Singleflight

package main

import (
	"fmt"
	"sync"
	"sync/atomic"
	"time"

	"golang.org/x/sync/singleflight"
)

var (
	group      singleflight.Group
	fetchCount atomic.Int64
)

// Simulate a slow database query
func fetchFromDB(key string) (string, error) {
	fetchCount.Add(1)
	time.Sleep(100 * time.Millisecond)
	return fmt.Sprintf("value-for-%s", key), nil
}

func getData(key string) (string, error) {
	val, err, shared := group.Do(key, func() (any, error) {
		return fetchFromDB(key)
	})
	if err != nil {
		return "", err
	}
	fmt.Printf("key=%s shared=%v result=%s\n", key, shared, val)
	return val.(string), nil
}

func main() {
	var wg sync.WaitGroup

	// 10 goroutines request the same key at the same time
	for i := 0; i < 10; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			getData("user:42")
		}()
	}

	wg.Wait()
	fmt.Printf("\n10 concurrent requests, %d actual DB calls\n", fetchCount.Load())
}