common/database/metrics.go

package database

import (
	"context"
	"database/sql"
	"fmt"
	"time"

	"github.com/prometheus/client_golang/prometheus"
)

// DatabaseMetrics holds the Prometheus metrics for database connection pool monitoring
type DatabaseMetrics struct {
	ConnectionsOpen         prometheus.Gauge
	ConnectionsIdle         prometheus.Gauge
	ConnectionsInUse        prometheus.Gauge
	ConnectionsWaitCount    prometheus.Counter
	ConnectionsWaitDuration prometheus.Histogram
}

// NewDatabaseMetrics creates a new set of database metrics and registers them
func NewDatabaseMetrics(registerer prometheus.Registerer) *DatabaseMetrics {
	metrics := &DatabaseMetrics{
		ConnectionsOpen: prometheus.NewGauge(prometheus.GaugeOpts{
			Name: "database_connections_open",
			Help: "Number of open database connections",
		}),
		ConnectionsIdle: prometheus.NewGauge(prometheus.GaugeOpts{
			Name: "database_connections_idle",
			Help: "Number of idle database connections",
		}),
		ConnectionsInUse: prometheus.NewGauge(prometheus.GaugeOpts{
			Name: "database_connections_in_use",
			Help: "Number of database connections in use",
		}),
		ConnectionsWaitCount: prometheus.NewCounter(prometheus.CounterOpts{
			Name: "database_connections_wait_count_total",
			Help: "Total number of times a connection had to wait",
		}),
		ConnectionsWaitDuration: prometheus.NewHistogram(prometheus.HistogramOpts{
			Name:    "database_connections_wait_duration_seconds",
			Help:    "Time spent waiting for a database connection",
			Buckets: prometheus.DefBuckets,
		}),
	}

	if registerer != nil {
		registerer.MustRegister(
			metrics.ConnectionsOpen,
			metrics.ConnectionsIdle,
			metrics.ConnectionsInUse,
			metrics.ConnectionsWaitCount,
			metrics.ConnectionsWaitDuration,
		)
	}

	return metrics
}

// monitorConnectionPool runs a background goroutine to collect connection pool metrics
func monitorConnectionPool(ctx context.Context, db *sql.DB, registerer prometheus.Registerer) {
	if registerer == nil {
		return // No metrics collection if no registerer provided
	}

	metrics := NewDatabaseMetrics(registerer)
	ticker := time.NewTicker(30 * time.Second)
	defer ticker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
			stats := db.Stats()

			metrics.ConnectionsOpen.Set(float64(stats.OpenConnections))
			metrics.ConnectionsIdle.Set(float64(stats.Idle))
			metrics.ConnectionsInUse.Set(float64(stats.InUse))
			metrics.ConnectionsWaitCount.Add(float64(stats.WaitCount))

			if stats.WaitDuration > 0 {
				metrics.ConnectionsWaitDuration.Observe(stats.WaitDuration.Seconds())
			}

			// Log connection pool stats for high usage or waiting
			if stats.OpenConnections > 20 || stats.WaitCount > 0 {
				fmt.Printf("Connection pool stats: open=%d idle=%d in_use=%d wait_count=%d wait_duration=%s\n",
					stats.OpenConnections, stats.Idle, stats.InUse, stats.WaitCount, stats.WaitDuration)
			}
		}
	}
}