Prometheus Go Code Review

Review Checklist

• Metric types match measurement semantics (Counter/Gauge/Histogram)

• Labels have low cardinality (no user IDs, timestamps, paths)

• Metric names follow conventions (snake_case, unit suffix)

• Histograms use appropriate bucket boundaries

• Metrics registered once, not per-request

• Collectors don't panic on race conditions

• /metrics endpoint exposed and accessible

Metric Type Selection

Measurement Type Example

Requests processed Counter requests_total

Items in queue Gauge queue_length

Request duration Histogram request_duration_seconds

Concurrent connections Gauge active_connections

Errors since start Counter errors_total

Memory usage Gauge memory_bytes

Critical Anti-Patterns

1. High Cardinality Labels

// BAD - unique per user/request counter := promauto.NewCounterVec( prometheus.CounterOpts{Name: "requests_total"}, []string{"user_id", "path"}, // millions of series! ) counter.WithLabelValues(userID, request.URL.Path).Inc()

// GOOD - bounded label values counter := promauto.NewCounterVec( prometheus.CounterOpts{Name: "requests_total"}, []string{"method", "status_code"}, // <100 series ) counter.WithLabelValues(r.Method, statusCode).Inc()

2. Wrong Metric Type

// BAD - using gauge for monotonic value requestCount := promauto.NewGauge(prometheus.GaugeOpts{ Name: "http_requests", }) requestCount.Inc() // should be Counter!

// GOOD requestCount := promauto.NewCounter(prometheus.CounterOpts{ Name: "http_requests_total", }) requestCount.Inc()

3. Registering Per-Request

// BAD - new metric per request func handler(w http.ResponseWriter, r *http.Request) { counter := prometheus.NewCounter(...) // creates new each time! prometheus.MustRegister(counter) // panics on duplicate! }

// GOOD - register once var requestCounter = promauto.NewCounter(prometheus.CounterOpts{ Name: "http_requests_total", })

func handler(w http.ResponseWriter, r *http.Request) { requestCounter.Inc() }

4. Missing Unit Suffix

// BAD duration := promauto.NewHistogram(prometheus.HistogramOpts{ Name: "request_duration", // no unit! })

// GOOD duration := promauto.NewHistogram(prometheus.HistogramOpts{ Name: "request_duration_seconds", // unit in name })

Good Patterns

Metric Definition

var ( httpRequests = promauto.NewCounterVec( prometheus.CounterOpts{ Namespace: "myapp", Subsystem: "http", Name: "requests_total", Help: "Total HTTP requests processed", }, []string{"method", "status"}, )

httpDuration = promauto.NewHistogramVec(
    prometheus.HistogramOpts{
        Namespace: "myapp",
        Subsystem: "http",
        Name:      "request_duration_seconds",
        Help:      "HTTP request latencies",
        Buckets:   []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10},
    },
    []string{"method"},
)

)

Middleware Pattern

func metricsMiddleware(next http.Handler) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { timer := prometheus.NewTimer(httpDuration.WithLabelValues(r.Method)) defer timer.ObserveDuration()

    wrapped := &responseWriter{ResponseWriter: w, status: 200}
    next.ServeHTTP(wrapped, r)

    httpRequests.WithLabelValues(r.Method, strconv.Itoa(wrapped.status)).Inc()
})

}

Exposing Metrics

import "github.com/prometheus/client_golang/prometheus/promhttp"

func main() { http.Handle("/metrics", promhttp.Handler()) http.ListenAndServe(":9090", nil) }

Review Questions

• Are metric types correct (Counter vs Gauge vs Histogram)?

• Are label values bounded (no UUIDs, timestamps, paths)?

• Do metric names include units (_seconds, _bytes)?

• Are metrics registered once (not per-request)?

• Is /metrics endpoint properly exposed?