Values(patterns)), "|")}, nil
+}
+
+func (e *GitGrepExporter) Describe(ch chan<- *prometheus.Desc) {
+	for _, desc := range e.descs {
+		ch <- desc
+	}
 }
 
-func (g *GitGrepExporter) grepRepos() {
+func (g *GitGrepExporter) Collect(ch chan<- prometheus.Metric) {
 	for name, path := range g.repos {
 		slog.Info("starting grep", "path", path)
-		g.grepRepo(name)
+		g.collectCountsForRepo(name, ch)
 		slog.Info("finished grep", "path", path)
 	}
 }
 
-func (g *GitGrepExporter) grepRepo(name string) {
-	path := g.repos[name]
+func (g *GitGrepExporter) collectCountsForRepo(repo string, ch chan<- prometheus.Metric) {
+	path := g.repos[repo]
 	output, err := run("git", "-C", path, "grep", "-hE", g.combinedPattern, "HEAD")
 
 	if err != nil {
@@ -114,8 +109,8 @@ func (g *GitGrepExporter) grepRepo(name string) {
 			if exitErr.ExitCode() == 1 {
 				// no matches found
 
-				for _, gauge := range g.gauges {
-					gauge.With(map[string]string{"repo": name}).Set(0)
+				for _, desc := range g.descs {
+					ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, 0, repo)
 				}
 				return
 			}
@@ -138,8 +133,8 @@ func (g *GitGrepExporter) grepRepo(name string) {
 			}
 		}
 	}
-	for word, gauge := range g.gauges {
-		gauge.With(map[string]string{"repo": name}).Set(counts[word])
+	for word, desc := range g.descs {
+		ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, counts[word], repo)
 	}
 }
 
diff --git a/main_test.go b/main_test.go
index bd5130a..9fdecca 100644
--- a/main_test.go
+++ b/main_test.go
@@ -18,6 +18,7 @@ func TestGrepRepo(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	reg.MustRegister(te)
 
 	run = func(name string, args ...string) ([]byte, error) {
 		return []byte(
@@ -29,8 +30,6 @@ func TestGrepRepo(t *testing.T) {
 		), nil
 	}
 
-	te.grepRepo("project1")
-
 	want := "# HELP match_lines \n" +
 		"# TYPE match_lines gauge\n" +
 		"match_lines{repo=\"project1\"} 4\n" +
diff --git a/service.nix b/service.nix
index 0b5aa91..bc0bbdc 100644
--- a/service.nix
+++ b/service.nix
@@ -24,11 +24,6 @@ in
       type = lib.types.listOf lib.types.str;
       default = [];
     };
-
-    interval = lib.mkOption {
-      type = lib.types.nullOr lib.types.str;
-      default = "10m";
-    };
   };
 
   config = lib.mkIf cfg.enable {
@@ -42,7 +37,6 @@ in
         PORT = toString cfg.port;
         REPOS = builtins.toJSON cfg.repos;
         PATTERNS = builtins.toJSON cfg.patterns;
-        INTERVAL = cfg.interval;
       };
 
       wantedBy = ["multi-user.target"];