prometheus

## What I do - Set up Prometheus metrics collection - Write PromQL queries - Configure alerting rules - Use client libraries - Set up service discovery - Create Grafana dashboards - Handle metric cardinality ## When to use me When monitoring applications and infrastructure metrics. ## Configuration ```yaml global: scrape_interval: 15s evaluation_interval: 15s scrape_configs: - job_name: 'prometheus' static_configs: - targets: ['localhost:9090'] - job_name: 'node' static_configs: - targets: ['node-exporter:9100'] - job_name: 'myapp' metrics_path: '/metrics' static_configs: - targets: ['app:8080'] ``` ## Metrics Types ```go import "github.com/prometheus/client_golang/prometheus" var ( // Counter - only increases requestsTotal = prometheus.NewCounterVec( prometheus.CounterOpts{ Name: "http_requests_total", Help: "Total HTTP requests", }, []string{"method", "status"}, ) // Gauge - can go up and down temperature = prometheus.NewGauge( prometheus.GaugeOpts{ Name: "room_temperature_celsius", Help: "Current room temperature", }, ) // Histogram - distributions requestDuration = prometheus.NewHistogram( prometheus.HistogramOpts{ Name: "http_request_duration_seconds", Buckets: []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5}, }, ) // Summary - percentiles responseSize = prometheus.NewSummary( prometheus.SummaryOpts{ Name: "http_response_size_bytes", Objectives: map[float64]float64{0.5: 0.05, 0.95: 0.05, 0.99: 0.01}, }, ) ) func init() { prometheus.MustRegister(requestsTotal, temperature, requestDuration) } ``` ## PromQL Queries ```promql # Rate - per second increase rate(http_requests_total[5m]) # Increase over time window increase(http_requests_total[1h]) # Average over time avg(http_request_duration_seconds) # Percentiles histogram_quantile(0.95, rate(http_request_duration_seconds_bucket[5m])) # Count over time count(http_requests_total{status="200"}) # Sum by label sum by (method) (http_requests_total) # Top queries topk(10, http_requests_total) # Subqueries max_over_time(http_requests_total[1h:5m]) ``` ## Alerting Rules ```yaml groups: - name: example rules: - alert: HighErrorRate expr: rate(http_requests_total{status=~"5.."}[5m]) > 0.05 for: 5m labels: severity: critical annotations: summary: "High error rate on {{ $labels.method }}" - alert: InstanceDown expr: up == 0 for: 2m labels: severity: critical annotations: summary: "Instance {{ $labels.instance }} down" ``` ## Python Client ```python from prometheus_client import Counter, Gauge, Histogram, start_http_server REQUEST_COUNT = Counter('http_requests_total', 'Total HTTP requests', ['method', 'status']) ACTIVE_USERS = Gauge('active_users', 'Number of active users') REQUEST_LATENCY = Histogram('http_request_latency_seconds', 'Request latency') @app.route('/api') def handle_request(): with REQUEST_LATENCY.time(): # process request REQUEST_COUNT.labels(method='GET', status='200').inc() return 'OK' if __name__ == '__main__': start_http_server(8000) ```