feat: time database queries

internal/observability/db.go

@@ -0,0 +1,51 @@
+package observability
+
+import (
+	"context"
+	"database/sql"
+	"time"
+)
+
+type instrumentedDB struct {
+	db interface {
+		ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
+		PrepareContext(context.Context, string) (*sql.Stmt, error)
+		QueryContext(context.Context, string, ...interface{}) (*sql.Rows, error)
+		QueryRowContext(context.Context, string, ...interface{}) *sql.Row
+	}
+	metrics *Metrics
+}
+
+func InstrumentDB(db interface {
+	ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
+	PrepareContext(context.Context, string) (*sql.Stmt, error)
+	QueryContext(context.Context, string, ...interface{}) (*sql.Rows, error)
+	QueryRowContext(context.Context, string, ...interface{}) *sql.Row
+}, metrics *Metrics) *instrumentedDB {
+	return &instrumentedDB{db: db, metrics: metrics}
+}
+
+func (db *instrumentedDB) ExecContext(ctx context.Context, query string, args ...interface{}) (sql.Result, error) {
+	start := time.Now()
+	result, err := db.db.ExecContext(ctx, query, args...)
+	db.metrics.ObserveDBQuery("exec", time.Since(start), err)
+	return result, err
+}
+
+func (db *instrumentedDB) PrepareContext(ctx context.Context, query string) (*sql.Stmt, error) {
+	return db.db.PrepareContext(ctx, query)
+}
+
+func (db *instrumentedDB) QueryContext(ctx context.Context, query string, args ...interface{}) (*sql.Rows, error) {
+	start := time.Now()
+	rows, err := db.db.QueryContext(ctx, query, args...)
+	db.metrics.ObserveDBQuery("query", time.Since(start), err)
+	return rows, err
+}
+
+func (db *instrumentedDB) QueryRowContext(ctx context.Context, query string, args ...interface{}) *sql.Row {
+	start := time.Now()
+	row := db.db.QueryRowContext(ctx, query, args...)
+	db.metrics.ObserveDBQuery("query_row", time.Since(start), nil)
+	return row
+}

internal/observability/metrics.go

@@ -56,6 +56,7 @@ type Metrics struct {
 	jikanRequests      *counterVec
 	jikanRequestErrors *counterVec
 	jikanLatency       *histogramVec
+	dbQueryLatency     *histogramVec
 	workerTicks        *counterVec
 	cacheOperations    *counterVec
 }
@@ -67,6 +68,7 @@ func NewMetrics() *Metrics {
 		jikanRequests:      newCounterVec("endpoint", "status"),
 		jikanRequestErrors: newCounterVec("endpoint", "status"),
 		jikanLatency:       newHistogramVec(defaultDurationBuckets, "endpoint", "status"),
+		dbQueryLatency:     newHistogramVec(defaultDurationBuckets, "operation", "result"),
 		workerTicks:        newCounterVec("worker", "result"),
 		cacheOperations:    newCounterVec("cache", "result"),
 	}
@@ -95,6 +97,14 @@ func (m *Metrics) ObserveJikanRequest(endpoint string, status int, duration time
 	}
 }
 
+func (m *Metrics) ObserveDBQuery(operation string, duration time.Duration, err error) {
+	result := "success"
+	if err != nil {
+		result = "error"
+	}
+	m.dbQueryLatency.Observe(duration.Seconds(), operation, result)
+}
+
 func (m *Metrics) ObserveWorkerTick(worker string, err error) {
 	if err != nil {
 		m.workerTicks.Inc(worker, "failure")
@@ -113,6 +123,7 @@ func (m *Metrics) writePrometheus(w http.ResponseWriter) {
 	writeCounterMetric(w, "mal_jikan_upstream_requests_total", "Total upstream Jikan requests by endpoint and status.", m.jikanRequests.snapshot())
 	writeCounterMetric(w, "mal_jikan_upstream_errors_total", "Total upstream Jikan errors by endpoint and status.", m.jikanRequestErrors.snapshot())
 	writeHistogramMetric(w, "mal_jikan_upstream_request_duration_seconds", "Upstream Jikan request latency in seconds.", m.jikanLatency.snapshot(), m.jikanLatency.bounds)
+	writeHistogramMetric(w, "mal_db_query_duration_seconds", "Database query latency in seconds.", m.dbQueryLatency.snapshot(), m.dbQueryLatency.bounds)
 	writeCounterMetric(w, "mal_worker_ticks_total", "Total background worker ticks by worker and result.", m.workerTicks.snapshot())
 	writeCounterMetric(w, "mal_cache_operations_total", "Total cache hits and misses by cache name.", m.cacheOperations.snapshot())
 }

internal/observability/metrics_test.go

@@ -2,18 +2,22 @@ package observability
 
 import (
 	"context"
+	"database/sql"
 	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
 	"time"
+
+	_ "github.com/mattn/go-sqlite3"
 )
 
 func TestMetricsHandlerRendersPrometheusFamilies(t *testing.T) {
 	metrics := NewMetrics()
 	metrics.ObserveHTTPRequest(http.MethodGet, "/anime/:id", http.StatusOK, 125*time.Millisecond)
 	metrics.ObserveJikanRequest("/anime/{id}", http.StatusTooManyRequests, 800*time.Millisecond, assertErr{})
+	metrics.ObserveDBQuery("query", 25*time.Millisecond, nil)
 	metrics.ObserveWorkerTick("episodes_availability", nil)
 	metrics.ObserveCache("jikan", "hit")
 	metrics.ObserveCache("episode_availability", "miss")
@@ -32,10 +36,39 @@ func TestMetricsHandlerRendersPrometheusFamilies(t *testing.T) {
 	assertContains(t, text, `mal_http_request_duration_seconds_count{method="GET",route="/anime/:id",status="200"} 1`)
 	assertContains(t, text, `mal_jikan_upstream_requests_total{endpoint="/anime/{id}",status="429"} 1`)
 	assertContains(t, text, `mal_jikan_upstream_errors_total{endpoint="/anime/{id}",status="429"} 1`)
+	assertContains(t, text, `mal_db_query_duration_seconds_count{operation="query",result="success"} 1`)
 	assertContains(t, text, `mal_worker_ticks_total{result="success",worker="episodes_availability"} 1`)
 	assertContains(t, text, `mal_cache_operations_total{cache="episode_availability",result="miss"} 1`)
 }
 
+func TestInstrumentDBRecordsQueryLatency(t *testing.T) {
+	metrics := NewMetrics()
+	sqlDB, err := sql.Open("sqlite3", ":memory:")
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	defer func() { _ = sqlDB.Close() }()
+
+	instrumented := InstrumentDB(sqlDB, metrics)
+	if _, err := instrumented.ExecContext(context.Background(), `CREATE TABLE item (id INTEGER PRIMARY KEY)`); err != nil {
+		t.Fatalf("create table: %v", err)
+	}
+	if _, err := instrumented.QueryContext(context.Background(), `SELECT id FROM item`); err != nil {
+		t.Fatalf("query table: %v", err)
+	}
+	if _, err := instrumented.QueryContext(context.Background(), `SELECT id FROM missing_table`); err == nil {
+		t.Fatal("expected missing table query to fail")
+	}
+
+	samples := metrics.dbQueryLatency.snapshot()
+	if len(samples) != 3 {
+		t.Fatalf("db samples = %d, want 3", len(samples))
+	}
+	assertHistogramSample(t, samples, "exec", "success")
+	assertHistogramSample(t, samples, "query", "success")
+	assertHistogramSample(t, samples, "query", "error")
+}
+
 type assertErr struct{}
 
 func (assertErr) Error() string { return "boom" }
@@ -46,3 +79,13 @@ func assertContains(t *testing.T, text string, want string) {
 		t.Fatalf("missing metric line %q in:\n%s", want, text)
 	}
 }
+
+func assertHistogramSample(t *testing.T, samples []histogramSample, operation string, result string) {
+	t.Helper()
+	for _, sample := range samples {
+		if sample.labels["operation"] == operation && sample.labels["result"] == result && sample.count == 1 {
+			return
+		}
+	}
+	t.Fatalf("missing db histogram sample operation=%q result=%q in %#v", operation, result, samples)
+}