# Monitoring: Prometheus, Grafana & Loki

**Prometheus:** `prometheus-kube-prom-prometheus.monitoring.svc.cluster.local:9090`  
**Grafana:** `https://grafana.riotpiao.homelab.com`  
**Loki:** `loki.logging.svc.cluster.local:3100`  
**Namespaces:** `monitoring`, `logging`

## When to Use

- **Metrics** — CPU, memory, latency, request rate, custom business metrics (Prometheus)
- **Logs** — Pod logs, kernel logs, trace events (Loki)
- **Dashboards** — Real-time visualization (Grafana)
- **Alerts** — Page on high error rate, latency spike, or resource exhaustion

## Quick Start

**1. Access Grafana:**
```bash
# Browser: https://grafana.riotpiao.homelab.com
# Login: admin / GRAFANA_ADMIN_PASSWORD (from .env)
# Or via Authentik SSO

# Port-forward (if no external access)
make pf-grafana  # localhost:3000
```

**2. View dashboards:**
```
Grafana → Dashboards → Homelab folder
- Service Availability (uptime probes + cert expiry)
- Latency & Golden Signals (request rate/error %/duration)
- Hardware Statistics (per-node CPU/mem/disk)
- Kube-Controller Health (API server metrics)
- Service Internals (per-service deep-dive)
```

**3. Add Prometheus data source:**
```
Grafana → Settings → Data Sources → Add
- Type: Prometheus
- URL: http://prometheus-kube-prom-prometheus.monitoring.svc.cluster.local:9090
- Save & test
```

**4. Create dashboard:**
```json
{
  "dashboard": {
    "title": "My Service",
    "panels": [
      {
        "targets": [
          {
            "expr": "rate(http_requests_total{service='myapp'}[5m])"
          }
        ],
        "title": "Request Rate"
      }
    ]
  }
}
```

## Instrumentation Patterns

**Go (Prometheus):**
```go
import "github.com/prometheus/client_golang/prometheus"

// Counter (monotonic increase)
requestsTotal := prometheus.NewCounterVec(
  prometheus.CounterOpts{
    Name: "http_requests_total",
    Help: "Total requests",
  },
  []string{"method", "status"},
)

// Gauge (can go up or down)
activeConnections := prometheus.NewGauge(prometheus.GaugeOpts{
  Name: "active_connections",
})

// Histogram (latency buckets)
requestDuration := prometheus.NewHistogramVec(
  prometheus.HistogramOpts{
    Name: "http_request_duration_seconds",
    Buckets: []float64{.001, .01, .1, 1},
  },
  []string{"method", "path"},
)

// Record metric
requestsTotal.WithLabelValues("POST", "200").Inc()
requestDuration.WithLabelValues("POST", "/api/users").Observe(0.042)
```

**Node.js (prom-client):**
```javascript
const prometheus = require('prom-client');

const httpRequestDuration = new prometheus.Histogram({
  name: 'http_request_duration_seconds',
  help: 'Duration of HTTP requests in seconds',
  labelNames: ['method', 'path', 'status'],
  buckets: [0.001, 0.01, 0.1, 1],
});

// Middleware
app.use((req, res, next) => {
  const start = Date.now();
  res.on('finish', () => {
    const duration = (Date.now() - start) / 1000;
    httpRequestDuration.labels(req.method, req.path, res.statusCode).observe(duration);
  });
  next();
});

// Expose metrics
app.get('/metrics', (req, res) => {
  res.set('Content-Type', prometheus.register.contentType);
  res.end(prometheus.register.metrics());
});
```

## ServiceMonitor (Auto-Scrape)

**Prometheus automatically discovers ServiceMonitor resources:**
```yaml
apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: myapp
  namespace: myapp-ns
spec:
  selector:
    matchLabels:
      app: myapp
  endpoints:
  - port: metrics
    interval: 30s
    path: /metrics
```

**Deploy & verify:**
```bash
kubectl apply -f myapp-servicemonitor.yaml

# Check Prometheus targets
kubectl port-forward -n monitoring svc/prometheus-kube-prom-prometheus 9090:9090
# http://localhost:9090/targets → should show myapp
```

## PrometheusRule (Alerting)

**Define alert rules:**
```yaml
apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: myapp-alerts
  namespace: myapp-ns
spec:
  groups:
  - name: myapp
    rules:
    - alert: HighErrorRate
      expr: rate(http_requests_total{status=~"5.."}[5m]) > 0.05  # > 5% error rate
      for: 5m
      annotations:
        summary: "High error rate for {{ $labels.service }}"
        
    - alert: HighLatency
      expr: histogram_quantile(0.99, http_request_duration_seconds_bucket) > 1  # p99 > 1s
      for: 10m
      annotations:
        summary: "High latency for {{ $labels.path }}"
```

## Logging (Loki)

**View pod logs in Grafana:**
```
Grafana → Explore → Loki
Query: {namespace="story-crater-backend"} | json | level="error"

Label filters:
- namespace
- pod_name
- container
- level (error, warn, info)
```

**Log retention:**
- 10-day default (configurable)
- Older logs deleted automatically
- Chunks stored in MinIO (s3://loki-chunks)

## Monitoring Checklist

**For every service:**
- [ ] Export `/metrics` (Prometheus format)
- [ ] Add ServiceMonitor resource
- [ ] Add PrometheusRule (errors, latency)
- [ ] Create Grafana dashboard (6 rows: Availability, Resources, Domain, Logs, SLO, Related)
- [ ] Set up Slack/PagerDuty integration (if on-call)

## Key Queries

**Request rate by status:**
```promql
sum(rate(http_requests_total[5m])) by (status)
```

**Error rate %:**
```promql
sum(rate(http_requests_total{status=~"5.."}[5m])) / sum(rate(http_requests_total[5m])) * 100
```

**P99 latency:**
```promql
histogram_quantile(0.99, rate(http_request_duration_seconds_bucket[5m]))
```

**Memory usage %:**
```promql
(1 - (node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes)) * 100
```

## Troubleshooting

**Prometheus can't scrape ServiceMonitor:**
```bash
# Check ServiceMonitor exists
k get servicemonitor -A | grep myapp

# Verify Service has correct labels
k get svc -A -o json | jq '.items[] | select(.metadata.name=="myapp") | .metadata.labels'

# Check Prometheus targets
kubectl port-forward -n monitoring svc/prometheus-kube-prom-prometheus 9090:9090
# http://localhost:9090/targets → look for "Down" targets with error
```

**Metrics missing from Grafana:**
```bash
# Verify pod is exporting metrics
k port-forward -n myapp-ns pod/myapp-0 8080:8080
curl http://localhost:8080/metrics | grep my_metric_name

# Check query syntax in Grafana
# Hover over query → "Query Inspector" to see response
```

**Alert not firing:**
```bash
# Check PrometheusRule is loaded
k get prometheusrule -n myapp-ns

# Verify query in Prometheus
kubectl port-forward -n monitoring svc/prometheus-kube-prom-prometheus 9090:9090
# http://localhost:9090 → Graph tab → paste query
```

See `/TROUBLESHOOTING.md` for full incident guide.